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    CAMBRIDGE BIOLOGICAL SERIES.

    GENERAL EDITOR:--ARTHUR E. SHIPLEY, M.A.

    FELLOW AND TUTOR OF CHRIST'S COLLEGE, CAMBRIDGE.




    THE

    VERTEBRATE SKELETON.




    London: C.J. CLAY AND SONS,
    CAMBRIDGE UNIVERSITY PRESS WAREHOUSE,
    AVE MARIA LANE,

    AND

    H.K. LEWIS,
    136, GOWER STREET, W.C.

    [Illustration]

    Glasgow: 50, WELLINGTON STREET.

    Leipzig: F.A. BROCKHAUS.

    New York: THE MACMILLAN COMPANY.

    Bombay and Calcutta: MACMILLAN AND CO., LTD.


    [_All Rights reserved._]




    THE

    VERTEBRATE SKELETON

    BY

    SIDNEY H. REYNOLDS, M.A.,

    TRINITY COLLEGE, CAMBRIDGE;
    LECTURER AND DEMONSTRATOR IN GEOLOGY AND ZOOLOGY AT UNIVERSITY
    COLLEGE, BRISTOL.

    Cambridge:

    AT THE UNIVERSITY PRESS.

    1897

    [_All Rights reserved._]




    =Cambridge=:

    PRINTED BY J. & C.F. CLAY,
    AT THE UNIVERSITY PRESS.




PREFACE.


IN the following pages the term skeleton is used in its widest sense,
so as to include exoskeletal or tegumentary structures, as well as
endoskeletal structures. It was thought advisable to include some
account of the skeleton of the lowest Chordata--animals which are not
strictly vertebrates, but it seemed undesirable to alter the title of
the book in consequence.

The plan adopted in the treatment of each group has been to give first
an account of the general skeletal characters of the group in question
and of its several subdivisions; secondly to describe in detail the
skeleton of one or more selected types; and thirdly to treat the
skeleton as developed in the group organ by organ.

A beginner is advised to commence, not with the introductory chapter,
but with the skeleton of the Dogfish, then to pass to the skeletons of
the Newt and Frog, and then to that of the Dog. After that he might
pass to the introductory chapter and work straight through the book. I
have endeavoured to make the account of each type skeleton complete in
itself; this has necessitated a certain amount of repetition,--a
fault that I have found it equally difficult to avoid in other parts
of the book.

Throughout the book generic names are printed in italics; and italics
are used in the accounts of the type skeletons for the names of
membrane bones. Clarendon type is used to emphasise certain words. In
the classificatory table the names of extinct genera only, are printed
in italics.

In a book in which an attempt is made to cover to some extent such a
vast field, it would be vain to hope to have avoided many errors both
of omission and commission, and I owe it to the kindness of several
friends that the errors are not much more numerous. I cannot however
too emphatically say that for those which remain I alone am
responsible. Messrs C.W. Andrews, E. Fawcett, S.F. Harmer, J. Graham
Kerr, and B. Rogers have all been kind enough to help me by reading
proofs or manuscript, while the assistance that I have received from
Dr Gadow during the earlier stages and from Prof. Lloyd Morgan and Mr
Shipley throughout the whole progress of the work has been very great.
To all these gentlemen my best thanks are tendered.

All the figures except 1, 35, 55, and 84 were drawn by Mr Edwin
Wilson, to whose care and skill I am much indebted. The majority are
from photographs taken by my sister Miss K.M. Reynolds or by myself in
the British Museum and in the Cambridge University Museum of Zoology,
and I take this opportunity of thanking Sir W.H. Flower and Mr S.F.
Harmer for the facilities they have afforded and for permission to
figure many objects in the museums respectively under their charge. I
have also to thank (1) Prof. von Zittel for permission to reproduce
figs. 27, 41, 52, 69, 70, 80, 106 A, and 107 C; (2) Sir W.H. Flower
and Messrs A. and C. Black for figs. 1 and 84; (3) Prof. O.C. Marsh
and Dr H. Woodward for fig. 35; (4) Dr C.H. Hurst and Messrs Smith,
Elder, and Co. for fig. 55.

A few references are given, but no attempt has been made to give
anything like a complete list. The abbreviations of the titles of
periodicals are those used in the _Zoological Record_.

I have always referred freely to the textbooks treating of the
subjects dealt with, and in particular I should like to mention that
the section devoted to the skeleton of mammals is, as it could hardly
fail to be, to a considerable extent based on Sir W.H. Flower's
_Osteology of the Mammalia_.

    SIDNEY H. REYNOLDS.

    _March 10, 1897._




                             CONTENTS.


                                                                  PAGE
                              CHAPTER I.

  Introductory account of the skeleton in general                    1


                             CHAPTER II.

  Classification                                                    30


                            CHAPTER III.

  Skeleton of Hemichordata, Urochordata and Cephalochordata         50


                             CHAPTER IV.

  Skeletal characters of the Vertebrata. The skeleton in the
     Cyclostomata                                                   53


                              CHAPTER V.

  Skeletal characters of the Ichthyopsida. Characters of the
     several groups of Pisces                                       59


                             CHAPTER VI.

  The skeleton of the Dogfish (Scyllium canicula)                   71


                            CHAPTER VII.

  The skeleton of the Codfish (_Gadus morrhua_) and the skull of
     the Salmon (_Salmo salar_)                                     83


                           CHAPTER VIII.

  General account of the skeleton in Fishes                        104


                             CHAPTER IX.

  Characters of the several groups of Amphibia                     133


                              CHAPTER X.

  The skeleton of the Newt (_Molge cristata_)                      138


                             CHAPTER XI.

  The skeleton of the Frog (_Rana temporaria_)                     151


                            CHAPTER XII.

  General account of the skeleton in Amphibia                      168


                           CHAPTER XIII.

  Skeletal characters of the Sauropsida. Characters of the
     several groups of Reptiles                                    189


                            CHAPTER XIV.

  The skeleton of the Green Turtle (_Chelone midas_)               214


                             CHAPTER XV.

  The skeleton of the Crocodile (_Crocodilus palustris_)           237


                            CHAPTER XVI.

  General account of the skeleton in Reptiles                      270


                           CHAPTER XVII.

  Characters of the several groups of Birds                        295


                          CHAPTER XVIII.

  The skeleton of the Wild Duck (_Anas boschas_)                   302


                            CHAPTER XIX.

  General account of the skeleton in Birds                         328


                             CHAPTER XX.

  Characters of the several groups of Mammalia                     343


                            CHAPTER XXI.

  The skeleton of the Dog (_Canis familiaris_)                     374


                           CHAPTER XXII.

  General account of the skeleton in Mammalia. The exoskeleton
     and vertebral column                                          416


                          CHAPTER XXIII.

  General account of the skeleton in Mammalia (_continued_). The
     skull and appendicular skeleton                               455




                        LIST OF ILLUSTRATIONS.


  FIG.                                                            PAGE

  1   Diagrammatic sections of various forms of teeth                6

  2   Cervical vertebrae of an Ox (_Bos taurus_)                    15

  3   Diagram of the skeleton of _Amphioxus lanceolatus_            51

  4   Dorsal, lateral, and ventral views of the skull of
        _Petromyzon marinus_                                        56

  5   Skull of a male _Chimaera monstrosa_                          65

  6   Lateral view of the skull of a Dogfish (_Scyllium canicula_)  75

  7   Semidorsal view of the pectoral girdle and fins of a Dogfish
        (_Scyllium canicula_)                                       80

  8   Dorsal view of the pelvic girdle and fins of a male Dogfish
        (_Scyllium canicula_)                                       81

  9   Dorsal and ventral views of the cranium of a Salmon (_Salmo
        salar_) from which most of the membrane bones have been
        removed                                                     88

  10   Lateral view of the chondrocranium of a Salmon (_Salmo
          salar_)                                                   90

  11   Lateral view of the skull of a Salmon (_Salmo salar_)        92

  12   Mandibular and hyoid arches of a Cod (_Gadus morrhua_)       99

  13   Right half of the pectoral girdle and right pectoral fin of
          a Cod (_Gadus morrhua_)                                  102

  14   Diagram of a section through the jaw of a Shark
          (_Odontaspis americanus_) showing the succession of
          teeth                                                    107

  15   Part of the lower jaw of a Shark (_Galeus_)                 108

  16   Skulls of _Notidanus_ and _Cestracion_                      118

  17   Dorsal view of the branchial arches of _Heptanchus_         120

  18   Lateral view of the skull of a Sturgeon
          (_Acipenser sturio_)                                     122

  19   Dorsal and ventral views of the cranium of
          _Ceratodus miolepis_                                     125

  20   Lateral view of the skeleton of _Ceratodus miolepis_        128

  21   Dorsal, ventral and lateral views of the skull of a Newt
          (_Molge cristata_)                                       142

  22   Ventral and lateral views of the shoulder-girdle and
          sternum of an old male Crested Newt (_Molge cristata_)   146


  23   Right posterior and anterior limbs of a Newt (_Molge
          cristata_)                                               148

  24   Dorsal and ventral views of the cranium of a Common Frog
          (_Rana temporaria_)                                      155

  25   Dorsal and ventral views of the cranium of a Common Frog
          (_Rana temporaria_) from which the membrane bones have
          mostly been removed                                      157

  26   Lateral view of the skull and posterior view of the
          cranium of a Common Frog (_Rana temporaria_)             159

  27   Dorsal view of the skull of a Labyrinthodont (_Capitosaurus
          nasutus_)                                                176

  28   Ventral view of the cranium, and lateral view of the
          cranium and mandible of _Siphonops annulatus_            178

  29   Visceral arches of Amphibia: A, _Molge cristata_; B, _Rana
          temporaria_, adult; C, Tadpole of _Rana_; D, _Siredon
          pisciformis_                                             181

  30   Shoulder-girdle and sternum of an adult male Common Frog
          (_Rana temporaria_), and of an adult female
          _Docidophryne gigantea_                                  183

  31   A, Right antibrachium and manus of a larval Salamander
          (_Salamandra maculosa_); B, Right tarsus and adjoining
          bones of _Molge sp._                                     186

  32   Lateral and dorsal views of the skull of an _Ichthyosaurus_ 196

  33   Lateral view and longitudinal section of the skull of a
          Lizard (_Varanus varius_)                                201

  34   Lateral view of the shoulder-girdle of a Lizard (_Varanus_) 202

  35   Restored skeleton of _Ceratosaurus nasicornis_              206

  36   Dorsal and ventral views of the carapace of a Loggerhead
          Turtle (_Thalassochelys caretta_)                        216

  37   Plastron of a Green Turtle (_Chelone midas_)                218

  38   The skull of a Green Turtle (_Chelone midas_)               223

  39   Longitudinal vertical section through the cranium of a
          Green Turtle (_Chelone midas_)                           226

  40   Anterior limb of a young Hawksbill Turtle (_Chelone
          imbricata_), and posterior limb of a large Green Turtle
          (_Chelone midas_)                                        234

  41   The first four cervical vertebrae of a Crocodile
          (_Crocodilus vulgaris_)                                  239

  42   Anterior view of a late thoracic and the first sacral
          vertebrae of a Crocodile (_Crocodilus palustris_)        242


  43   Palatal aspect of the cranium and mandible of an Alligator
          (_Caiman latirostris_)                                   245

  44   Lateral view of the skull of an Alligator (_Caiman
          latirostris_)                                            248

  45   Longitudinal section through the skull of an Alligator
          (_Caiman latirostris_)                                   253

  46   Sternum and associated membrane bones of a Crocodile
          (_Crocodilus palustris_)                                 261

  47   Left half of the pectoral girdle of an Alligator
          (_Caiman latirostris_)                                   262

  48   Right anterior and posterior limbs of an Alligator
          (_Caiman latirostris_)                                   264

  49   Pelvis and sacrum of an Alligator (_Caiman latirostris_)    267

  50   Preparation of part of the right mandibular ramus of
          _Crocodilus palustris_                                   274

  51   Dorsal and ventral views of the skull of a Common Snake
          (_Tropidonotus natrix_)                                  279

  52   Skull of Hatteria (_Sphenodon punctatus_)                   282

  53   Hyoids of an Alligator (_Caiman latirostris_), and of a
          Green Turtle (_Chelone midas_)                           285

  54   Ventral view of the shoulder-girdle and sternum of
          _Loemanctus longipes_                                    287

  55   Left half of the skeleton of a Common Fowl (_Gallus
          bankiva_)                                                301
  56   The wing of a Wild Duck (_Anas boschas_)                    304

  57   Wings of a Wild Duck with the coverts removed (_Anas
          boschas_)                                                305

  58   Dorsal and ventral views of the pelvis and sacrum of a Duck
          (_Anas boschas_)                                         311

  59   Skull of a Duck (_Anas boschas_)                            312

  60   A, Ventral view of the cranium of a Duck (_Anas boschas_);
          B, Cranium and mandible seen from the left side          313

  61   Lateral view of the pelvis and sacrum of a Duck (_Anas
          boschas_)                                                325
  62   Third cervical vertebra of an Ostrich (_Struthio camelus_)  331

  63   Shoulder-girdle and sternum of A, Black Vulture (_Vultur
          cinereus_); B, Peacock (_Pavo cristatus_); C, Pelican
          (_Pelicanus conspicillatus_)                             337

  64   Bones of the right wing of A, a Penguin; B, an Ostrich
          (_Struthio camelus_) and C, a Gannet (_Sula alba_)       339

  65   Pelvic girdle and sacrum of A, Cassowary (_Casuarius
          galeatus_); B, Owen's Apteryx (_A. oweni_); C,
          Broad-billed Rhea (_R. macrorhyncha_); D, Ostrich
          (_Struthio camelus_)                                     340

  66   Ventral view of the shoulder-girdle and sternum of a
          Duckbill (_Ornithorhynchus paradoxus_)                   347

  67   Cervical vertebrae of a Ca'ing Whale (_Globicephalus
       melas_)                                                     354

  68   Dentition of a Dog (_Canis familiaris_)                     375

  69   Atlas and axis vertebrae of a Dog (_Canis familiaris_)      379

  70   Second thoracic and second lumbar vertebrae of a Dog
          (_Canis familiaris_)                                     382

  71   Diagram of the relations of the principal bones in the
          Mammalian skull                                          385

  72   Vertical longitudinal section through skull of a Dog
          (_Canis familiaris_)                                     387

  73   Dorsal view of the cranium of a Dog (_Canis familiaris_)    389

  74   Diagram of the mammalian tympanic cavity and associated
          bones                                                    391

  75   Ventral view of the cranium of a Dog (_Canis familiaris_)   396

  76   Sternum and sternal ribs of a Dog (_Canis familiaris_)      403

  77   Bones of the left upper arm and fore-arm of a Dog (_Canis
          familiaris_)                                             407

  78   Right innominate bone, A, of a full-grown Terrier; B, of a
          Collie Puppy                                             410

  79   Left leg bones of a Dog (_Canis familiaris_)                411

  80   A, Right manus; B, Right pes of a Dog (_Canis familiaris_)  413

  81   Skull of a young Indian Rhinoceros (_R. unicornis_) showing
          the change of the dentition                              421

  82   Palatal aspect of the cranium and mandible of a Donkey
          (_Equus asinus_)                                         431

  83   Skull of _Procavia (Dendrohyrax) dorsalis_                  433

  84   Carnassial or sectorial teeth of Carnivora                  436

  85   Mandible of Isabelline Bear (_Ursus isabellinus_)           438

  86   Left mandibular ramus of the Sea Leopard (_Ogmorhinus
          leptonyx_)                                               439

  87   Cervical vertebrae of a young Fin Whale (_Balaenoptera
          musculus_)                                               444

  88   Atlas and axis vertebrae of an Ox (_Bos taurus_)            445

  89   First and second thoracic vertebrae of an Ox (_Bos taurus_) 449

  90   Skulls of Tasmanian Wolf (_Thylacinus cynocephalus_) and
          Hairy-nosed Wombat (_Phascolomys latifrons_)             456

  91   Skull of Two-fingered Sloth (_Choloepus didactylus_)        458

  92   Skull of _Rhytina stelleri_                                 460


  93   Lateral view and longitudinal section of the skull of a
          young Ca'ing Whale (_Globicephalus melas_)               463

  94   Cranium and mandible of a Pig (_Sus scrofa_)                466

  95   Mandible of a Hippopotamus (_Hippopotamus amphibius_)       467

  96   Skull of a young Indian Elephant (_Elephas indicus_)        474

  97   Longitudinal section of the skull of a young Indian
          Elephant (_Elephas indicus_)                             475

  98   Half-front view of the skull of a Porcupine (_Hystrix
          cristata_)                                               477
  99   Skulls of an old and of a young Gorilla (_Gorilla savagei_) 483

  100   Malleus, stapes, and incus of Man, Dog, and Rabbit         485

  101   Skeleton of a Cape Buffalo (_Bubalus caffer_)              492

  102   Lateral and dorsal views of the shoulder-girdle and part
           of the sternum of the Spiny Anteater (_Echidna
           aculeata_)                                              494

  103   Skeleton of a Llama (_Auchenia glama_)                     496

  104   Dorsal view of the sternum and right half of the shoulder
           girdle of _Mus sylvaticus_                              498

  105   Anterior surface of the right humerus of a Wombat
           (_Phascolomys latifrons_)                               500

  106   Manus of Perissodactyles: A, Left manus of _Tapirus_; B,
           Right manus of _Titanotherium_; C, Left manus of
           _Chalicotherium giganteum_                              508

  107   Left manus of A, _Coryphodon hamatus_; B, _Phenacodus
           primaevus_; C, _Procavia (Dendrohyrax) arboreus_        510

  108   Left anterior and posterior limbs and limb girdles of
           _Uintatherium mirabile_                                 516

  109   Left femur of an Ox (_Bos taurus_) and of a Sumatran
           Rhinoceros (_Rhinoceros sumatrensis_)                   518

  110   Pes of A, a Tapir (_Tapirus americanus_); B, a Rhinoceros
           (_Rhinoceros sumatrensis_); C, _Hipparion gracile_; D,
           a Horse (_Equus caballus_)                              524




ERRATA.


     p. 172, note, _for_ 14 _read_ 15.

     p. 279, description of figure, _for_ Tropidinotus _read_
     Tropidonotus.

     p. 287, description of figure, _for_ shoulder-girdle of sternum
     _read_ shoulder-girdle and sternum.

     p. 393, middle of page, _for_ VIII _read_ VII.

     p. 427, line 2, _for_ Grampus _read_ Killer.




CHAPTER I.

INTRODUCTORY ACCOUNT OF THE SKELETON IN GENERAL.


BY the term =skeleton= is meant the hard structures whose function is
to support or to protect the softer tissues of the animal body.

The skeleton is divisible into

A. The EXOSKELETON, which is external;

B. The ENDOSKELETON, which is as a rule internal; though in some
cases, e.g. the antlers of deer, endoskeletal structures become, as
development proceeds, external.

In Invertebrates the hard, supporting structures of the body are
mainly =exoskeletal=, in Vertebrates they are mainly =endoskeletal=;
but the endoskeleton includes, especially in the skull, a number of
elements, the =dermal= or =membrane= bones, which are shown by
development to have been originally of external origin. These membrane
bones are so intimately related to the true endoskeleton that they
will be described with it. The simplest and lowest types of both
vertebrate and invertebrate animals have unsegmented skeletons; with
the need for flexibility however segmentation arose both in the case
of the invertebrate exoskeleton and the vertebrate endoskeleton. The
exoskeleton in vertebrates is phylogenetically older than the
endoskeleton, as is indicated by both palaeontology and embryology.
Palaeontological evidence is afforded by the fact that all the lower
groups of vertebrates--Fish, Amphibia, and Reptiles--had in former
geological periods a greater proportion of species protected by
well-developed dermal armour than is the case at present.
Embryological evidence tends the same way, inasmuch as dermal
ossifications appear much earlier in the developing animal than do the
ossifications in the endoskeleton.

Skeletal structures may be derived from each of the three germinal
layers. Thus =hairs= and =feathers= are =epiblastic= in origin,
=bones= are =mesoblastic=, and the =notochord= is =hypoblastic=.

The different types of skeletal structures may now be considered and
classified more fully.


A. EXOSKELETAL STRUCTURES.

I. EPIBLASTIC (epidermal).

Exoskeletal structures of epiblastic origin may be developed on both
the inner and outer surfaces of the Malpighian layer of the
epidermis[1]. Those developed on the outer surface include =hairs=,
=feathers=, =scales=, =nails=, =beaks= and =tortoiseshell=; and are
specially found in vertebrates higher than fishes. Those developed on
the inner surface of the Malpighian layer include only the =enamel= of
teeth and some kinds of scales. With the exception of feathers, which
are partly formed from the horny layer, all these parts are mainly
derived from the Malpighian layer of the epidermis.

=Hairs= are slender, elongated structures which arise by the
proliferation of cells from the Malpighian layer of the epidermis.
These cells in the case of each hair form a short papilla, which sinks
inwards and becomes imbedded at the bottom of a follicle in the
dermis. Each hair is normally composed of an inner cellular pithy
portion containing much air, and an outer denser cortical portion of a
horny nature. Sometimes, as in Deer, the hair is mainly formed of the
pithy portion, and is then easily broken. Sometimes the horny part
predominates, as in the bristles of Pigs. A highly vascular dermal
papilla projects into the base of the hair.

=Feathers=, like hairs, arise from epidermal papillae which become
imbedded in pits in the dermis. But the feather germ differs from the
hair germ, in the fact that it first grows out like a cone on the
surface of the epidermis, and that the horny as well as the Malpighian
layer takes part in its formation.

=Nails=, =claws=, =hoofs=, and the =horns of Oxen= are also epidermal,
as are such structures as the =scales= of reptiles, of birds' feet,
and of _Manis_ among mammals, the =rattle= of the rattlesnake, the
=nasal horns= of _Rhinoceros_, and the =baleen= of whales. All these
structures will be described later.

=Nails= arise in the interior of the epidermis by the thickening and
cornification of the stratum lucidum. The outer border of the nail
soon becomes free, and growth takes place by additions to the inner
surface and attached end.

When a nail tapers to a sharp point it is called a =claw=. In many
cases the nails more or less surround the ends of the digits by which
they are borne.

Horny =beaks= of epidermal origin occur casing the jaw-bones in
several widely distinct groups of animals. Thus among reptiles they
are found in Chelonia (tortoises and turtles) as well as in some
extinct forms; they occur in all living birds, in _Ornithorhynchus_
among mammals, and in the larvae of many Amphibia.

In a few animals, such as Lampreys and _Ornithorhynchus_, the jaws
bear horny tooth-like structures of epidermal origin.

The =enamel= of teeth and of placoid scales is also epiblastic in
origin[2], and it may be well at this point to give some account of
the structure of teeth, though they are partly mesoblastic in origin.
The simplest teeth are those met with in sharks and dogfish, where
they are merely the slightly modified scales developed in the
integument of the mouth. They pass by quite insensible gradations into
normal placoid scales, such as cover the general surface of the body.
A =placoid scale=[3] is developed on a papilla of the dermis which
projects outwards and backwards, and is covered by the columnar
Malpighian layer of the epidermis. The outer layer of the dermal
papilla then gradually becomes converted into dentine and bone, while
enamel is developed on the inner side of the Malpighian layer, forming
a cap to the scale. The Malpighian and horny layers of the epidermis
get rubbed off the enamel cap, so that it comes to project freely on
the surface of the body.

As regards their attachment teeth may be (1) attached to the fibrous
integument of the mouth, or (2) fixed to the jaws or other bones of
the mouth, or (3) planted in grooves, or (4) in definite sockets in
the jaw-bones (see p. 107).

Teeth in general consist of three tissues, =enamel=, =dentine= and
=cement=, enclosing a central pulp-cavity containing blood-vessels and
nerves. Enamel is, however, often absent, as in all living Edentates.

=Enamel= generally forms the outermost layer of the crown or visible
part of the tooth; it is the hardest tissue occurring in the animal
body and consists of prismatic fibres arranged at right angles to the
surface of the tooth. It is characterised by its bluish-white
translucent appearance.

II. MESOBLASTIC (mesodermal).

=Dentine= or =ivory= generally forms the main mass of a tooth. It is a
hard, white substance allied to bone. When examined microscopically
dentine is seen to be traversed by great numbers of nearly parallel
branching tubules which radiate outwards from the pulp-cavity. In
fishes as a rule, and sometimes in other animals, a variety of dentine
containing blood-vessels occurs, this is called =vasodentine=.

=Cement= or =crusta petrosa= forms the outermost layer of the root of
the tooth. In composition and structure it is practically identical
with bone. In the more complicated mammalian teeth, besides enveloping
the root, it fills up the spaces between the folds of the enamel.

The hard parts of a tooth commonly enclose a central pulp-cavity into
which projects the pulp, a papilla of the dermis including
blood-vessels and nerves. As long as growth continues the outer layers
of this pulp become successively calcified and added to the substance
of the dentine. In young growing teeth the pulp-cavity remains widely
open, but in mammals the general rule is that as a tooth gets older
and the crown becomes fully formed, the remainder of the pulp becomes
converted into one or more tapering roots which are imbedded in the
alveolar cavities of the jaws. The opening of the pulp-cavity is then
reduced to a minute perforation at the base of each root. A tooth of
this kind is called a =rooted= tooth.

But it is not only in young teeth that the pulp-cavity sometimes
remains widely open; for some teeth, such as the tusks of Elephants
and the incisor teeth of Rodents, form no roots and continue to grow
throughout the animal's life. Such teeth are said to be rootless or to
have persistent pulps.

An intermediate condition is seen in some teeth, such as the grinding
teeth of Horses. These teeth grow for a very long time, their crowns
wearing away as fast as their bases are produced; finally however
definite roots are formed and growth ceases.

[Illustration FIG. 1. DIAGRAMMATIC SECTIONS OF VARIOUS FORMS OF TEETH
(from FLOWER).

I. Incisor or tusk of elephant, with pulp-cavity persistently open at
base. II. Human incisor during development with root imperfectly
formed, and pulp-cavity widely open at base. III. Completely formed
human incisor, with pulp-cavity contracted to a small aperture at the
end of the root. IV. Human molar with broad crown and two roots. V.
Molar of Ox, with the enamel covering the crown, deeply folded and the
depressions filled with cement. The surface is worn by use, otherwise
the enamel coating would be continuous at the top of the ridges. In
all the figures the enamel is black, the pulp white, the dentine
represented by horizontal lines, and the cement by dots.]

The teeth of any animal may be =homodont=, that is, all having the
same general character, or =heterodont=, that is, having different
forms adapted to different functions. The dentition is heterodont in a
few reptiles and the majority of mammals.

SUCCESSION OF TEETH. In most fishes, and many amphibians and reptiles
the teeth can be renewed indefinitely. In sharks, for example,
numerous rows of reserve teeth are to be seen folded back behind those
in use (see fig. 15). The majority of mammals have only two sets of
teeth, and are said to be =diphyodont=; some have only a single series
(=monophyodont=).

DEVELOPMENT OF TEETH. A brief sketch of the method in which
development of teeth takes place in the higher vertebrates may here be
given. Along the surface of the jaws a thickening of the epiblastic
epithelium takes place, giving rise to a ridge, which sinks inwards
into the tissue of the jaw, and it is known as the primary enamel
organ. At the points where teeth are to be developed special ingrowths
of this primary enamel organ take place, and into each there projects
a vascular dental papilla from the surrounding mesoblast of the jaw.
Each ingrowth of the enamel organ forms an =enamel cap=, which
gradually embraces the dental papilla, and at the same time appears to
be pushed on one side, owing to the growth not being uniform. The
external layer of the dental papilla is composed of long nucleated
cells, the =odontoblasts=, and it is by these that the dentine is
formed. Similarly the internal layer of the enamel organ is formed of
columnar enamel cells, which give rise to the enamel. The mesoblastic
cells surrounding the base of the tooth give rise to the cement.

=Bone= is in many cases exoskeletal, but it will be most conveniently
described with the endoskeleton.

The =scales of fish= are wholly or in part mesoblastic in origin,
being totally different from those of reptiles. The =cycloid= and
=ctenoid= scales of Teleosteans (see p. 105) are thin plates coated
with epidermis. They are sometimes bony, but as a rule are simply
calcified. =Ganoid= scales are flat plates of bone coated with an
enamel-like substance, and articulating together with a peg and socket
arrangement; they are probably identical with enlarged and flattened
placoid scales.

The =armour plates= of fossil Ganoids, Labyrinthodonts, and Dinosaurs,
and of living Crocodiles, some Lizards and Armadillos, are composed of
bone. They are always covered by a layer of epidermis.

The =antlers of deer= are also composed of bone; they will be more
fully described in the chapter on mammals. It may perhaps be well to
mention them here, though they really belong to the endoskeleton,
being outgrowths from the frontal bones.


B. ENDOSKELETAL STRUCTURES.

I. HYPOBLASTIC.

(_a_) The =notochord= is an elastic rod formed of large vacuolated
cells, and is surrounded by a membranous sheath of mesoblastic origin.
It is the primitive endoskeleton in the Chordata, all of which possess
it at some period of their existence; while in many of the lower forms
it persists throughout life. Even in the highest Chordata it is the
sole representative of the axial skeleton for a considerable part of
the early embryonic life. A simple unsegmented notochord persists
throughout life in the Cephalochordata, Cyclostomata, and some Pisces,
such as Sturgeons and Chimaeroids.

(_b_) The enamel of the pharyngeal teeth of the Salmon and many other
Teleosteans is hypoblastic in origin. The epiblast of the stomodaeum,
in which the other teeth are developed, passes into the hypoblast of
the mesenteron in which these pharyngeal teeth are formed.


II. MESOBLASTIC.

The most primitive type of a mesoblastic endoskeleton consists of a
membranous sheath surrounding the notochord, as in _Myxine_ and its
allies. The first stage of complication is by the development of
cartilage in the notochordal sheath, as in _Petromyzon_. Often the
cartilage becomes calcified in places, as in the vertebral centra of
_Scyllium_ and other Elasmobranchs. Lastly, the formation of bone
takes place; it generally constitutes the most important of the
endoskeletal structures.

=Bone= may be formed in two ways:--

(1) by the direct ossification of pre-existing cartilage, when it is
known as =cartilage bone= or =endochondral bone=;

(2) by independent ossification in connective tissue; it is then known
as =membrane= or =dermal= or =periosteal bone=.

With the exception of the _clavicle_[4] all the bones of the trunk and
limbs, together with a large proportion of those of the skull, are
preformed in the embryo in cartilage, and are grouped as cartilage
bones; while the clavicle and most of the roofing and jaw-bones of the
skull are not preformed in cartilage, being developed simply in
connection with a membrane. Hence it is customary to draw a very
strong line of distinction between these two kinds of bone; in reality
however this distinction is often exaggerated, and the two kinds pass
into one another, and as will be shown immediately, the permanent
osseous tissue of many of those which are generally regarded as
typical cartilage bones, is really to a great extent of periosteal
origin. The palatine bone, for instance, of the higher vertebrates in
general is preceded by a cartilaginous bar, but is itself almost
entirely a membrane bone.

Before describing the development of bone it will be well to briefly
describe the structure of adult bone and cartilage.

The commonest kind of =cartilage=, and that which preforms so many of
the bones of the embryo, is =hyaline= cartilage. It consists of oval
nucleated cells occupying cavities (=lacunae=) in a clear
intercellular semitransparent matrix, which is probably secreted by
the cells. Sometimes one cell is seen in each lacuna, sometimes
shortly after cell-division a lacuna may contain two or more cells.
The free surface of the cartilage is invested by a fibrous membrane,
the =perichondrium=.

=Bone= consists of a series of lamellae of ossified substance between
which are oval spaces, the =lacunae=, giving rise to numerous fine
channels, the =canaliculi=, which radiate off in all directions. The
lacunae are occupied by the =bone cells= which correspond to cartilage
cells, from which if the bone is young, processes pass off into the
canaliculi. It is obvious that the ossified substance of bone is
intercellular in character, and corresponds to the matrix of
cartilage.

Bone may be compact, or loose and spongy in character, when it is
known as =cancellous bone=. In compact bone many of the lamellae are
arranged concentrically round cavities, the =Haversian canals=, which
in life are occupied by blood-vessels. Each Haversian canal with its
lamellae forms a =Haversian system=. In spongy bone instead of
Haversian canals there occur large irregular spaces filled with
marrow, which consists chiefly of blood-vessels and fatty tissue. The
centre of a long bone is generally occupied by one large continuous
marrow cavity. The whole bone is surrounded by a fibrous connective
tissue membrane, =the periosteum=.

THE DEVELOPMENT OF BONE.

=Periosteal ossification.= An example of a bone entirely formed in
this way is afforded by the parietal. The first trace of ossification
is shown by the appearance, below the membrane which occupies the
place of the bone in the early embryo, of calcareous spicules of bony
matter, which are laid down round themselves by certain large cells,
the =osteoblasts=. These osteoblasts gradually get surrounded by the
matter which they secrete and become converted into bone cells, and in
this way a mass of spongy bone is gradually produced. Meanwhile a
definite periosteum has been formed round the developing bone, and on
its inner side fresh osteoblasts are produced, and these with the
others gradually render the bone larger and more and more compact.
Finally, the middle layer of the bone becomes again hollowed out and
rendered spongy by the absorption of part of the bony matter.

=Endochondral ossification=[5]. This is best studied in the case of a
long bone like the femur or humerus. Such a long bone consists of a
shaft, which forms the main part, and two terminal portions, which
form the =epiphyses=, or portions ossifying from centres distinct from
that forming the shaft or main part of the bone.

In the earliest stage the future bone consists of hyaline cartilage
surrounded by a vascular sheath, the perichondrium.

Then, starting from the centre, the cartilage becomes permeated by a
number of channels into which pass vessels from the perichondrium and
osteoblasts. In this way the centre of the developing shaft becomes
converted into a mass of cavities separated by bands or trabeculae of
cartilage. This cartilage next becomes calcified, but as yet is not
converted into true bone. The osteoblasts in connection with the
cavities now begin to deposit true endochondral spongy bone, and then
after a time this becomes absorbed by certain large cells, the
osteoclasts, and resolved into marrow or vascular tissue loaded with
fat. So that the centre of the shaft passes from the condition of
hyaline cartilage to that of calcified cartilage, thence to the
condition of spongy bone, and finally to that of marrow. At the same
time beneath the perichondrium osteoblasts are developed which also
begin to give rise to spongy bone. The perichondrium thus becomes the
periosteum, and the bone produced by it, is periosteal or membrane
bone. So that while a continuous marrow cavity is gradually being
formed in the centre of the shaft, the layer of periosteal bone round
the margin is gradually thickening, and becoming more and more compact
by the narrowing down of its cavities to the size of Haversian canals.
The absorption of endochondral and formation of periosteal bone goes
on, till in time it comes about that the whole of the shaft, except
its terminations, is of periosteal origin. At the extremities of the
shaft, however, and at the epiphyses, each of which is for a long time
separated from the shaft by a pad of cartilage, the ossification is
mainly endochondral, the periosteal bone being represented only by a
thin layer.

Until the adult condition is reached and growth ceases, the pad of
cartilage between the epiphysis and the shaft continues to grow, its
outer (epiphysial) half growing by the formation of fresh cartilage as
fast as its inner half is encroached on by the growth of bone from the
shaft. The terminal or articular surfaces of the bone remain
throughout life covered by layers of articular cartilage.

Even after the adult condition is reached the bone is subject to
continual change, processes of absorption and fresh formation going on
for a time and tending to render the bone more compact.

METHODS IN WHICH BONES ARE UNITED TO ONE ANOTHER.

The various bones composing the endoskeleton are united to one another
either by =sutures= or by movable =joints=.

When two bones are suturally united, their edges fit closely together
and often interlock, being also bound together by the periosteum.

In many cases this sutural union passes into fusion or =ankylosis=,
ossification extending completely from one bone to the other with the
obliteration of the intervening suture. This feature is especially
well marked in the cranium of most birds.

The various kinds of joints or articulations[6] may be subdivided into
imperfect joints and perfect joints.

In =imperfect joints=, such as the intervertebral joints of mammals,
the two contiguous surfaces are united by a mass of fibrous tissue
which allows only a limited amount of motion.

In =perfect joints= the contiguous articular surfaces are covered with
cartilage, and between them lies a synovial membrane which secretes a
viscid lubricating fluid.

The amount of motion possible varies according to the nature of the
articular surfaces; these include--

_a._ =ball and socket joints=, like the hip and shoulder, in which the
end of one bone works in a cup provided by another, and movements can
take place in a variety of planes.

_b._ =hinge joints=, like the elbow and knee, in which as in
ball-and-socket joints one bone works in a cup provided by another,
but movements can take place in one plane only.


THE ENDOSKELETON.

The endoskeleton is divisible into =axial= and =appendicular= parts;
and the =axial= skeleton into--

    1. the spinal column,

    2. the skull {_a._ the cranium,
                 {_b._ the jaws and visceral skeleton,

    3. the ribs and sternum[7].


I. THE AXIAL SKELETON.

1. THE SPINAL COLUMN.

The spinal column in the simplest cases consists of an unsegmented
rod, the notochord, surrounded by the =skeletogenous layer=, a sheath
of mesoblastic origin, which also envelops the nerve cord. Several
intermediate stages connect this simple spinal column with the
vertebral column characteristic of higher vertebrates. A typical
vertebral column may be said to consist of (1) a series of
cartilaginous or bony blocks, the vertebral =centra=, which arise in
the sheath surrounding the notochord. They cause the notochord to
become constricted and to atrophy to a varying extent, though a
remnant of it persists, either permanently or for a long period,
within each centrum or between successive centra. (2) From the dorsal
surface of each centrum arise a pair of processes which grow round the
spinal cord and unite above it, forming a =dorsal= or =neural arch=.
(3) A similar pair of processes arising from the ventral surface of
the centrum form the =ventral= or =haemal arch=. To the ventral arch
the ribs strictly belong, and it tends to surround the ventral
blood-vessels and the body cavity with the alimentary canal and other
viscera.

A =neural spine= or spinous process commonly projects upwards from the
dorsal surface of the neural arch, and a pair of =transverse
processes= project outwards from its sides. When, as is commonly the
case, the two halves of the haemal arch do not meet, the ventral
surface of the centrum often bears a downwardly-projecting
=hypapophysis=.

The character of the surfaces by which vertebral centra articulate
with one another varies much. Sometimes both surfaces are concave, and
the vertebra is then said to be =amphicoelous=; sometimes a centrum is
convex in front and concave behind, the vertebra is then
=opisthocoelous=, sometimes concave in front and convex behind, when
the vertebra is =procoelous=. Again, in many vertebrae both faces of
the centra are flat, while in others they are saddle-shaped, as in the
neck vertebrae of living birds, or biconvex, as in the case of the
first caudal vertebra of crocodiles.

In the higher vertebrates pads of fibrocartilage--the =intervertebral
discs=--are commonly interposed between successive centra, these or
parts of them often ossify, especially in the trunk and tail, and are
then known as =inter centra=.

[Illustration FIG. 2. CERVICAL VERTEBRAE OF AN OX (_Bos taurus_).

A, is the fifth; B, the fourth; C, the third. x 1/4 (Camb. Mus.)

     1. neural spine.
     2. transverse process.
     3. hypapophysis.
     4. convex anterior face of the centrum.
     5. concave posterior face of the centrum.
     6. prezygapophysis.
     7. postzygapophysis.
     8. vertebrarterial canal.
     9. neural canal.
    10. inferior lamella of transverse process.]

The vertebrae of the higher forms can generally be arranged in the
following five groups, each marked by certain special characteristics:

1. The =cervical= or =neck vertebrae=. These connect the skull with
the thorax, and are characterised by relatively great freedom of
movement. They often bear small ribs, but are distinguished from the
succeeding thoracic vertebrae by the fact that their ribs do not reach
the sternum. The first cervical vertebra which articulates with the
skull is called the =atlas=, but a study of the nerve exits shows that
the first vertebra is not serially homologous throughout the
Ichthyopsida, so that it is best to reserve the term atlas for the
first vertebra in Sauropsida and Mammalia.

2. The =thoracic vertebrae= (often called dorsal) bear movably
articulated ribs which unite ventrally with the sternum.

3. The =lumbar vertebrae= are generally large, and are often more
movable on one another than are the thoracic vertebrae. They bear no
ribs.

4. The =sacral vertebrae= are characterised by the fact that they are
firmly fused together, and are united with the pelvic girdle by means
of their transverse processes and rudimentary ribs.

5. The =caudal= or =tail vertebrae= succeed the sacral. The anterior
ones are often fused with one another and with the sacrals, but they
differ from true sacral vertebrae in that there are no rudimentary
ribs between their transverse processes and the pelvic girdle. They
often bear V-shaped =chevron bones=.

In fish and snakes the vertebral column is divisible into only two
regions, an anterior trunk region, whose vertebrae bear ribs, and a
posterior tail region, whose vertebrae are ribless.


2. THE SKULL.

Before giving a general account of the adult skull it will be well to
briefly describe its development.


GENERAL DEVELOPMENT OF THE CRANIUM[8].

Shortly after its appearance, the central nervous system becomes
surrounded by a membranous mesodermal investment which in the region
of the spinal cord is called the =skeletogenous layer= or =perichordal
sheath=, while in the region of the brain it is called the
=membranous cranium=. Ventral to the central nervous system is the
notochord, which extends far into the region of the future cranium,
and like the nervous system, is enclosed by the skeletogenous layer.
The primitive cartilaginous cranium is formed by histological
differentiation within the substance of the membranous cranium and
always consists of the following parts:

(_a_) the =parachordals=. These are a pair of flat curved plates of
cartilage, each of which has its inner edge grooved where it comes in
contact with the notochord. The parachordals, together with the
notochord, form a continuous plate, which is known as the =basilar
plate=. The basilar plate is the primitive floor below the hind- and
mid-brain. In front the parachordals abut upon another pair of
cartilaginous bars, the trabeculae, the two pairs of structures being
sometimes continuous with one another from the first;

(_b_) the =trabeculae= which meet behind and embrace the front end of
the notochord. Further forwards they at first diverge from one
another, and then converge again, enclosing a space, the =pituitary
space=. After a time they generally fuse with one another in the
middle line, and, with the parachordals behind, form an almost
continuous basal plate. The trabeculae generally appear before the
parachordals. They form the primitive floor below the fore-brain;

(_c_) the cartilaginous =capsules= of the three pairs of =sense
organs=. At a very early stage of development involutions of the
surface epiblast give rise to the three pairs of special sense
organs--the olfactory or nasal organs in front, the optic in the
middle, and the auditory behind. The olfactory and auditory organs
always become enclosed in definite cartilaginous capsules, the eyes
often as in the Salmon, become enclosed in cartilaginous sclerotic
capsules, while sometimes, as in mammals, their protecting capsules
are fibrous.

Each pair of sense capsules comes into relation with part of the
primitive cranium, and greatly modifies it. Thus the auditory or
periotic capsules press on the parachordals till they come to be more
or less imbedded in them. Perhaps owing to the pressure of the nasal
capsules the trabeculae fuse in front, and then grow out into an
anterior pair of processes, the =cornua trabeculae=, and a posterior
pair, the =antorbital processes=, which together almost completely
surround the nasal capsules. The sclerotic capsules of the eyes
greatly modify the cranium, although they never become completely
united with it.

The cartilaginous cranium formed of the basal plate, together with the
sense capsules, does not long remain merely as a floor. Its sides grow
vertically upwards, forming the =exoccipital= region of the cranium
behind, and the =alisphenoidal= and =orbitosphenoidal= regions further
forwards. In many forms, such as Elasmobranchs, all these upgrowths
meet round the brain, roofing it in and forming an almost complete
cartilaginous cranium. But in most vertebrata, while in the occipital
region, the cartilaginous cranium is completed dorsally, in the
alisphenoidal and orbitosphenoidal regions the cartilage merely forms
the lateral walls of the cranium, the greater part of the brain having
dorsal to it a wide space, closed by merely membranous tissue in
connection with which the large frontal and parietal bones are
subsequently formed.


The SKULL includes

_a._ the cranium,

_b._ the jaws and visceral skeleton.

The =cranium= can be further subdivided into

(1) an axial portion, the =cranium proper= or =brain case=;

(2) =the sense capsules.= The capsules of the auditory and olfactory
sense organs are always present, and as has been already mentioned,
in many animals the eye likewise is included in a cartilaginous
capsule.


(1) THE CRANIUM PROPER OR BRAIN CASE.

The cranium varies much in form and structure. In lower vertebrates,
such as Sharks and Lampreys, it remains entirely cartilaginous and
membranous, retaining throughout life much of the character of the
embryonic rudiment of the cranium of higher forms. The dogfish's
cranium, described on pp. 73 to 76, is a good instance of a cranium of
this type. But in the majority of vertebrates the cartilage becomes
more or less replaced by cartilage bone, while membrane bones are also
largely developed and supplant the cartilage.

The cranium of most vertebrates includes a very large number of bones
whose arrangement varies much, but one can distinguish a definite
=basicranial axis= formed of the basi-occipital, basisphenoid, and
presphenoid bones, which is a continuation forwards of the axis of the
vertebral column. From the basicranial axis a wide arch arises,
composed of a number of bones, which form the sides and roof of the
brain-case These bones are arranged in such a manner that if both
cartilage and membrane bones are included they can be divided into
three rings or segments. The hinder one of these segments is the
occipital, the middle the parietal, and the anterior one the frontal.

The occipital segment is formed of four cartilage bones, the
=basi-occipital= below, two =exoccipitals= at the sides, and the
=supra-occipital= above. The parietal segment is formed of the
=basisphenoid= below, two =alisphenoids= at the sides and two membrane
bones, the _parietals_ above, and the frontal segment in like manner
consists of the =presphenoid= below, the two =orbitosphenoids= at the
sides, and two membrane bones, the _frontals_, above. The parietals
and frontals, being membrane bones, are not comparable to the
supra-occipital, in the way that the presphenoid and basisphenoid are
to the basi-occipital.

The cartilage bones of the occipital segments are derived from the
parachordals of the embryonic skull, those of the parietal and frontal
segments from the trabeculae.

In front of the presphenoid the basicranial axis is continued by the
=mesethmoid=.


(2) THE SENSE CAPSULES.

These enclose and protect the special sense organs.

(_a_) =Auditory capsule.=

The basisphenoid is always continuous with the basi-occipital, but the
alisphenoid is not continuous with the exoccipital as the =periotic=
or =auditory capsule= is interposed between them. Each periotic
capsule has three principal ossifications; an anterior bone, the
=pro-otic=, a posterior bone, the =opisthotic=, and a superior bone,
the =epi-otic=.

These bones may severally unite, or instead of uniting with one
another they may unite with the neighbouring bones. Thus the epi-otic
often unites with the supra-occipital, and the opisthotic with the
exoccipital.

Two other bones developed in the walls of the auditory capsule are
sometimes added, as in Teleosteans; these are the =pterotic= and
=sphenotic=.

(_b_) =Optic capsule.=

The eye is frequently enclosed in a cartilaginous sclerotic capsule,
and in this a number of scale-like bones are often developed.

Several membrane bones are commonly formed around the orbit or cavity
for the eye. The most constant of these is the _lachrymal_ which lies
in the anterior corner; frequently too, as in Teleosteans, there is a
_supra-orbital_ lying in the upper part of the orbit, or as in many
Reptiles, a _postorbital_ lying in the posterior part of the orbit.

(_c_) =Nasal capsule.=

In relation to the nasal capsules various bones occur.

The basicranial axis in front of the presphenoid is ossified, as the
=mesethmoid=, dorsal to which there sometimes, as in Teleosteans,
occur a _median ethmoid_ and a pair of =lateral ethmoids=[9]. Two
pairs of membrane bones very commonly occur in this region, viz. the
_nasals_ which lie dorsal to the mesethmoid, and the _vomers_
(sometimes there is only one) which lie ventral to it.

The part of the skull lying immediately in front of the cranial cavity
and in relation to the nasal capsules constitutes the =ethmoidal
region=.

There remain certain other membrane bones which are often found
connected with the cranium. Of these, one of the largest is the
_parasphenoid_ which, in Ichthyopsids, is found underlying the
basicranial axis. _Prefrontals_ often, as in most reptiles, occur
lying partly at the sides and partly in front of the frontal, and
_postfrontals_ similarly occur behind the orbit lying partly behind
the frontals and partly at their sides. Lastly a _squamosal bone_ is,
as in Mammals, very commonly developed, and lies external and partly
dorsal to the auditory capsules.

THE JAWS AND VISCERAL SKELETON.

In the most primitive fish these consist of a series of cartilaginous
rings or arches placed one behind another and encircling the anterior
end of the alimentary canal. Originally they are mainly concerned with
branchial respiration.

The first or =maxillo-mandibular= arch forms the upper jaw and the
lower jaw or mandible.

The second or =hyoid= arch bears gills and often assists in attaching
the jaws to the cranium. The remaining arches may bear gills, though
the last is commonly without them.

The above condition is only found in fishes, in higher animals the
visceral skeleton is greatly reduced and modified.

The first or maxillo-mandibular arch is divisible into a dorsal
portion, the =palato-pterygo-quadrate bar=, which forms the primitive
upper jaw and enters into very close relations with the cranium, and a
ventral portion, =Meckel's cartilage=, which forms the primitive lower
jaw. The cartilaginous rudiments of both these portions disappear to a
greater or less extent and become partly ossified, partly replaced by
or enveloped in membrane bone.

The posterior part of the palato-pterygo-quadrate bar becomes ossified
to form the =quadrate=, the anterior part to form the palatine and
pterygoid, or the two latter may be formed partially or entirely of
periosteal bone, developed round the cartilaginous bar. Two pairs of
important membrane bones, the _premaxillae_ and _maxillae_ form the
anterior part of the upper jaw, and behind the maxilla lies another
membrane bone, the _jugal_ or _malar_, which is connected with the
quadrate by a _quadratojugal_. The premaxillae have a large share in
bounding the external nasal openings or anterior nares.

In lower vertebrates the nasal passage leads directly into the front
part of the mouth cavity and opens by the posterior nares. In some
higher vertebrates, such as mammals and crocodiles, processes arise
from the premaxillae and palatines, and sometimes from the pterygoids,
which meet their fellows in the middle line and form the palate,
shutting off the nasal passage from the mouth cavity and causing the
posterior nares to open far back.

The cartilage of the lower jaw is in all animals with ossified
skeletons, except the Mammalia, partly replaced by cartilage bone
forming the =articular=, partly overlain by a series of membrane bones
the _dentary_, _splenial_, _angular_, _supra-angular_ and _coronoid_.
In many sharks large paired accessory cartilages occur at the sides of
the jaws; and in a few reptiles and some Amphibia, such as the Frog,
the ossified representative of the anterior of these structures occurs
forming the =mento-meckelian= bone. In mammals the lower jaw includes
but a single bone.

The quadrate in all animals with ossified skeletons, except the
Mammalia, forms the suspensorium of the mandible or the skeletal link
between the jaw and the cranium; in the Mammalia, however, the
mandible articulates with the squamosal, while the quadrate is greatly
reduced, and is now generally considered to be represented by the
tympanic ring of the ear.

The second visceral or hyoid arch in fishes consists of two pieces of
cartilage, a proximal[1] piece the =hyomandibular=, and a distal[10]
piece the =cerato-hyal=. The cerato-hyals of the two sides are commonly
united by a median ventral plate, the =basi-hyal=. The hyoid arch
bears gills on its posterior border, but its most important function
in most fishes is to act as the suspensorium. In higher vertebrates
the representative of the hyomandibular is much reduced in size, and
comes into relation with the ear forming the =auditory ossicles=; the
cerato-hyal looses its attachment to the hyomandibular and becomes
directly attached to the cranium, forming a large part of the hyoid
apparatus of most higher vertebrates.

Behind the hyoid arch come the branchial arches. They are best
developed in fishes, in which they are commonly five in number and
bear gills. Their ventral ends are united in pairs by median pieces,
the =copulae=.

In higher vertebrates they become greatly reduced, and all except the
first and second completely disappear. In the highest vertebrates, the
mammals, the second has disappeared, but in birds and many reptiles it
is comparatively well developed.

3. THE RIBS AND STERNUM.

The =ribs= are a series of segmentally arranged cartilaginous or bony
rods, attached to the vertebrae; they tend to surround the body
cavity, and to protect the organs contained within it. Ribs are very
frequently found attached to the transverse processes of the
vertebrae, but a study of their origin in fish shows that they are
really the cut off terminations of the ventral arch, not of the
transverse processes which are outgrowths from the dorsal arch. In the
tail their function is to surround and protect structures like the
ventral blood-vessels which do not vary much in size, consequently
they meet one another, and form a series of complete ventral or haemal
arches. But the trunk contains organs like the lungs and stomach which
are liable to vary much in size at different times, consequently the
halves of the haemal arch do not meet ventrally, and then the ribs
become detached from the rest of the haemal arch. Having once become
detached, they are able to shift about and unite themselves to various
points of the vertebra. They frequently, as has been already
mentioned, become entirely attached to the transverse process, or they
may be attached to the transverse process by a dorsal or =tubercular=
portion and to the centrum or to the ventral arch by a ventral or
=capitular= portion.

In all animals above fishes the distal ends of the thoracic ribs unite
with a median breast bone or sternum which generally has the form of a
segmented rod. The =sternum= is really formed by the fusion of the
distal ends of a series of ribs. In many animals elements of the
shoulder girdle enter into close relation with the rib elements of the
sternum.

II. THE APPENDICULAR SKELETON.

This consists of the skeleton of the anterior or =pectoral=, and the
posterior or =pelvic= limbs, and their girdles. In every case (except
in Chelonia) the parts of the appendicular skeleton lie external to
the ribs.

1. THE LIMB GIRDLES.

=The Pectoral girdle=[11]. In the simplest case the pectoral or
shoulder girdle consists of a hoop of cartilage incomplete dorsally.
It is attached by muscle to the vertebral column, and is divided on
either side into dorsal and ventral portions by a cavity, the =glenoid
cavity=, at the point where the anterior limb articulates. In higher
fishes this hoop is distinctly divided into right and left halves; it
becomes more or less ossified, and a pair of important bones, the
clavicles, are developed in connection with its ventral portion.

In higher vertebrates ossification sets up in the cartilage and gives
rise on each side to a dorsal bone, the =scapula=, and frequently to
an anterior ventral bone, the =precoracoid=, and a posterior ventral
bone, the =coracoid=. The precoracoid is often not ossified, and upon
it is developed the clavicle which more or less replaces it. In some
forms a =T= shaped _interclavicle_ occurs, in others =epicoracoids= are
found in front of the coracoids. In all vertebrata above fish, except
the great majority of mammals, the coracoids are large and articulate
with the sternum. But in mammals the coracoids are nearly always quite
vestigial, and the pectoral girdle is attached to the axial skeleton
by the clavicle or sometimes by muscles and ligaments only.

The =Pelvic girdle=[12] like the pectoral consists primitively of a
simple rod or hoop of cartilage, which in vertebrata above fishes is
divided into dorsal and ventral portions, by a cavity, the
=acetabulum=, with which the posterior limb articulates. In the pelvic
girdle as in the pectoral one dorsal, and (commonly) two ventral
ossifications take place. The dorsal bone is the =ilium= and
corresponds to the scapula. The posterior ventral bone is the
=ischium= corresponding to the coracoid. The anterior ventral bone is
the =pubis= and is generally compared to the precoracoid, but in some
cases a fourth pelvic element, the =acetabular= or =cotyloid= bone is
found, and this may correspond to the precoracoid.

The pelvic girdle differs from the pectoral in the fact that the
dorsal bones--the ilia--are nearly always firmly united to transverse
processes of the sacral vertebrae, by means of rudimentary ribs. The
pubes and ischia generally meet in ventral symphyses.

2. THE LIMBS.

It will be most convenient to defer a discussion of the limbs of
fishes to chap. VIII.

All vertebrates above fishes have the limbs divisible into three main
segments:--

                     =Anterior or Fore limb.=  =Posterior or Hind limb.=

  Proximal segment.    upper arm or _brachium_.          thigh.
  Middle segment.    fore-arm or _antibrachium_.         shin or _crus_.
  Distal segment.             _manus_.                       _pes_.

The proximal segments each contain one bone, the =humerus= in the case
of the upper arm, and the =femur= in the case of the thigh. The middle
segments each contain two bones, the =radius= and =ulna= in the case
of the fore-arm, and the =tibia= and =fibula= in the case of the shin.

       *       *       *       *       *

The manus and pes are further subdivided into

(_a_) two or three proximal rows of bones forming the wrist or
=carpus= in the case of the manus, and the ankle or =tarsus= in the
case of the pes.

(_b_) a middle row called respectively the =metacarpus= and
=metatarsus=.

(_c_) a number of distal bones called the =phalanges= which form the
skeleton of the fingers and toes, or =digits=.

Typically the manus and pes both have five digits (pentedactylate).
The first digit of the manus is commonly called the =pollex=, and the
first digit of the pes the =hallux=.

In a very simple =carpus= such as that of _Chelydra_, there are nine
bones. They are arranged in a proximal row of three, the radiale,
intermedium, and ulnare,--the first being on the radial side of the
limb, and a distal row of five called respectively carpale 1, 2, 3, 4,
5, beginning on the radial side. Between these two rows is a single
bone the centrale, or there may be two.

Similarly there are nine bones in a simple =tarsus= such as that of
_Salamandra_. They form a proximal row of three, the tibiale,
intermedium and fibulare, and a distal row of five, called
respectively tarsale 1, 2, 3, 4, 5, beginning on the tibial side.
Between the two rows there is a centrale as in the carpus, or there
may be two.

       *       *       *       *       *

The following names derived from human anatomy are commonly applied to
the various carpal and tarsal bones:

       =Carpus.=

    radiale = scaphoid
    intermedium = lunar
    ulnare = cuneiform
    centrale = central
    carpale 1 = trapezium
       "    2 = trapezoid
       "    3 = magnum
       "    4 } = unciform
       "    5 }

       =Tarsus.=

    tibiale     }
    intermedium }      astragalus
    fibulare = calcaneum
    centrale = navicular
    tarsale 1 = internal cuneiform
       "    2 = middle       "
       "    3 = external     "
       "    4 } = cuboid
       "    5 }

     NOTE. The above is the view commonly accepted concerning the
     homology of the carpal and tarsal bones. But with regard to the
     proximal row of tarsal bones there is difference of opinion.
     All anatomists are agreed that the calcaneum is the fibulare
     and that the intermedium is contained in the astragalus, but
     while the majority regard the astragalus as the fused tibiale
     and intermedium, Baur considers that a small bone found on the
     tibial side of the tarsus in _Procavia_, many Rodents,
     Insectivores, and the male _Ornithorhynchus_, is the vestigial
     tibiale, and regards the astragalus as the intermedium
     alone[13]. He also considers that the mammalian scaphoid
     represents a centrale.


MODIFICATIONS IN THE POSITIONS OF THE LIMBS[14].

In their primitive position the limbs are straight and are extended
parallel to one another at right angles to the axis of the trunk. Each
limb then has a dorsal surface, a ventral surface, an anterior or
=pre-axial= edge, and a posterior or =postaxial= edge.

In the anterior limb the radius and the pollex are pre-axial, the ulna
and the fifth finger are postaxial. In the posterior limb the tibia
and the hallux are pre-axial, the fibula and the fifth toe are
postaxial. The Cetacea and various extinct reptiles, such as
_Ichthyosaurus_ and _Plesiosaurus_, have their limbs in practically
this primitive position.

The first modification from it is produced by the bending ventrally of
the middle segments of both limbs upon the proximal segments, while
the distal segment is bent in the opposite direction on the middle
segment. Then the ventral surfaces of the antibrachium and crus come
to look inwards, and their dorsal surfaces to look outwards. The
brachium and manus, thigh and pes still have their dorsal surfaces
facing upwards and their ventral surfaces facing downwards as before,
and the relations of their pre-and postaxial borders remain as they
were. Many Amphibians and Reptiles, such as tortoises, carry their
limbs in this position.

In all higher vertebrates, however, a further change takes place, each
limb is rotated as a whole from its proximal end, the rotation taking
place in opposite directions in the fore and hind limbs respectively.
The anterior limb is rotated backwards from the shoulder, so that the
brachium lies nearly parallel to the body, and the elbow points
backwards, the antibrachium downwards, and the manus backwards; the
pre-axial surface of the whole limb with the radius and pollex now
faces outwards, and the postaxial surface with the ulna and fifth
finger now faces inwards. In the Walrus and, to a certain extent, in
the Sea lions the anterior limb remains throughout life in this
position. The posterior limb is also rotated, but the rotation in this
case takes place forwards, so that the thigh lies nearly parallel to
the body, the knee-joint pointing forwards; the crus downwards and the
pes forwards. The pre-axial surface of the whole limb with the tibia
and hallux looks towards the middle of the body, the postaxial surface
with the fibula and fifth toe looks outwards. This is the position in
which the hind limb is carried in nearly all mammals.

In nearly all mammals a further change takes place in the position of
the anterior limb. The radius and ulna have hitherto been parallel to
one another, but now the lower end of the radius, carrying with it the
manus, comes to be rotated forwards round the ulna, so that the manus,
as well as the pes, comes to be forwardly-directed, and its pre-axial
surface faces inwards.

In the majority of mammals the radius and ulna are permanently fixed
in this, which is known as the =prone= position, but in man and some
other mammals the manus can be pronated or turned into this position
at will. When the radius and ulna are parallel throughout their whole
length the manus is said to be in the =supine= position.

The =extensor= side of a limb is that to which the muscles which
straighten it are attached, the =flexor= side is that to which the
muscles which bend it are attached.


FOOTNOTES:

[1] The skin consists of an outer layer of epiblastic origin, the
epidermis, and an inner layer of mesoblastic origin, the dermis. The
epidermis is divided into two principal layers, an outer one, the
horny layer or _stratum corneum_, and an inner one, the _stratum
Malpighii_. The innermost part of the stratum corneum is distinguished
as the _stratum lucidum_, and the outermost part of the stratum
Malpighii as the _stratum granulosum_.

[2] The enamel of the pharyngeal teeth of some Teleosteans is
hypoblastic in origin.

[3] See also p. 71.

[4] It is usual to regard the clavicle as a membrane bone, but
Kölliker has shown that in rabbit embryos of about the 17th day it is
cartilaginous.

[5] In compiling these paragraphs on Histology, free use has been made
of Klein and Noble Smith's _Atlas of Histology_, the small Histologies
of Klein and Schäfer, Huxley's _Elementary Physiology_, and Lloyd
Morgan's _Animal Biology_.

[6] See Huxley's _Elementary Physiology_, Revised edition, London,
1886, p. 180.

[7] Strictly speaking the jaws, visceral skeleton, ribs and sternum do
not form part of the axis, but it is convenient to group them as parts
of the axial skeleton.

[8] F.M. Balfour, _Comparative Embryology_, vol. II., London, 1881, p.
465. W.K. Parker and G.T. Bettany, _The Morphology of the Skull_,
London, 1877.

[9] Sometimes also called ectethmoids or parethmoids.

[10] The _proximal_ end of anything is the one nearest the point of
origin or attachment, the _distal_ end is the one furthest from the
point of origin or attachment.

[11] W.K. Parker, _A Monograph of the Shoulder Girdle and Sternum_,
Ray Soc. London, 1868.

[12] See R. Wiedersheim, _Zeitschr. wiss. Zool._ vol. LIII. suppl. p.
43, 1892.

[13] G. Baur, _Beiträge zur Morphogenie des Carpus und Tarsus der
Vertebraten_, Theil 1. Batrachia. Jena, 1888, and _Amer. Natural._,
vol. XIX. 1885 (several papers).

[14] This account is based on Chapter XX. of Flower's _Osteology of
the Mammalia_. London 1876.




CHAPTER II.

CLASSIFICATION.


THE following classification includes _only the forms mentioned in the
succeeding pages_. The relative value of some of the terms employed in
classification is not identical throughout the book. This remark
applies specially to the term _group_, which is a convenient one,
owing to its not having such a hard and fast zoological meaning as has
the term _family_, for instance. The term _group_ is applied in this
book to divisions of the animal kingdom of very different
classificatory importance.

    PHYLUM CHORDATA.

    SUBPHYLUM A. HEMICHORDATA.

          Balanoglossus.
          Cephalodiscus.
          Rhabdopleura.
          ? Phoronis.
          (? Actinotrocha--larval Phoronis).

    SUBPHYLUM B. UROCHORDATA (TUNICATA).

          Group LARVACEA and others.

    SUBPHYLUM C. CEPHALOCHORDATA.

          Amphioxus--lancelet.

     NOTE. In this chapter all the generic names printed in italics
     are those of extinct animals.

    SUBPHYLUM D. VERTEBRATA.

    DIVISION (I). CYCLOSTOMATA.

    Order 1. MARSIPOBRANCHII.

    Family =Myxinoidei=. Myxine--hag-fish.
                          Bdellostoma.

    Family =Petromyzontidae=. Petromyzon--lamprey.
                            (Ammocoetes--larval lamprey.)

    Family =Palaeospondylidae=. _Palaeospondylus._

    Order 2. OSTRACODERMI.

            Suborder 1. HETEROSTRACI.

    Family =Pteraspidae=. _Pteraspis._

            Suborder 2. OSTEOSTRACI.

    Family =Cephalaspidae=. _Cephalaspis._

            Suborder 3. ANTIARCHA.

    Family =Asterolepidae=. _Pterichthys._
                            _Asterolepis._


    DIVISION (II). GNATHOSTOMATA.

    A. ICHTHYOPSIDA.

    CLASS I. PISCES.

    Order 1. ELASMOBRANCHII.

            Suborder (1). ICHTHYOTOMI.

    Family =Pleuracanthidae=. _Xenacanthus._

            Suborder (2). PLEUROPTERYGII.
                  _Cladoselache._

            Suborder (3). SELACHII.

                    Group SQUALIDAE.

    Family =Notidanidae=. Heptanchus.
                          Hexanchus.
                        Chlamydoselache--frill-gilled shark.

    Family =Cochliodontidae=. _Cochliodus._

    Family =Cestraciontidae=. Cestracion--Port Jackson shark.
                                    _Acrodus._

    Family =Scylliidae=. Scyllium--spotted dogfish.

    Family =Lamnidae=. Odontaspis.

    Family =Carcharidae=. Galeus--tope.

    Family =Spinacidae=. Acanthias--spiny dogfish.
                               Scymnus.

    Family =Squatinidae=. Squatina (Rhina)--angel fish.

    Group BATOIDEI.

    Family =Pristidae=. Pristis--saw-fish.

    Family =Raiidae=. Raia--skate.

    Family =Myliobatidae=. Myliobatis--eagle ray.

    Family =Trygonidae=. Trygon--sting ray.

    Family =Torpedinidae=. Torpedo--electric ray.

            Suborder (4). ACANTHODII.

    Family =Acanthodidae=. _Acanthodes._

    Family =Diplacanthidae=. _Diplacanthus._

    Order 2. HOLOCEPHALI.

    Family =Chimaeridae=. Chimaera--rabbit fish.
                               Harriotta.
                               Callorhynchus.
                               _Ischyodus._

    Order 3. GANOIDEI.

            Suborder (1). CHONDROSTEI.

    Family =Palaeoniscidae=. _Palaeoniscus._
                                  _Trissolepis._

    Family =Acipenseridae=. Acipenser--sturgeon.
                                  Scaphirhynchus.

    Family =Polyodontidae=. Polyodon (Spatularia)--spoon-beaked sturgeon.
                                 Psephurus--slender-beaked sturgeon.

            Suborder (2). CROSSOPTERYGII.

    Family =Holoptychiidae=. _Holoptychius._

    Family =Rhizodontidae=. _Rhizodus._

    Family =Osteolepidae=. _Osteolepis._

    Family =Polypteridae=. Polypterus--bichir.
                                 Calamoichthys--reed-fish.

            Suborder (3). HOLOSTEI.

    Family =Lepidosteidae=. Lepidosteus--gar pike.

    Family =Semionotidae=. _Lepidotus._

    Family =Amiidae=. Amia--bow-fin.

    Order 4. TELEOSTEI.

            Suborder (1). PLECTOGNATHI.

    Family =Balistidae=. Balistes--file-fish.

    Family =Gymnodontidae=. Diodon--globe-fish.

    Family =Ostracionidae=. Ostracion--coffer-fish.

            Suborder (2). PHYSOSTOMI.

    Family =Siluridae=.--cat-fishes.

    Family =Cyprinidae=. Cyprinus--carp.

    Family =Esocidae=. Esox--pike.

    Family =Salmonidae=. Salmo--salmon.

    Family =Clupeidae=. Clupeus--herring.
                             Exocaetus--'flying fish'.

    Family =Muraenidae=. Anguilla--eel.

            Suborder (3). ANACANTHINI.

    Family =Gadidae=. Gadus--cod, haddock, whiting.

    Family =Pleuronectidae=. Solea--sole.

            Suborder (4). PHARYNGOGNATHI.

    Family =Labridae=. Labrus--wrasse.
                            Scarus--parrot fish.

            Suborder (5). ACANTHOPTERYGII.

    Family =Cataphracti=. Dactylopterus--flying gurnard.

    Family =Percidae=. Perca--perch.

                Order 5. DIPNOI.

            Suborder (1). SIRENOIDEI.

    Family =Dipteridae=. _Dipterus._

    Family =Monopneumona=. Ceratodus--barramunda.

    Family =Dipneumona=. Protopterus--African mud-fish.
                         Lepidosiren.

            Suborder (2). ARTHRODIRA.

    Family =Coccosteidae=. _Coccosteus._
                           _Dinichthys._

     NOTE. Palaeontological research has disclosed the existence of
     a great number of forms which seem to connect with one another
     almost all the orders of fishes as usually recognised. Forms
     connecting the living Ganoids with the Teleosteans have been
     especially numerous, so that these terms Ganoid and Teleostean
     can hardly be any longer used in a precise and scientific
     sense. This has rendered the subject of the classification of
     fishes a very difficult one. Though unsuitable for adoption in
     a work like the present, by far the most natural classification
     hitherto proposed seems to be that of Smith Woodward[15]. He
     considers that the course of development of fishes has followed
     two distinct lines, the autostylic and hyostylic (see p. 119),
     and groups the various forms as follows:

         HYOSTYLIC.                         AUTOSTYLIC.
    Subclass 1. ELASMOBRANCHII.        Subclass 3. HOLOCEPHALI.
      1. Ichthyotomi.                    1. (unknown).
      2. Selachii.                       2. Chimaeroidei.
      3. Acanthodii.                     3. (unknown).

    Subclass 2. TELEOSTOMI.            Subclass 4. DIPNOI.
      1. Crossopterygii (Palaeozoic      1. Sirenoidei.
          and Mesozoic).
      2. Crossopterygii (Cainozoic).     2. (unknown).
      3. Actinopterygii.                 3. Arthrodira.

     The primitive forms in each of these four subclasses have the
     fins archipterygia (see p. 127).

              CLASS II. AMPHIBIA.

                Order 1. URODELA.

            Suborder (1). ICHTHYOIDEA.

          Group A. PERENNIBRANCHIATA.

    Family =Menobranchidae=. Menobranchus.

    Family =Proteidae=. Proteus--olm.

    Family =Sirenidae=. Siren.

          Group B. DEROTREMATA.

    Family =Amphiumidae=. Megalobatrachus.
                           Cryptobranchus (Menopoma).
                           Amphiuma.

            Suborder (2). SALAMANDRINA.

    Family =Salamandridae=. Salamandra--salamander.
                             Molge--newt.
                             Onychodactylus.
                             Amblystoma.
                             (Siredon--axolotl, larval Amblystoma).
                             Batrachoseps.
                             Spelerpes (Gyrinophilus).

            Order 2. LABYRINTHODONTIA.

          Group =Lepospondyli=. _Branchiosaurus._

          Group =Temnospondyli=. _Archegosaurus._
                                  _Nyrania._
                                  _Euchirosaurus._

          Group =Stereospondyli=. _Capitosaurus._
                                   _Mastodonsaurus._

            Order 3. GYMNOPHIONA.

    Family =Caeciliidae=. Siphonops.
                           Epicrium.

    Order 4. ANURA.

    Suborder (1). AGLOSSA.

    Family =Xenopidae=. Xenopus.

    Family =Pipidae=. Pipa--Surinam toad.

    Suborder (2). PHANEROGLOSSA.

    Group ARCIFERA.

    Family =Discoglossidae=. Discoglossus--painted frog.
    Bombinator--fire-bellied frog.
    Alytes--midwife frog.

    Family =Pelobatidae=. Pelobates--toad frog.

    Family =Hylidae=. Hyla--green tree-frog.

    Family =Bufonidae=. Bufo--toad.
    Docidophryne.

    Family =Cystignathidae=. Ceratophrys--horned frog.

    Group FIRMISTERNIA.

    Family =Ranidae=. Rana--common and edible frogs.

    Family =Engystomatidae=. Brachycephalus.

    B. SAUROPSIDA.

    CLASS I. REPTILIA[16].

    Order 1. THEROMORPHA.

    Group =Anomodontia=. _Dicynodon._
    _Udenodon._

    Group =Placodontia=. _Placodus._

    Group =Pariasauria=. _Pariasaurus._
    _Elginia._

    Group =Theriodontia=. _Dimetrodon._
    _Galesaurus._
    _Cynognathus._

    Order 2. SAUROPTERYGIA.

    Family =Mesosauridae=. _Mesosaurus._

    Family =Nothosauridae=. _Nothosaurus._

    Family =Plesiosauridae=. _Plesiosaurus._
    _Pliosaurus._

    Order 3. CHELONIA.

    Suborder (1). TRIONYCHIA.

    Family =Trionychidae=. Trionyx--snapping turtle.

    Suborder (2). CRYPTODIRA.

    Family =Dermochelydidae=. Dermochelys (Sphargis)--leathery
    turtle.

    Family =Chelonidae=. Chelone--green turtle.

    Family =Chelydridae=. Chelydra--terrapin.

    Family =Chersidae=. Testudo--tortoise.

    Suborder (3). PLEURODIRA.

    Family =Chelydae=. Chelys.

    Order 4. ICHTHYOSAURIA.

    Family =Ichthyosauridae=. _Ichthyosaurus._

    Order 5. RHYNCHOCEPHALIA.

    Suborder (1). RHYNCHOCEPHALIA VERA.

    Family =Sphenodontidae=. Sphenodon (Hatteria).

    Family =Rhynchosauridae=. _Hyperodapedon._

    Suborder (2). PROGANOSAURIA.

    Family =Proterosauridae=. _Proterosaurus._

    Order 6. SQUAMATA.

    Suborder (1). LACERTILIA.

    Group =Lacertilia vera=.

    Family =Geckonidae=. Gecko.

    Family =Pygopodidae=. Lialis--scale-foot.

    Family =Agamidae=. Draco--flying lizard.
                       Agama.

    Family =Iguanidae=. Iguana.

    Family =Anguidae=. Ophisaurus (Bipes, Pseudopus).
                       Anguis--blindworm.

    Family =Varanidae=. Varanus--monitor.

    Family =Amphisbaenidae=. Chirotes.
                             Amphisbaena.

    Family =Scincidae=. Tiliqua (Cyclodus).
                        Scincus--skink.
                        Chalcides (Seps).


    Group =Rhiptoglossa=.

    Family =Chamaeleonidae=. Chamaeleon.

    Suborder (2). OPHIDIA.

    Family =Typhlopidae=. Typhlops--blind snake.

    Family =Boidae=. Python.

    Family =Colubridae=. Tropidonotus--ringed snake.

    Family =Hydrophidae=--sea snakes.

    Family =Crotalidae=. Crotalus--rattlesnake.

    Suborder (3). PYTHONOMORPHA.

    Family =Mosasauridae=. _Mosasaurus._

    Order 7. DINOSAURIA.

    Suborder (1). SAUROPODA.

    Family =Atlantosauridae=. _Brontosaurus._

    Family =Cetiosauridae=. _Morosaurus._

    Suborder (2). THEROPODA.

    Family =Megalosauridae=. _Megalosaurus_ (_Ceratosaurus_).

    Family =Compsognathidae=. _Compsognathus._

    Suborder (3). ORTHOPODA.

    Section (_a_). STEGOSAURIA.

    Family =Scelidosauridae=. _Polacanthus._

    Family =Stegosauridae=. _Stegosaurus._

    Section (_b_). CERATOPSIA.

    Family =Ceratopsidae=. _Polyonax_ (_Ceratops_).

    Section (_c_). ORNITHOPODA.

    Family =Camptosauridae=. _Hypsilophodon._

    Family =Iguanodontidae=. _Iguanodon._

    Family =Hadrosauridae=. _Hadrosaurus._

    Order 8. CROCODILIA.

    Suborder (1). PARASUCHIA.

    Family =Phytosauridae=. _Phytosaurus_ (_Belodon_).

    Suborder (2). EUSUCHIA.

    Family =Teleosauridae=. _Teleosaurus._
    _Metriorhynchus._

    Family =Goniopholidae=. _Goniopholis._

    Family =Alligatoridae=. Alligator.
    Caiman.
    Jacare.

    Family =Crocodilidae=. Crocodilus.

    Family =Garialidae=. Garialis (Gavialis).

    Order 9. PTEROSAURIA.

    Family =Pterodactylidae=. _Pterodactylus._

    Family =Rhamphorhynchidae=. _Rhamphorhynchus._

    Family =Pteranodontidae=. _Pteranodon._


FOOTNOTES:

[15] A. Smith Woodward, _Catalogue of Fossil Fishes in the British
Museum_, Part II., Introduction, p. xii.

[16] This classification of reptiles is mainly based on that of
Lydekker (_Catalogue of Fossil Reptiles in the British Museum_) but in
some respects that of von Zittel has been followed.




    CLASS II. AVES[17].


    Subclass (I). ARCHAEORNITHES.
    _Archaeopteryx._

    Subclass (II). NEORNITHES.

    Order 1. RATITAE.

    Group =Æpyornithes=. _Æpyornis._

    Group =Apteryges=. Apteryx--kiwi.

    Group =Dinornithes=. Moas.

    Group =Megistanes=. Casuarius--cassowary.
    Dromaeus--emeu.

    Group =Rheornithes=. Rhea--American ostrich.

    Group =Struthiornithes=. Struthio--ostrich.

    Order 2. ODONTOLCAE.
    _Hesperornis._

    Order 3. CARINATAE.

    Group =Ichthyornithiformes=.
    _Ichthyornis._
    _Apatornis._
    _Odontopteryx._

    Group =Colymbiformes=.
    Subgroup Colymbi--divers.

    Group =Sphenisciformes=.
    Subgroup Sphenisci--penguins.


    Group =Ciconiiformes=.

      Subgroup Steganopodes. Sula--gannet.
                             Pelicanus--pelican.
                             Phaëthon--frigate bird.
                             Phalacrocorax--cormorant.

      Subgroup Ardeae. Ardea--heron

      Subgroup Ciconiae. Leptoptilus--adjutant.
                         Ciconia--white stork.

    Group =Anseriformes=.

      Subgroup Palamedeae. Palamedea }
                                     } screamers.
                           Chauna    }

      Subgroup Anseres. Anas--wild duck.
                        Anser--goose.
                        Plectropterus--spur-winged goose.
                        Cygnus--swan.
                        Mergus--merganser.

    Group =Falconiformes=.

      Subgroup Cathartae. Cathartes--American vulture.

      Subgroup Accipitres. Falco--falcon.
                           Vultur--vulture.
                           Harpagus.
                           Gypogeranus--secretary bird.

    Group =Tinamiformes=.

      Subgroup Tinami. Tinamus.

    Group =Galliformes=.

      Subgroup Galli. Gallus--fowl.
                      Pavo--peacock.

      Subgroup Opisthocomi. Opisthocomus--hoatzin.

    Group =Gruiformes=.
                            Gruidae--cranes.

    Group =Stereornithes=. _Phororhacos._

    Group =Charadriiformes=.

      Subgroup Limicolae. Charadriidae--plovers.
                          Parra--jacana.

      Subgroup Lari. Laridae--gulls.
                     Alcidae--auks.

      Subgroup Pteroclidae. Pterocles--sandgrouse.

      Subgroup Columbidae. Columbae--pigeons.
                           _Didus_--dodo.
                           _Pezophaps_--solitaire.

    Group =Cuculiformes=.

      Subgroup Cuculi. Scythrops.

      Subgroup Psittaci. Stringops--owl-parrot.

    Group =Coraciiformes=.

      Subgroup Coraciae. Coracias--roller.
                         Buceros--hornbill.
                         Upupa--hoopoe.

      Subgroup Striges. Owls.

      Subgroup Cypseli. Cypselidae--swifts.
                        Trochilidae--humming-birds.

      Subgroup Trogonidae. Trogons.

      Subgroup Pici. Rhamphastos--toucan.
                     Picus--woodpecker.

    Group =Passeriformes=. Crows, finches, larks, warblers,
                                  and many others.

C. MAMMALIA[18].

Class MAMMALIA.

Subclass (I). ORNITHODELPHIA or PROTOTHERIA.

Order. MONOTREMATA.

    Family =Ornithorhynchidae=. Ornithorhynchus--duck-bill.

    Family =Echidnidae=. Echidna--spiny ant-eater.

    Group =Multituberculata=. _Tritylodon._

    Subclass (II). DIDELPHIA or METATHERIA.

    Order. MARSUPIALIA.

    Suborder (1). POLYPROTODONTIA.

    Family =Amphitheriidae=. _Phascolotherium._

    Family =Didelphyidae=. Didelphys--opossum.

    Family =Dasyuridae=. Thylacinus--Tasmanian wolf.
                              Sarcophilus--Tasmanian devil.
                              Dasyurus.

    Family =Peramelidae=. Perameles--bandicoot.
                               Choeropus.

    Family =Notoryctidae=. Notoryctes--marsupial mole.

    Suborder (2). DIPROTODONTIA.

    Family =Phascolomyidae=. Phascolomys--wombat.

    Family Phalangeridae. Tarsipes.
                          Phalanger--cuscus.
                          Phascolarctus--koala.
                          _Thylacoleo._

    Family =Diprotodontidae=. _Diprotodon._

    Family =Nototheriidae=. _Nototherium._

    Family =Macropodidae=. Macropus--kangaroo.

    Family =Epanorthidae=. Coenolestes.

    Subclass (III). MONODELPHIA or EUTHERIA.

    Order 1. EDENTATA.

    Family =Bradypodidae=. Bradypus }
                                          }--sloths.
                                Choloepus }

    Family =Megatheriidae=. _Megatherium_--ground sloth.

    Family =Myrmecophagidae=. Myrmecophaga--great ant-eater.
                                   Cycloturus--two-toed ant-eater.

    Family =Dasypodidae=. Chlamydophorus }
                               Dasypus        }--armadillos.
                               Priodon        }
                               Tatusia        }

    Family =Glyptodontidae=. _Glyptodon._

    Family =Manidae=. Manis--pangolin.

    Family =Orycteropodidae=. Orycteropus--aard-vark.

    Order 2. SIRENIA.

    Family =Manatidae=. Manatus--manatee.

    Family =Rhytinidae=. _Rhytina_--Steller's sea-cow.

    Family =Halicoridae=. Halicore--dugong.

    Family =Halitheriidae=. _Halitherium._

    Order 3. CETACEA.

    Suborder (1). ARCHAEOCETI.

    Family =Zeuglodontidae=. _Zeuglodon._

    Suborder (2). MYSTACOCETI or BALAENOIDEA.

    Family =Balaenidae=. Balaena--right whale.
                              Megaptera--humpbacked whale.
                              Balaenoptera--rorqual.

    Suborder (3). ODONTOCETI.

    Family =Physeteridae=. Physeter--sperm whale.
                                Hyperoödon--bottlenose.
                                Ziphius.
                                Mesoplodon.

    Family =Physodontidae=. _Physodon._

    Family =Squalodontidae=. _Squalodon._

    Family =Platanistidae=. Platanista--Gangetic dolphin.
                                 Inia.
                                 Pontoporia.

    Family =Delphinidae=. Monodon--narwhal.
                               Phocaena--porpoise.
                               Orca--killer.
                               Globicephalus--Ca'ing whale.
                               Grampus.
                               Lagenorhynchus.
                               Delphinus--dolphin.
                               Tursiops.
                               Prodelphinus.

    Order 4. UNGULATA.

    Division A. UNGULATA VERA.

    Suborder (1). ARTIODACTYLA.

    Section (_a_). SUINA.

    Family =Hippopotamidae=. Hippopotamus.

    Family =Suidae=. Sus--pig.
                          Babirussa.
                          Phacochaerus--wart hog.
                          _Hyotherium._

    Family =Cotylopidae=. _Cotylops (Oreodon)._
                               _Cyclopidius._

    Family =Agriochoeridae=. _Agriochoerus._

    Family =Anoplotheriidae=. _Anoplotherium._

    Section (_b_). TYLOPODA.

    Family =Camelidae=. Camelus--camel.
                             Auchenia--llama.

    Section (_c_). TRAGULINA.

    Family =Tragulidae=. Dorcatherium (Hyomoschus)--chevrotain.

    Section (_d_). RUMINANTIA or PECORA.

    Family =Cervidae=. Moschus--musk deer.
                            Cervus--deer.
                            Cervulus--muntjac.
                            Hydropotes--Chinese water deer.

    Family =Giraffidae=. Giraffa--giraffe.
                              _Sivatherium._

    Family =Antilocapridae=. Antilocapra--prongbuck.

    Family =Bovidae=. Tetraceros--four-horned antelope.
                           Gazella--gazelle.
                           Bos--ox.
                           Bison.
                           Bubalus--buffalo.

    Suborder (2). PERISSODACTYLA.

    Family =Tapiridae=. Tapirus--tapir.

    Family =Lophiodontidae=. _Lophiodon._
                                  _Hyracotherium._

    Family =Palaeotheriidae=. _Palaeotherium._

    Family =Equidae=. _Hipparion._
                           Equus--horse.

    Family =Rhinocerotidae=. Rhinoceros.
                                  _Elasmotherium._

    Family =Titanotheriidae=. _Titanotherium (Brontops)._
                                   _Palaeosyops._

    Family =Chalicotheriidae=. _Chalicotherium._

    Family =Macraucheniidae=. _Macrauchenia._

    Division B. SUBUNGULATA.

    Suborder (1). TOXODONTIA.

    Family =Astrapotheriidae=. _Astrapotherium._

    Family =Nesodontidae=. _Nesodon._

    Family =Toxodontidae=. _Toxodon._

    Family =Typotheriidae=. _Typotherium._


              Suborder (2). CONDYLARTHRA.

    Family =Phenacodontidae=. _Phenacodus._

              Suborder (3). HYRACOIDEA.

    Family =Hyracidae=. Procavia (Hyrax).

              Suborder (4). AMBLYPODA.

    Family =Coryphodontidae=. _Coryphodon._

    Family =Uintatheriidae=. _Uintatherium_ (_Dinoceras_).

              Suborder (5). PROBOSCIDEA.

    Family =Dinotheriidae=. _Dinotherium._

    Family =Elephantidae=. _Mastodon._
                           Elephas--elephant.

    Group Tillodontia.

                  Order 5. RODENTIA.

              Suborder (1). SIMPLICIDENTATA.

                  Section SCIUROMORPHA.

    Family =Castoridae=. Castor--beaver.

                  Section MYOMORPHA.

    Family =Lophiomyidae=. Lophiomys.

    Family =Muridae=. Hydromys.
                      Acanthomys--spiny mouse.
                      Mus--mouse.

    Family =Spalacidae=. Bathyergus.

    Family =Dipodidae=. Dipus--jerboa.
                        Pedetes--Cape jumping-hare.

                  Section HYSTRICOMORPHA.

    Family =Hystricidae=. Hystrix--porcupine.

    Family =Chinchillidae=. Chinchilla.
                            Lagostomus--viscacha.

    Family =Dasyproctidae=. Coelogenys--paca.
                                 Dasyprocta--agouti.

    Family =Caviidae=. Cavia--guinea-pig.
                            Hydrochaerus--capybara.

    Suborder (2). DUPLICIDENTATA.

    Family =Leporidae=. Lepus--hare and rabbit.

    Order 6. CARNIVORA.

    Suborder (1). CREODONTA.

    Family =Hyaenodontidae=. _Hyaenodon._

    Suborder (2). CARNIVORA VERA or FISSIPEDIA.

    Section ÆLUROIDEA.

    Family =Felidae=. Felis--cat, lion, tiger.
                           _Machaerodus_--sabre-toothed lion.

    Family =Viverridae=. Viverra--civet.
                              Paradoxurus--palm civet.

    Family =Protelidae=. Proteles--aard wolf.

    Family =Hyaenidae=. Hyaena.

    Section CYNOIDEA.

    Family =Canidae=. Canis--dog, wolf, fox.

    Section ARCTOIDEA.

    Family =Ursidae=. Ursus--bear.

    Family =Mustelidae=. Latax--sea otter.

    Suborder (3). PINNIPEDIA.

    Family =Otariidae=. Otaria--sea lion.

    Family =Trichechidae=. Trichechus--walrus.

    Family =Phocidae=. Ogmorhinus--sea leopard.

    Order 7. INSECTIVORA.

    Suborder (1). DERMOPTERA.

    Family =Galeopithecidae=. Galeopithecus--'flying lemur'.

    Suborder (2). INSECTIVORA VERA.

    Family =Macroscelidae=. Macroscelides--jumping shrew.

    Family =Erinaceidae=. Erinaceus--hedgehog.
                               Gymnura.

    Family =Soricidae=. Sorex--shrew.

    Family =Talpidae=. Talpa--mole.

    Family =Potamogalidae=. Potamogale.

    Family =Solenodontidae=. Solenodon.

    Family =Centetidae=. Microgale.
                              Centetes--tenrec.

    Family =Chrysochloridae=. Chrysochloris--golden mole.

    Order 8. CHIROPTERA.

    Suborder (1). MEGACHIROPTERA.

    Family =Pteropidae=. Pteropus--flying fox.

    Suborder (2). MICROCHIROPTERA.

    Family =Rhinolophidae=. Horseshoe bats.

    Family =Phyllostomatidae=. Desmodus--vampire.

    Order 9. PRIMATES.

    Suborder (1). LEMUROIDEA.

    Family =Tarsiidae=. Tarsius--tarsier.

    Family =Chiromyidae=. Chiromys--aye aye.

    Suborder (2). ANTHROPOIDEA.

    Family =Hapalidae=. Hapale--marmoset.

    Family =Cebidae=. Mycetes--howling monkey.
                           Ateles--spider monkey.

    Family =Cercopithecidae=. Cynocephalus--baboon.
                                   Macacus.
                                   Colobus.

    Family =Simiidae=. Hylobates--gibbon.
                            Simia--orang.
                            Gorilla.
                            Anthropopithecus--chimpanzee.

    Family =Hominidae=. Homo--man.


FOOTNOTES:

[17] This classification of birds is essentially that of Gadow and
Selenka in Bronn's _Classen und Ordnungen des Thierreichs_, Band VI.,
Abth. IV., Vögel. Leipzig, 1891.

[18] The classification adopted is almost entirely that given in
Flower and Lydekker's _Mammals Living and Extinct_. London, 1891.




CHAPTER III.

SKELETON OF HEMICHORDATA, UROCHORDATA, AND CEPHALOCHORDATA.


SUBPHYLUM A. HEMICHORDATA.

THE subphylum includes three genera, _Balanoglossus_[19],
_Cephalodiscus_ and _Rhabdopleura_; and perhaps a fourth, _Phoronis_.

The skeletal structures found in _Balanoglossus_[20] are all
endoskeletal. They include:

(1) The =notochord=. This arises as a diverticulum from the alimentary
canal which grows forwards into the proboscis and extends beyond the
front end of the central nervous system. It is hypoblastic in origin
and arises in the same way as does the notochord of _Amphioxus_. Its
cells become highly vacuolated and take on the typical notochordal
structure[21]. The cavity of the primitive diverticulum becomes
obliterated in front, but behind it opens throughout life into the
alimentary canal.

(2) The =axial skeletal rods=. These are a pair of chitinous rods
which lie ventral to the notochord and in the collar region unite to
form a single mass.

(3) The =branchial skeleton=. The gill bars separating the gill slits
from one another are strengthened by chitinous rods in a way closely
similar to that in _Amphioxus_. But between one primary forked rod and
the next there are two secondary unforked rods--not one, as in
_Amphioxus_.

(4) The =chondroid tissue=. This is of mesoblastic origin and may be
regarded as an imperfect sheath for the notochord.

In _Cephalodiscus_ and _Rhabdopleura_ as in _Balanoglossus_ the
notochord forms a small diverticulum growing forwards from the
alimentary canal into the proboscis stalk.

Recent researches on _Phoronis_[22] show the existence in the collar
region of the larva (_Actinotrocha_) of a paired organ, which is
regarded by its discoverer as representing a double notochord.


SUBPHYLUM B. UROCHORDATA (TUNICATA).

Skeletal structures of epiblastic and hypoblastic origin occur in the
Urochordata. Most Tunicates are invested by a thick gelatinous test
which often contains calcareous spicules, and serves as a supporting
organ for the soft body. The cells of this test are mesodermal in
origin.

In larval Tunicata and in adults of the group Larvacea the tail is
supported by a typical notochord, which is confined to the tail. In
all Tunicata except Larvacea all trace of the notochord is lost in the
adult.


SUBPHYLUM C. CEPHALOCHORDATA.

[Illustration FIG. 3. DIAGRAM OF THE SKELETON OF _Amphioxus
lanceolatus_ × 3 (after a drawing in the Index collection at the Brit.
Mus.).

    1. skeleton of dorsal fin.
    2. notochord.
    3. neural tube.
    4. buccal skeleton.
    5. branchial skeleton.
    6. septa separating the myotomes.
    7. skeleton of ventral fin.]

This subphylum includes the well-known genus _Amphioxus_[23]. In
_Amphioxus_ the skeleton is very simple. It contains no trace of
cartilage or bone and remains throughout life in a condition
corresponding to a very early stage in Vertebrata. The skeleton of
_Amphioxus_ is partly hypoblastic, partly mesoblastic in origin.

(_a_) =Hypoblastic skeleton.=

The =notochord= (fig. 3, 2) is an elastic rod extending along the
whole length of the body past the anterior end of the nerve cord. It
lies ventral to the nerve cord, and shows no trace of segmentation. It
is chiefly made up of greatly vacuolated cells containing lymph, but
near the dorsal and ventral surfaces the cells are less vacuolated.
The notochord is immediately surrounded by a structureless cuticular
layer, the _chordal sheath_, and outside this comes the mesoblastic
_skeletogenous layer_, which also surrounds the nerve cord.

The =branchial skeleton=. This consists of a series of chitinous
elastic rods which strengthen the gill bars and are alternately forked
and unforked ventrally. The forked rods are primary, and are U-shaped
in section, the unforked rods are secondary, and are circular in
section. All these rods are united at intervals by transverse rods.

(_b_) =Mesoblastic skeleton.=

The =buccal skeleton=. On each side of the mouth there is a curved bar
resembling the notochord in structure. The bars are segmented, and
each segment bears a smaller rod which supports a tentacle, the whole
forming the buccal skeleton (fig. 3, 4).

The notochord is enclosed in a thick =sheath= of connective tissue
continuous with a thinner sheath round the nerve cord. The sheaths of
the notochord and nerve cord together form the skeletogenous layer,
and prolongations of it form the myomeres or septa between the
myotomes or segments of the great lateral muscles of the body.

The =skeleton of each median fin= consists of small cubical masses of
a gelatinous substance arranged in rows (fig. 3, 1 and 7), and serving
to strengthen the fins.


FOOTNOTES:

[19] The name _Balanoglossus_ is used here in its widest sense to
include all the Enteropneusta.

[20] See W. Bateson, _Quart. J. Micr. Sci._ n. s. vol. XXIV. 1884, p.
208 and later; also E.W. Macbride, _Ibid._ vol. XXXVI. 1894, p. 385.

[21] See p. 52.

[22] A.T. Masterman, _P.R. Soc. Edinb._ 1895-96, p. 59; and _Anat.
Anz._ 1896, p. 266.

[23] See E. Ray Lankester, _Quart. J. Micr. Sci._ vol. XXIX. n. s.
1889, p. 365. W.B. Benham, _Ibid._ vol. XXXV. n. s. 1893, p. 97. J.W.
Kirkaldy, _Ibid_. vol. XXXVII. n. s. 1895, p. 303. The last-named
writer divides the genus into three subgenera.




CHAPTER IV.

SUBPHYLUM D. VERTEBRATA.


THE animals included in this great group all possess an internal axial
skeleton forming the vertebral column or back-bone; and a dorsal
spinal cord. The vertebral column is developed from the skeletogenous
layer, which surrounds the spinal cord together with the notochord and
its sheath; and in the great majority of cases the notochord becomes
more or less modified and reduced in the adult. In some cases the
notochord remains unmodified and the skeletogenous layer surrounding
it is not segmented to form vertebrae, but in every case the neural
arches which protect the spinal cord are segmented. The notochord
never extends further forwards than the mid-brain.

All true vertebrates possess a cranium or skeletal box enclosing the
brain.


(I.) CYCLOSTOMATA.

The mouth in living forms is suctorial and is not supported by jaws.
In some fossil forms the character of the mouth is unknown.


_Order I._ MARSIPOBRANCHII[24].

In these animals limbs and limb girdles are always completely absent.
They have no exoskeleton except horny teeth.

The endoskeleton, excluding the notochord, is entirely cartilaginous
or membranous. The axial skeleton consists of a cartilaginous cranium
without jaws, succeeded by a thick persistent notochord enveloped in
a sheath. The notochord in living forms is unsegmented, but segmented
cartilaginous neural arches are present in some cases. A complicated
series of cartilaginous elements occurs in relation to the mouth,
gills, and sense organs. The median fins are supported by
cartilaginous pieces, the radiale. The order includes the Lampreys and
Hags.


_Order II._ OSTRACODERMI[25].

The forms included in this group have long been extinct, being known
only from beds of Upper Silurian and Lower Devonian age. They differ
much from all other known animals. The exoskeleton is always greatly
developed and includes (1) large bony plates covering the anterior
region; (2) scales covering the posterior region. The plates are
deeply marked by canals belonging to dermal sense organs. Jaws are
unknown, and arches for the support of the appendicular skeleton are
rudimentary or absent. The tail is heterocercal (see p. 60).


_Suborder_ (1). HETEROSTRACI.

The exoskeleton consists principally of calcifications forming dorsal
and ventral shields which cover the head and abdominal region; the
dorsal shield is formed of a few plates firmly united, the ventral
shield of a single plate. The shields are composed of three layers,
the middle layer being traversed by canals belonging to the dermal
sense organs which open to the exterior by a series of pores. The tail
is sometimes covered by scales. The orbits are widely separated and
laterally placed. Paired appendages are absent. These curious forms
are found in beds of Upper Silurian and Lower Devonian age. One of the
best known genera is _Pteraspis_.


_Suborder_ (2). OSTEOSTRACI.

The exoskeleton as in the Heterostraci consists of shields and scales,
the shields being divisible into three layers. The anterior part of
the body is covered dorsally by a single large shield which differs
from those of the Heterostraci in having the inner layer ossified. The
middle layer contains canals for the passage of blood vessels, but the
exoskeleton shows no impressions of dermal sense organs. The posterior
part of the body is covered by large quadrangular scales. Paired
appendages are absent, but median dorsal and caudal fins occur
supported by scales, not fin-rays. _Cephalaspis_, the best known of
these animals, occurs in beds of Lower Devonian age.


_Suborder_ (3). ANTIARCHA.

The exoskeleton is formed of bony plates, the dorsal and ventral
shields each consisting of several symmetrically arranged pieces. The
tail may be covered with small scales or may be naked. The head is
articulated with the trunk, and its angles are drawn out into a pair
of segmented paddle-like appendages, covered with dermal plates. The
orbits are close together. A dorsal fin and traces of mouth parts
occur in _Pterichthys_, but the endoskeleton is unknown. The best
known forms _Pterichthys_[26] and _Asterolepis_ occur in beds of Lower
Devonian age.


GENERAL ACCOUNT OF THE SKELETON OF MARSIPOBRANCHII.

The Marsipobranchii are worm-like animals. The living forms include
two families, the Myxinoidei (Hags)--genera _Myxine_ and
_Bdellostoma_--and the Petromyzontidae (Lampreys).

Three species of _Petromyzon_ are known, _P. fluviatilis_, _P.
marinus_ and _P. planeri_. The larval forms were for a long time
thought to belong to a separate genus and were called _Ammocoetes_.

The Myxinoids, although very highly specialised in their own way, are
at distinctly a lower stage of development than the adult Lamprey, and
come nearer to the larval Lamprey or Ammocoete.

SPINAL COLUMN.

[Illustration FIG. 4. A, DORSAL; B, LATERAL AND C, VENTRAL VIEW OF THE
SKULL OF _Petromyzon marinus_ × 1 (after PARKER).

    1. horny teeth.
    2. labial cartilage.
    3. anterior dorsal cartilage.
    4. posterior dorsal cartilage.
    5. nasal capsule.
    6. auditory capsule.
    7. dorsal portion of trabeculae.
    8. lateral distal mandibular.
    9. lingual cartilage.
    10. branchial basket.
    11. cartilaginous cup supporting pericardium.
    12. sheath of notochord.
    13. neural plate.]

In Myxinoids and larval lampreys, the notochord is enclosed in a thick
chordal sheath, in connection with which in the tail region there
occur cartilaginous pieces forming neural arch elements. In the trunk
region, however, no cartilage occurs in connection with the spinal
column, the only cartilage present being that forming the radiale of
the dorsal fin. On the other hand in most species of lamprey
(_Petromyzon_) cartilaginous pieces forming imperfect neural arches
(fig. 4, B, 13) are found lying in the tough skeletogenous layer
dorsal to the notochord, and extending throughout the whole length of
the trunk and tail. Two of these pieces, which are probably homologous
with the neural plates (see p. 72) of Elasmobranchs, occur to each
_neuromere_, or segment as determined by the spinal nerves. The
dorsal and caudal fins are supported by paired cartilaginous radiale
which are connected proximally with the skeletogenous layer.

THE SKULL.

In Myxinoids the cranium is a mere cartilaginous floor without side
walls or roof, and the trabeculae[27] end without growing forwards
into cornua. In Lampreys the trabeculae grow forwards and send up
plates of cartilage which meet above (fig. 4, 7) and form side walls
and a roof for part of the brain case. In Lampreys a labial suctorial
apparatus is well developed, including a large ring-like piece of
cartilage (fig. 4, 2) which supports the oral funnel and bears a large
armament of horny teeth. In Myxinoids on the other hand the labial
skeleton is small and consists merely of barbels round the mouth.

The olfactory organ of Myxinoids has a very curious skeleton. It is
covered with a kind of grating of cartilage which is prolonged in
front into a tube composed of a series of imperfect cartilaginous
rings. In Lampreys the olfactory organ opens merely by a short
membranous passage. In correlation with the small development of the
labial suctorial apparatus in Myxinoids the lingual apparatus is very
greatly developed. The tongue in _Myxine_ has been said to 'dominate
the whole body' (Parker). It is supported by a great median
cartilaginous bar which when followed forwards first becomes bifid and
still further forwards becomes four-cleft.

The horny teeth in Myxinoids are chiefly borne on the very large
supralingual apparatus. They form a double series arranged in the form
of an arch. In _Myxine_ there are seven large teeth and nine small
ones on each side. In _Bdellostoma_ the teeth of the two rows are more
equal in size. In _Bdellostoma_ and _Myxine_ it has been shown that
imperfect calcified teeth occur below the horny teeth.

In Lampreys the lingual apparatus (fig. 4, C, 9) is well developed,
but not excessively so. It consists of a long median cartilaginous bar
which ends in front with a semicircular piece of cartilage supporting
the median part of the tongue.

In both Myxinoids and Lampreys there is a complicated branchial basket
apparatus, but while in Myxinoids the basket apparatus is
interbranchial, formed deep within the head near the hypoblastic
lining of the throat, in Lampreys it is extra-branchial and formed
outside the head cavities (fig. 4, 10). The two sides of the basket
apparatus in _Myxine_ are not symmetrical. In the interbranchial
basket apparatus of Myxinoids the hyoid and first and second branchial
arches can be recognised. Traces of the interbranchial skeleton of
Myxinoids can be detected in Lampreys, and similarly in Myxinoids,
there are indications of the extra-branchial skeleton of _Petromyzon_.
The branchial basket in Lampreys forms at its posterior end a kind of
cup which supports the pericardium (fig. 4, 11).

A remarkable Cyclostome named _Palaeospondylus_[28] has recently been
described from the Scottish Old Red Sandstone. It differs however from
all living Cyclostomes, in having a spinal column formed of distinct
vertebrae with well-developed neural arches. The caudal fin is well
developed and the dorsal radiale are forked as in lampreys. The skull
is well calcified and the auditory capsules are specially large. The
mouth is very similar to that of lampreys, being circular and without
jaws; it is provided with barbels or cirri. There is no trace of limbs
and the average length is only about 1-1-1/2 inches.


FOOTNOTES:

[24] See W.K. Parker On the skeleton of the Marsipobranch fishes,
_Phil. Trans._ 1883, London.

[25] See A. Smith Woodward, _Catalogue of Fossil Fish in the British
Museum_, Part II., 1891. A. Smith Woodward, _Nat. Sci._ vol. I. 1892,
p. 596.

[26] See R.H. Traquair, _Ann. Nat. Hist._, ser. 6, vol. II. 1888, p.
485.

[27] See p. 17.

[28] R.H. Traquair, _Ann. Nat. Hist._ vol. VI. 1890, p. 485; _P. Phys.
Soc. Edinb._ vol. XII. 1892-93, pp. 87-94, and 312-320. A. Smith
Woodward, _Nat. Sci._ vol. III. p. 128, 1893.




CHAPTER V.

(II.) GNATHOSTOMATA.


THE mouth is supported by definite jaws.


ICHTHYOPSIDA.

The epiblastic exoskeleton is generally unimportant, the mesoblastic
exoskeleton is usually well developed.

The notochord with its membranous sheath (1) may remain unmodified, or
(2) may be replaced by bone or cartilage derived from the
skeletogenous layer, or (3) may be calcified to a varying extent.

The first vertebra is not homologous throughout the whole series and
so is not strictly comparable to the atlas of Sauropsids and Mammals.

The centra of the vertebrae have no epiphyses. The skull may be (_a_)
incomplete and membranous, or (_b_) more or less cartilaginous, or
(_c_) bony. Membrane bones are not included in the cranial walls, and
there are large unossified tracts in the skull. When membrane bones
are developed in connection with the skull, a large parasphenoid
occurs. The basisphenoid is always small or absent. The skull may be
immovably fixed to the vertebral column, or may articulate with it by
a single or double occipital condyle. When the occipital condyle is
double, it is formed by the exoccipitals, and the basi-occipital is
small or unossified. The mandible may be (_a_) cartilaginous, (_b_)
partially ossified, or (_c_) membrane bones may be developed in
connection with it,--if so, there is usually more than one membrane
bone developed in connection with each half.

There are at least four pairs of branchial arches present during
development. The sternum, if present, is not costal in origin.


CLASS I. PISCES.

The exoskeleton is in the form of scales, which may be entirely
mesoblastic or dermal in origin (e.g. _cycloid_ and _ctenoid_ scales),
or may be formed of both mesoblast and epiblast (e.g. _placoid_ and
_ganoid_ scales). Large bony plates may be derived from both these
types of scale. In general fish with a greatly developed dermal armour
have the endoskeleton poorly developed; and the converse also holds
good.

The integument of the dorsal and ventral surfaces is commonly
prolonged into longitudinal unpaired fins, supported by an internal
skeleton. These fins are distinguished according to their position as
dorsal, caudal and anal fins. The dorsal and anal fins are used
chiefly as directing organs, the caudal fin is however a most
important organ of propulsion.

Three types of tail are found in fishes, viz.:--

1. The =diphycercal=, in which the axis is straight and the tail is
one-bladed and symmetrical, an equal proportion of radiale[29] being
attached to the upper and lower surfaces of the axis.

2. The =heterocercal=, in which the tail is asymmetrical and the axis
is bent upwards, the proportion of radiale or of fin-rays attached to
its upper surface being much smaller than that attached to its lower
surface.

3. The =homocercal=, in which the tail though externally symmetrical,
so far resembling the diphycercal type, is internally really
heterocercal, the great majority of the radiale or of the fin-rays
being attached to the lower surface of the axis.

The cranium in the simplest cases (e.g. Selachii) forms a
cartilaginous box enclosing the brain and sense organs; in bony fishes
it is greatly complicated. When palatine or pterygoid bones are
present they are formed by the ossification of cartilage; in
Sauropsida and Mammalia they are laid down as membrane bones. There is
no tympanic cavity or auditory ossicle in relation to the ear.

There are two principal types of suspensorium by means of which the
jaws are attached to the cranium:--

(1) The =Autostylic=. This is the primitive condition in which the
mandibular arch articulates with the base of the cranium in front of
the hyoid and in a similar manner.

(2) The =Hyostylic=. In this case the mandibular arch becomes
connected with the hyomandibular and supported by the hyoid arch.
These terms are more fully discussed in Chapter VIII.

There is always an internal framework supporting the gills; it usually
consists of the hyoid arch and five, rarely six or seven, pairs of
branchial arches. The limbs are represented by two pairs of fins, the
pectoral and the pelvic; they are not divided into proximal, middle
and distal portions. The ribs do not unite with a median ventral
sternum, or meet in the mid-ventral line in any other way in the trunk
region.


_Order I._ ELASMOBRANCHII.

The exoskeleton is in the form of placoid scales which are sometimes
so numerous as to give the whole skin a rough surface forming
shagreen. In some cases the placoid scales are enlarged to form plates
or spines capped or coated with enamel. These spines may be imbedded
in the flesh in front of the paired or unpaired fins, or may be
attached to the tail. They are specially characteristic of the
suborder Acanthodii. The endoskeleton is cartilaginous and true bone
is never found. Much of the skeleton, especially of the vertebral
column, is however often calcified, this being especially well seen in
the anterior part of the vertebral column of Rays (Raiidae). In living
forms cartilaginous biconcave vertebrae are always well developed, but
in some extinct forms the notochord persists unconstricted. Neural and
haemal arches are however always developed; they sometimes remain
separate, sometimes fuse with the centra. Ribs are often wanting and
when present are often not separated off from the vertebrae. The
cranium is a simple cartilaginous box whose most prominent parts are
the capsules which enclose the sense organs. The skull is sometimes
immovably fixed to the vertebral column, sometimes articulates with it
by means of two condyles. There is no operculum and no representative
of the maxilla or premaxillae. The teeth are very variable. Large
pectoral and pelvic fins always occur.

The Elasmobranchii may be divided into four suborders:--

    (1) Ichthyotomi.
    (2) Pleuropterygii.
    (3) Selachii.
    (4) Acanthodii.


_Suborder_ (1). ICHTHYOTOMI[30].

The members of this suborder range from the Devonian to the Permian
and so have long been extinct.

The endoskeletal cartilage has granular calcifications evenly
distributed throughout it. The notochord is unconstricted, but the
neural and haemal arches are well-developed, and the neural spines are
long and slender. There is a continuous dorsal fin with separate
basalia and radiale. The tail is diphycercal, and the pectoral fins
are typical archipterygia[31]. The pelvic fins of the male are
prolonged to form claspers.

The best known of these primitive Elasmobranchs are the
Pleuracanthidae.


_Suborder_ (2). PLEUROPTERYGII.

This suborder was formed for the reception of _Cladoselache_, an
Elasmobranch found in the Lower Carboniferous of Ohio[32].

The exoskeleton is in the form of small, thickly-studded dermal
denticles. The vertebral centra are unossified, and the tail is
strongly heterocercal. There were certainly five, perhaps seven gill
slits, and the suspensorium is apparently hyostylic. The paired fins
are, according to the view which derives them by concentration from
continuous lateral folds, the most primitive known (see p. 129) and
claspers are absent.


_Suborder_ (3). SELACHII.

Cartilaginous or partially calcified biconcave vertebrae are always
well developed; they constrict the notochord intervertebrally. The
neural and haemal arches and spines are stout and intercalary
cartilages (interdorsalia) are present. The tail is heterocercal, but
in some cases (_Squatina_) approaches the diphycercal condition. In
most cases the suspensorium is hyostylic, the jaws being attached to
the cranium by means of the hyomandibular, and the palato-pterygo
quadrate bar not being fused to the cranium. There are generally five
pairs of branchial arches, and gill rays are borne on the posterior
surface of the hyoid arch, and on both the anterior and posterior
surfaces of the first four branchial arches. The Notidanidae differ
from most Selachians in two respects, first as regards the
suspensorium,--Meckel's cartilage articulating directly with the
palato-pterygo-quadrate bar, and not being connected with the hyoid
arch; and secondly as regards the number of branchial arches,--six
pairs occurring in _Hexanchus_ and seven in _Heptanchus_.

The pectoral fins are without the segmented axis of the
archipterygium. In most cases they are sharply marked off from the
body and lie almost at right angles to it; but in the Rays they have
the form of lateral expansions in the same plane as the body, from
which they are not sharply marked off. The pelvic fins in the male
bear long grooved cartilaginous rods which are accessory copulatory
organs or claspers.

There are two principal groups of Selachii, the Squalidae or Sharks
and Dogfish, and the Batoidei or Skates and Rays. The Squalidae have
the shape of ordinary fish, the pectoral fins are vertically placed
and the body ends in a powerful heterocercal tail. The Batoidei have
flattened bodies owing to the great size and horizontal position of
the pectoral fins. The tail is long and thin and is often armed with
spines. The teeth in Selachii differ much in character in the
different forms, and are always arranged in numerous rows. They are
generally pointed and triangular or conical in the Squalidae, while in
the Batoidei they are often broad and flattened.


_Suborder_ (4). ACANTHODII.

The fishes included in this group are all extinct and in some respects
are intermediate between Elasmobranchii and Ganoidei. The body is
elongated and closely covered with small scales consisting of dentine
enamelled at the surface. The notochord is persistent and the
calcification of the endoskeletal cartilage is only superficial. The
tail is heterocercal. The jaws bear small conical teeth, or in some
cases are toothless. The skeleton of all the fins differs from that of
modern Elasmobranchs in having the cartilaginous radiale much reduced,
and the fins are nearly always each provided with an anterior spine,
which except in the case of the pectoral fins is merely inserted
between the muscles. These spines are really enormous dermal fin-rays;
the pectoral fin-spine is articulated to the pectoral girdle.

The suborder includes many well-known extinct forms like _Acanthodes_
and _Diplacanthus_; it ranges from the Devonian to the Permian.


PISCES, HOLOCEPHALI.

_Order II._ HOLOCEPHALI.

This order includes a single suborder only.


_Suborder._ CHIMAEROIDEI.

[Illustration FIG. 5. SKULL OF A MALE _Chimaera monstrosa_ (after
HUBRECHT).

    1. nasal capsule.
    2. cartilaginous appendage to the fronto-nasal region.
    3. erectile appendage.
    4. foramen by which the ophthalmic nerves leave the orbit.
    5. foramen by which the ophthalmic branch of the Vth nerve enters
        the orbit.
    6. auditory capsule.
    7. interorbital septum.
    8. mandible articulating with an outgrowth from the posterior part
       of the palato-pterygo-quadrate.
    9. teeth.
    10. labial cartilage.
    II. III. V. VII. IX. X. foramina for the passage of cranial nerves.]

These singular fish have the skin smooth and in living forms almost or
quite scaleless. The palato-pterygo-quadrate bar and hyomandibular are
fused to the cranium, and Meckel's cartilage articulates directly with
the part corresponding to the quadrate. The skull is distinctly
articulated with the spinal column, the notochord is persistent and
unconstricted, and the skeletogenous layer shows no trace of metameric
segmentation, though in the neural arches this segmentation is
readily traceable. The neural arches of the first few vertebrae are
fused together and completely surround the notochord, while they do
not in other parts of the body. The tail is diphycercal. Of the living
genera, in _Callorhynchus_ there is no trace of calcification in the
skeletogenous layer, while in _Chimaera_ rings of calcification are
found, there being three to five for each vertebra as indicated by the
foramina for the exit of the spinal nerves. The pelvic fins are
produced into claspers. Besides the living genera _Chimaera_,
_Harriotta_ and _Callorhynchus_ a fair number of fossil forms are
known, e.g. _Ischyodus_.


_Order_ III. GANOIDEI.

The fishes included under the term Ganoidei form a very heterogeneous
group, some of which closely approach the Dipnoi, others the
Elasmobranchii, others the Teleostei. The great majority of them are
extinct, only eight living genera being known; these are all
inhabitants of the northern hemisphere, and with the exception of
_Acipenser_, which is both fluviatile and marine, are entirely
confined to fresh water.

The following is a list of the living genera of Ganoids with their
respective habitats:--

     _Acipenser._ Rivers and seas of the northern hemisphere.

     _Scaphirhynchus._ Mississippi and rivers of Central Asia.

     _Polyodon_ (_Spatularia_). Mississippi.

     _Psephurus._ Yan-tse-kiang, and Hoangho.

     _Polypterus._ Rivers of tropical Africa.

     _Calamoichthys._ Some rivers of West Africa.

     _Lepidosteus._ Freshwaters of Central and North America and
     Cuba.

     _Amia._ Rivers of Carolina.

The exoskeleton is very variable, thus the body may be:--

(_a_) Naked or with minute stellate ossifications as in the
Polyodontidae. (_b_) Partially covered with large detached bony
plates as in _Scaphirhynchus_ and _Acipenser_. (_c_) Entirely covered
with rhomboidal ganoid scales as in _Lepidosteus_, _Polypterus_,
_Palaeoniscus_ and many extinct forms. (_d_) Covered with rounded
scales shaped like the cycloid scales of Teleosteans as in _Amia_.
(_e_) Having the trunk and part of the tail covered with rhomboidal
scales, and the remainder of the tail with rounded scales as in
_Trissolepis_.

The teeth also are very variable. The endoskeleton shows every stage
of transition from an almost entirely cartilaginous state as in
_Acipenser_ to a purely bony state as in _Lepidosteus_. Sometimes, as
in _Acipenser_, the notochord persists, and its sheath is unsegmented;
sometimes, as in _Lepidosteus_, there are fully formed vertebrae. The
tail may be heterocercal, as in _Acipenser_, or diphycercal as in
_Polypterus_. The cartilaginous cranium is always covered with
external membrane bone to a greater or less extent, and the
suspensorium is markedly hyostylic. The pectoral girdle is formed of
two parts, one endoskeletal and cartilaginous, corresponding with the
pectoral girdle of Elasmobranchs, and one exoskeletal and formed of
membrane bones, corresponding with the clavicular bones of
Teleosteans. The pelvic fins are always abdominal. The fins often, as
in _Polypterus_, have spines (fulcra) attached to their anterior
borders.

The order Ganoidei may be divided into three suborders:

     (1) CHONDROSTEI. Living genera _Acipenser_, _Scaphirhynchus_,
     _Polyodon_ and _Psephurus_.

     (2) CROSSOPTERYGII. Living genera _Polypterus_ and
     _Calamoichthys_.

     (3) HOLOSTEI. Living genera _Lepidosteus_ and _Amia_.


_Suborder_ (1). CHONDROSTEI.

The skull is immovably fixed to the vertebral column. By far the
greater part of the skeleton is cartilaginous. The notochord is
persistent and unconstricted, its sheath is membranous, but
cartilaginous neural and haemal arches are developed. Intercalary
pieces (interdorsalia) occur between the neural arches, and similar
pieces (interventralia) between the haemal arches. The cranium is
covered with membrane bone, and teeth are but slightly developed. The
tail is heterocercal. Gill rays occur on the hyoid arch, and the gills
are protected by a bony operculum attached to the hyomandibular. The
skin (1) may be almost or quite naked, (2) may carry bony plates
arranged in rows, or may be covered (3) with rhomboidal scales, or (4)
partly with rhomboidal, partly with cycloidal scales.


_Suborder_ (2). CROSSOPTERYGII.

The exoskeleton has the form of cycloidal or rhomboidal scales. The
condition of the vertebral column differs in the different genera.
Sometimes, as in _Polypterus_, there are well-developed ossified
vertebrae; sometimes, as in many extinct forms, the notochord persists
and is unconstricted. The tail may be diphycercal or heterocercal. The
pectoral and sometimes the pelvic fins consist of an endoskeletal axis
bearing a fringe of dermal rays.


_Suborder_ (3). HOLOSTEI.

The exoskeleton has the form of cycloidal or rhomboidal scales. The
notochord is constricted and its sheath is segmented and ossified,
forming distinct vertebrae, which are generally biconcave, sometimes
opisthocoelous (_Lepidosteus_). The cartilaginous cranium is largely
replaced by bone, and in connection with it we find not only membrane
bone, but cartilage bone, as the basi-occipital, exoccipitals, and
pro-otic are ossified. The tail is heterocercal. The suspensorium
resembles that of Teleosteans, consisting of a proximal ossification,
the hyomandibular, which is movably articulated to the skull and a
distal ossification, the symplectic. The two are separated by some
unossified cartilage. The cartilaginous upper and lower jaws are to a
great extent surrounded and replaced by a series of membrane bones.


_Order_ IV. TELEOSTEI.

The exoskeleton is sometimes absent but generally consists of
overlapping cycloid or ctenoid scales. Bony plates are sometimes
present, as in the Siluridae, or the body may be encased in a complete
armour of calcified plates, as in _Ostracion_. Enamel is however never
present, and the plates are entirely mesodermal. The skeleton is bony,
but in the skull much cartilage generally remains. The vertebral
centra are usually deeply biconcave, and the tail is of the masked
heterocercal type distinguished as _homocercal_. In the skull the
occipital region is always completely ossified, while the sphenoidal
region is generally less ossified. The skull has usually a very large
number of membrane bones developed in connection with it. The teeth
vary much in character in the different members of the order, but are
as a rule numerous and pointed, and are ankylosed to the bone. The
suspensorium is hyostylic and the jaws have much the same arrangement
as in the Holostei. There are five pairs of branchial arches, of which
all except the last bear gill rays. A series of dermal opercular bones
is developed in connection with these arches. The pectoral girdle
consists almost entirely of dermal clavicular bones. The pelvic girdle
has disappeared, its place being taken by the enlarged and ossified
dermal fin-rays of the pelvic fins.

The group includes the vast majority of living fish (see p. 33).


_Order_ V. DIPNOI.

The exoskeleton is of two types; dermal bones are largely developed in
the head region, while the tail and posterior part of the body may be
naked or may be covered with overlapping scales. The cranium remains
chiefly cartilaginous, the palato-pterygo-quadrate bar is fused with
the cranium, and the suspensorium is autostylic. The gill clefts are
feebly developed and open into a cavity covered by an operculum. The
notochord is persistent and unconstricted, and the limbs are
archipterygia. The pelvic fins are without claspers.


_Suborder_ (1). SIRENOIDEI[33].

The head has well developed membrane bones. The trunk is covered with
overlapping scales and bears no bony plates. Three pairs of teeth are
present, two in the upper and one in the lower jaw, the two principal
pairs of teeth are borne on the palato-pterygoids and splenials, while
the third pair are found in the vomerine region. The tail is
diphycercal in living forms. In the extinct Dipteridae it is
heterocercal. The pectoral girdle includes both membrane and cartilage
bones. The pelvic girdle consists of a single bilaterally symmetrical
piece of cartilage.

This suborder is represented by the living genera _Ceratodus_,
_Protopterus_ and _Lepidosiren_, and among extinct forms by the
Dipteridae and others.


_Suborder_ (2). ARTHRODIRA.

Bony plates are developed not only on the head but also on the
anterior part of the trunk, where they consist of a dorsal, a ventral,
and a pair of lateral plates which articulate with the cranial shield.
The posterior part of the trunk is naked. The tail is diphycercal. The
jaws are shear-like, and their margins are usually provided with
pointed teeth whose bases fuse with the tissue of the jaw and
constitute dental plates. There seem to have been three pairs of these
plates, arranged as in the Sirenoidei, the principal ones in the upper
jaw being borne on the palato-pterygoids. Small pelvic fins are
present, but pectoral fins are unknown.

The Arthrodira occur chiefly in beds of Devonian and Carboniferous
age. Two of the best known genera are _Coccosteus_ from the European
Devonian and _Dinichthys_, a large predatory form from the lower
Carboniferous of Ohio.


FOOTNOTES:

[29] See p. 79.

[30] For this and other groups of extinct fish see A. Smith Woodward,
_Catalogue of Fossil Fish in the British Museum_, Parts I.-III.
London, 1889-95.

[31] See p. 127.

[32] See B. Dean, _J. Morphol._ vol. IX. pp. 87-114, 1894, and _Nat.
Sci._ vol. VIII. p. 245, 1896.

[33] A. Günther, _Phil. Trans._ vol. 161, Part II. 1871, p. 511. T.H.
Huxley, "On Ceratodus and the classification of fishes," _P.Z.S._
1876, p. 24.




CHAPTER VI.

THE SKELETON OF THE DOGFISH[34].

_Scyllium canicula._


I. EXOSKELETON.

The exoskeleton of the dogfish is mainly composed of placoid scales,
each of which consists of a little bony base imbedded in the skin,
bearing a small backwardly-directed spine formed of dentine capped
with enamel. The scales are larger on the dorsal than on the ventral
surface, and on the jaws they are specially large and regularly
arranged in rows, there forming the teeth. The margins of the jaws or
lips are without scales.

A second exoskeletal structure is found in the fins, all of which,
both paired and unpaired, have, in addition to their cartilaginous
endoskeleton, large numbers of long slender horny fibres, the
fin-rays, which are of exoskeletal origin.


II. ENDOSKELETON.

The endoskeleton of the dogfish consists almost entirely of cartilage,
which however may become calcified in places, e.g. the centrum of each
vertebra is lined by a layer of calcified tissue.

The endoskeleton is divisible into an =axial= portion consisting of
the vertebral column, skull, and skeleton of the median fins, and an
=appendicular= portion consisting of the skeleton of the paired fins
and their girdles.


1. THE AXIAL SKELETON.

A. THE VERTEBRAL COLUMN AND RIBS.

The vertebral column consists of a series of some hundred and thirty
vertebrae, each of which is united with its predecessor and successor
in such a way as to allow a large amount of flexibility.

These vertebrae are developed round an unsegmented rod, the
=notochord=, which forms the axial support of the embryo. The
notochord remains continuous throughout the whole vertebral column,
but is greatly constricted opposite the middle of each vertebra, and
thus rendered moniliform. The vertebrae are divided into two groups,
an anterior group of trunk vertebrae, and a posterior group of caudal
or tail vertebrae.

A typical vertebra consists of a middle portion, the =centrum=, a
dorsal portion, the =dorsal= or =neural arch=, which surrounds the
spinal cord, and a ventral portion, the =ventral= or =haemal arch=,
which similarly encloses a space.

The tail vertebrae of the dogfish have this typical arrangement, the
trunk vertebrae have the haemal arches modified.

Each =centrum= is a short cylinder of cartilage surrounding an
hourglass-shaped cavity occupied by the notochord. The =neural arches=
are composed of three separate elements, the =vertebral neural plates=
(basidorsalia), =intervertebral neural plates= (interdorsalia), and
=neural spines= (supradorsalia).

The =vertebral neural plates= are in the adult fused with their
respective centra, and are notched behind for the exit of the ventral
(motor) roots of the spinal nerves. The =intervertebral neural plates=
are polygonal pieces alternating with the vertebral neural plates;
they are notched behind, but at a more dorsal level than are the
vertebral neural plates, for the exit of the dorsal or sensory roots
of the spinal nerves.

The =neural spines= are small patches of cartilage filling up the gaps
between the dorsal ends of the neural plates.

The =haemal arches= (basiventralia) differ much in the trunk and tail
portions of the vertebral column. In the trunk portion the centra are
flattened below, and the two halves of the haemal arch diverge from
one another as blunt =ventri-lateral processes= to
which short cartilaginous rods, the =ribs=, are attached. Further back
at about vertebra 37, the two halves of the haemal arch project
downwards and meet forming a complete arch. Further back still,
towards the hind end of the tail, the haemal arches bear median
=haemal spines= (ventrispinalia).


B. THE SKULL.

The skull of the dogfish remains cartilaginous throughout the life of
the animal, and has consequently a far more simple structure than have
the skulls of higher animals, in which complication has been produced
by the development of bone.

The skull consists of the following parts:--

(1) a dorsal portion, the =cranium=, which lodges the brain, and to
the sides of which the capsules of the auditory and olfactory sense
organs are united. The cranium may be compared to an unsegmented
continuation of the vertebral column;

(2) a number of ventral structures, disconnected or only loosely
connected with the cranium. These together constitute the =visceral
skeleton= forming the jaws and supporting the gills.

(1) THE CRANIUM.

The =Cranium= is an oblong box, with a flattened floor and a more
irregular roof. Its sides are expanded in front owing to the
olfactory capsules, and behind owing to the auditory capsules, while
in the middle they are deeply hollowed to form the orbits.

(_a_) On the dorsal surface of the cranium the following points should
be noticed. First at the anterior end, the large thin-walled =nasal=
or =olfactory capsules= (fig. 6, 1), each of which is drawn out into a
narrow cartilaginous process.

The olfactory capsules have no ventral walls, and are separated from
one another by the =internasal septum=, which is drawn out into a
third slender process. These three processes together constitute the
=rostrum= (fig. 6, 2).

Behind the olfactory capsules comes a large, nearly circular, hole,
the =anterior fontanelle=, slightly behind which are the two
=ophthalmic foramina=. The dorsal and ventral boundaries of the orbits
are respectively formed by the prominent =supra-orbital= and
=suborbital ridges=. Behind are the =auditory capsules= (fig. 6, 8),
each of which is marked by a pair of prominent ridges, converging
towards the middle line to a pair of apertures. These apertures
communicate with two canals, the =aqueductus vestibuli=, which lead
into the internal ear. The two ridges lodge respectively the =anterior
and posterior vertical semicircular canals= of the ear.

(_b_) The principal structures to be noted in a side view of the
cranium are contained in the =orbit= or eye-cavity. Near the base of
the orbit at its anterior end is seen the small =orbitonasal foramen=
(fig. 6, 7), for the passage of blood-vessels, not nerves. Above it is
the large =ophthalmic foramen= (fig. 6, 5) so prominent in a dorsal
view of the skull; through it the ophthalmic branches of the fifth and
seventh nerves pass. Slightly further back near the ventral surface is
the large =optic foramen= (fig. 6, II.) for the passage of the second
nerve. Vertically above the optic foramen, near the dorsal surface, is
the very small =foramen for the fourth nerve= (fig. 6, IV.). Behind
and a little above the optic foramen is another small aperture, the
=foramen for the third nerve=. Behind and slightly below this is the
large =foramen for the sixth and main branches of the fifth and
seventh nerves= (fig. 6, V.). In front of and slightly below this
foramen are seen two other small apertures; the more anterior and
ventral of these (fig. 6, 4) is for the passage of a vessel connecting
the efferent artery of the hyoid gill with the internal carotid artery
inside the skull, the more posterior and dorsal is for the
=interorbital canal= (fig. 6, 3) which unites the two orbital sinuses.
Above and very slightly in front of the large foramen for the sixth
and main parts of the fifth and seventh nerves, are two small foramina
(fig. 6, Va., and VIIa.), through which the =ophthalmic branches of
the fifth and seventh nerves= enter the orbit. Behind and slightly
below the large foramen just mentioned is a small hole through which
the external carotid enters the orbit (fig. 6, 9).

[Illustration Fig. 6. LATERAL VIEW OF THE SKULL OF A DOGFISH
(_Scyllium canicula_) × 2/3.]

    1. nasal capsule.
    2. rostrum.
    3. interorbital canal.
    4. foramen for hyoidean artery.
    5. foramen for the exit of the
    ophthalmic branches of
    Vth and VIIth nerves.
    6. foramen through which the
    external carotid leaves the
    orbit.
    7. orbitonasal foramen.
    8. auditory capsule.
    9. foramen through which the
    external carotid enters the
    orbit.
    10. ethmo-palatine ligament.
    11. palato-pterygo-quadrate bar.
    12. Meckel's cartilage.
    13. hyomandibular.
    14. cerato-hyal.
    15. pharyngo-branchial.
    16. epi-branchial.
    17. cerato-branchial.
    18. gill filaments, nearly all have
    been cut off short for the
    sake of clearness.
    19. extra-branchial.
    20. pre-spiracular ligament.
    II. III. IV. V. Va. VIIa. foramina
    for passage of cranial nerves.

Behind the orbit is the =auditory capsule=. This is marked below by a
prominent =surface for the articulation of the hyomandibular=, above
which is the deep =postorbital groove= for the passage of a
blood-vessel, connecting the orbital and anterior cardinal sinuses.

(_c_) Passing to the posterior end of the cranium: in the centre is
seen the large =foramen magnum= through which the brain and spinal
cord communicate. The =notochord= enters the skull just below this
foramen, and on each side of the notochord is a projection, the
=occipital condyle=, by which the first vertebra articulates with the
skull.

External to the condyles are the prominent =pneumogastric foramina=
for the passage of the tenth nerves, and further to the sides, just
beyond the posterior vertical semicircular canals, are a pair of deep
pits in which lie the =foramina for the ninth nerves= (fig. 6, IX).

(_d_) The broad and flat ventral surface of the cranium is continued
in front as the =internasal septum= and terminated laterally by the
=suborbital ridges=. At a little behind the middle it is traversed by
two shallow grooves along which the internal carotid arteries run. At
the divergent ends of these grooves are seen two small apertures
through which the external carotids enter the orbit (fig. 6, 9), and
at the point where they meet is a single small aperture through which
the internal carotid enters the cranium.

(2) THE VISCERAL SKELETON.

The =Visceral skeleton= forms a series of seven cartilaginous arches
or hoops, surrounding the anterior part of the alimentary canal, and
enclosing a wide but rather shallow space.

(_a_) The first or =mandibular arch= is the largest of the series, and
forms the upper and lower jaws. Each half of the upper jaw or
=palato-pterygo-quadrate= bar is formed by a thick cartilaginous rod
which meets its fellow in the middle line in front, the two being
united by ligament. Each half is connected to the cranium just in
front of the orbit by the =ethmo-palatine ligament= (fig. 6, 10), and
at its hind end articulates with one of the halves of the lower jaw.
Each half of the lower jaw or =Meckel's cartilage= (fig. 6, 12) is a
cartilaginous bar, wide behind but narrow in front, where it is united
to its fellow by a median ligament. Imbedded in the tissue external to
the upper jaw are a pair of =labial cartilages=, and a similar but
smaller pair are imbedded in the tissue external to the lower jaw.

The jaws are developed from a structure whose dorsal and ventral
portions subsequently become of very different importance. The ventral
portion forms both upper and lower jaws, the former being developed as
an outgrowth from the latter. The dorsal portion forms only the
=pre-spiracular ligament= (fig. 6, 20), a strong fibrous band
containing a nodule of cartilage, and running from the anterior part
of the auditory capsule to the point where the jaws are connected with
the hyomandibular.

(_b_) The =hyoid arch= consists of a pair of cartilaginous rods which
are attached at their dorsal ends to the cranium, and are united
ventrally by a broad median plate of cartilage, the =basi-hyal=. Each
rod is divided into a dorsal portion, the =hyomandibular= and a
ventral portion, the =cerato-hyal=. The =hyomandibular= (fig. 6, 13)
is a short stout rod of cartilage projecting outwards, and somewhat
backwards and downwards from the cranium, with which it articulates
behind the orbit and below the postorbital groove. Its distal end
articulates with a rather long slender bar, the =cerato-hyal= (fig. 6,
14), which is in its turn attached to the side of the =basi-hyal=. The
=basi-hyal= is a broad plate, rounded in front and drawn out behind
into two processes to which the two halves of the first branchial arch
are attached. The posterior surfaces of both hyomandibular and
cerato-hyal bear slender cartilaginous processes, the =gill rays=. The
hyoid arch forms the main =suspensorium= or means by which the jaws
are attached to the cranium. This attachment is chiefly brought about
by a series of short ligaments which connect the posterior ends of
both upper and lower jaws with the hyomandibular, but there is also a
ligament connecting the lower jaw with the cerato-hyal. The attachment
of the jaws to the cranium is also partially effected by the
pre-spiracular and ethmo-palatine ligaments.

(_c_) Each of the five =branchial arches= is a hoop, incomplete above
and formed of four or more pieces of cartilage. The most dorsal
elements, the =pharyngo-branchials=, are flattened, pointed plates
whose free inner ends run obliquely backwards, and terminate below the
vertebral column. They are connected at their outer ends with the
short broad =epi-branchials= (fig. 6, 16) which lie at the sides of
the pharynx. From the epi-branchials arise the long =cerato
branchials= (fig. 6, 17) which run forwards and inwards along the
ventral wall of the pharynx. The first four cerato-branchials are
connected with small rods, the =hypo-branchials=, which run backwards
to meet one another in the middle line. The last two pairs of
hypo-branchials and the fifth cerato-branchials are connected with a
broad median plate, the =basibranchial=. Along the outer sides of the
second, third and fourth cerato-branchials are found elongated curved
rods, the =extra-branchials= (fig. 6, 19). The epi-branchials and
cerato-branchials bear gill rays along their posterior borders.


C. THE SKELETON OF THE MEDIAN FINS.

The =dorsal fins= have a skeleton consisting of a series of short
cartilaginous rods, the =basals= or basalia, which slope obliquely
backwards. Their bases are imbedded in the muscles of the back, while
their free ends bear a number of small polygonal cartilaginous plates,
the =radials= or radiale. Associated with this cartilaginous skeleton
are a number of long slender horny fibres, the fin-rays, which have
been already referred to in connection with the exoskeleton. The
skeleton of the other median fins mainly consists of these fibres, the
cartilaginous portion being reduced or absent.


2. THE APPENDICULAR SKELETON.

This includes the skeleton of the two pairs of limbs and of their
respective girdles.

THE PECTORAL GIRDLE forms a crescent-shaped hoop of cartilage,
incomplete above and lying just behind the visceral skeleton. The
mid-ventral part of the hoop is the thinnest portion, and is drawn out
in front into a short rounded process which is cupped dorsally and
supports part of the floor of the pericardium (fig. 7, 1). On each
side of this flattened mid-ventral portion the arch becomes very thick
and bears on its outer border a surface with which the three basal
cartilages of the fin articulate. The dorsal ends or scapular portions
of the girdle form a pair of gradually tapering horns.

THE PECTORAL FIN articulates with the pectoral girdle by means of
three basalia or basal cartilages, the =propterygium=, =meso
pterygium= and =meta-pterygium=. The most anterior and the smallest of
these is the =propterygium= (fig. 7, 5), while the most posterior
one, the =meta-pterygium= (fig. 7, 3), is much the largest. Along the
outer borders of the three basalia are arranged a series of close set
cartilaginous pieces, the =radiale=. The propterygium supports only a
single radial, which is however much larger than any of the others.
The meso-pterygium also supports only a single radial which divides
distally.

[Illustration FIG. 7. SEMIDORSAL VIEW OF THE PECTORAL GIRDLE AND FINS
OF A DOGFISH (_Scyllium canicula_) × 2/3.

The gaps between the radiale are blackened.

    1. hollow in the mid-ventral part
    of the pectoral girdle which
    supports the pericardium.
    2. dorsal (scapular portion) of
    pectoral girdle.
    3. meta-pterygium.
    4. meso-pterygium.
    5. propterygium.
    6. propterygial radial.
    7. meso-pterygial radial.
    8. meta-pterygial radial.
    9. outline of the distal part of
    the fin which is supported
    by horny fin-rays.]

The meta-pterygium bears about twelve long narrow radials, the first
nine of which are traversed by a transverse joint at about two-thirds
of the way from their origin. Succeeding the radials are a series of
small polygonal pieces of cartilage arranged in one or more rows and
attached to the ends of the radials, and finally the fin is completed
by the dermal fin-rays.

[Illustration FIG. 8. DORSAL VIEW OF THE PELVIC GIRDLE AND FINS OF A
MALE DOGFISH (_Scyllium canicula_).

    1. pelvic girdle.
    2. basi-pterygium.
    3. clasper.
    4. radiale.]

THE PELVIC GIRDLE is much smaller than the pectoral. It is formed of a
stout nearly straight bar of cartilage placed transversely across the
ventral region of the body. The bar has no dorsal or lateral
extensions, and is terminated by short blunt processes. It bears on
its posterior surface a pair of facets with which the pelvic fins
articulate.

THE PELVIC FIN is smaller and more simply constructed than is the
pectoral. It consists of a long, somewhat curved rod, the
=basi-pterygium= (fig. 8, 2), running directly backwards on the inner
side of the fin, and articulating in front with the pelvic girdle.
From its outer side arise a series of about fourteen parallel
cartilaginous radials which bear smaller polygonal pieces. The
anterior one or two of these radials may articulate independently with
the pelvic girdle. In the adult male dogfish the distal end of the
basi-pterygium bears a stout rod nearly as long as itself, and grooved
on the dorsal surface. This is the skeleton of the =clasper= (fig. 8,
3).

FOOTNOTE:

[34] See Marshall and Hurst's _Practical Zoology_, 4th ed. London,
1895, p. 214.




CHAPTER VII.

THE SKELETON OF THE CODFISH. (_Gadus morrhua._)


I. EXOSKELETON.

The exoskeleton includes

(1) =Scales.= These are of the type known as =cycloid= and consist of
flat rounded plates composed of concentrically arranged laminae of
calcified matter, with the posterior margin entire. The anterior end
of each scale is imbedded in the skin and is overlapped by the
preceding scales.

(2) The =teeth=. These are small, pointed, calcified structures
arranged in large groups on the premaxillae, mandible, vomer, and
superior and inferior pharyngeal bones.

(3) The =fin-rays=. These are delicate, nearly straight bony rods
which support the fins.


II. ENDOSKELETON.

The endoskeleton of the Codfish, though partially cartilaginous, is
mainly ossified.

It is divisible into an =axial portion=, including the skull,
vertebral column, ribs, and skeleton of the median fins, and an
=appendicular portion=, including the skeleton of the paired fins and
their girdles.

1. THE AXIAL SKELETON.

A. THE VERTEBRAL COLUMN.

This consists of a series of some fifty-two vertebrae, all completely
ossified.


It is divisible into two regions only, viz. the =trunk= region, the
vertebrae of which bear movable ribs, and the =caudal= or =tail=
region, the vertebrae of which do not bear movable ribs.

=Trunk vertebrae.=

These are seventeen in number; the ninth may be described as typical
of them all. It consists of a short deeply biconcave =centrum= whose
two cavities communicate by a narrow central canal. From the dorsal
surface of the anterior half of the centrum arise two strong plates,
the dorsal or =neural processes=, which are directed obliquely
backwards and meet forming the dorsal or =neural arch=. This is
produced into a long backwardly-directed dorsal or =neural spine=.

From the lower part of the anterior edge of each neural arch arise a
pair of blunt triangular projections which overhang the posterior half
of the preceding centrum, and bear a pair of flattened surfaces which
correspond to the anterior or =prezygapophyses= of most vertebrae,
they differ however from ordinary prezygapophyses in the fact that
they look downwards and outwards. From the posterior end of the
centrum arise a pair of short blunt processes each of which bears an
upwardly- and inwardly-directed articulating surface corresponding to
a =postzygapophysis=.

The two halves of the ventral arch form a pair of large
=ventri-lateral processes= which arise from the anterior half of the
centrum and pass outwards and slightly backwards and downwards.

Behind these there arises on each vertebra a second outgrowth which is
small and flattened, and like the ventri-lateral process serves to
protect the air-bladder. The surface of the centrum is marked by more
or less wedge-shaped depressions, one in the mid-dorsal line, and two
on the ventral surface immediately mesiad to the bases of the
ventri-lateral process. There are also a number of smaller
depressions.

The space between one centrum and the next is in the fresh skeleton
filled up by the gelatinous remains of the =notochord=.

The first few vertebrae differ from the others in having very short
centra and no ventri-lateral processes.

The first vertebra comes into very close relation to the posterior
part of the skull, articulating with the exoccipitals. In the next few
vertebrae the centra gradually lengthen, and at the fourth or fifth
vertebra the ventri-lateral processes appear and gradually increase in
size as followed back. They likewise gradually come to arise at a
lower level on the centrum, and also become more and more downwardly
directed, till at the last trunk vertebra they nearly meet.

The =neural spines= of the anterior trunk vertebrae are much longer
than those of the posterior ones, that of the first vertebra being the
largest and longest of all, and articulating with the skull. The
spinal nerves pass out through wide notches or spaces between the
successive neural arches.

=Caudal vertebrae.=

The caudal vertebrae are about thirty-five in number, each consists of
a centrum with a slender backwardly-directed dorsal or neural arch,
similar to those of the posterior trunk vertebrae. The two halves of
the ventral or haemal arch however do not form outwardly-directed
ventri-lateral processes, but arise on the ventral surface of the
centrum, and passing downwards meet and enclose a space; they thus
form a complete canal, and are prolonged into a backwardly-directed
ventral or =haemal spine=. The anterior haemal arches are much larger
than the corresponding neural arches, but when followed back they
gradually decrease in size, till at about the twenty-fourth caudal
vertebra they are nearly as small as the neural arches. The last
caudal vertebra is succeeded by a much flattened =hypural= bone or
=urostyle=, which together with the posterior neural and haemal spines
supports the tail-fin.

B. THE RIBS.

The =ribs= are slender, more or less cylindrical bones attached to the
poster-dorsal faces of the ventri-lateral processes of all the trunk
vertebrae except the first and second. The earlier ones are thicker
and more curved; the later ones thinner and more nearly straight. The
ribs are homologous with the distal parts of the haemal arches of the
caudal vertebrae.

Associated with the ribs are a second series of rib-like bones, the
=intermuscular bones=. These are slender, curved bones which arise
from the ribs or from the ventri-lateral processes at a distance of
about an inch from the centra, and curve upwards, outwards and
backwards. In the anterior region where the ventri-lateral processes
are short they arise from the ribs, further back they arise from the
ventri-lateral processes.

C. THE UNPAIRED OR MEDIAN FINS.

These are six in number, three being =dorsal=, one =caudal= and two
=anal=.

The =dorsal= and =anal= fins each consist of two sets of structures,
the =fin-rays= and the =interspinous bones=. Each fin-ray forms a
delicate, nearly straight, bony rod which becomes thickened and
bifurcated at its proximal or vertebral end, while distally it is
transversely jointed and flexible, frequently also becoming more or
less flattened.

The first dorsal fin has thirteen rays, the second, sixteen to
nineteen, the third, seventeen to nineteen. The first anal fin has
about twenty-two, the second anal fourteen. In each fin the posterior
rays rapidly decrease in size when followed back.

The =interspinous bones= of the dorsal and anal fins alternate with
the neural and haemal spines respectively, and form short,
forwardly-projecting bones, each attached proximally to the base of
the corresponding fin-ray.

The =caudal fin= consists of a series of about forty-three rays which
radiate from the posterior end of the vertebral column, being
connected with the urostyle or hypural bone, and with the posterior
neural and haemal spines without the intervention of interspinous
bones. Like the other fin-rays those forming the caudal fin are
transversely jointed, and are widened and frayed out distally.

The tail-fin in the Cod is =homocercal=, i.e. it appears to be
symmetrically developed round the posterior end of the vertebral
column, though in reality a much greater proportion is attached below
the end of the vertebral column than above it. It is a masked
heterocercal tail.

THE SKULL.

Owing to the fact that very little cartilage remains in the skull of
the adult Codfish, its relation to the completely cartilaginous skull
of the Dogfish is not easily seen. Before describing it therefore, the
skull of the Salmon will be described, as it forms an intermediate
type.

THE SKULL OF THE SALMON[36].

The Salmon's skull consists of (1) the =chondrocranium=, which remains
partly cartilaginous and is partly converted into cartilage bone,
especially in the occipital region, (2) a large series of plate-like
membrane bones.

THE CHONDROCRANIUM.

This is an elongated structure, wide behind owing to the fusion of the
large auditory capsules with the cranium, and elongated and tapering
considerably in front; in the middle it is much contracted by the
large orbital cavities.

DORSAL SURFACE OF THE CRANIUM.

In the centre of the posterior end of the dorsal surface is the
=supra-occipital= (fig. 9, A, 1) with a prominent posterior ridge.
It is separated by two tracts of unossified cartilage from the large
series of bones connected with the =auditory organ=. The first of
these is the =epi-otic= (fig. 9, 2), which is separated by only a
narrow tract of cartilage from the supra-occipital, and is continuous
laterally with the large =pterotic= (fig. 9, A, 3) which overlaps in
front a smaller bone, the =sphenotic= (fig. 9, 4). Both epi-otic and
pterotic are drawn out into rather prominent backwardly-projecting
processes.

[Illustration FIG. 9. A. DORSAL AND B. VENTRAL VIEW OF THE CRANIUM OF
A SALMON (_Salmo salar_) from which most of the membrane bones have
been removed (after PARKER). Cartilage is dotted.

    1. supra-occipital.
    2. epi-otic.
    3. pterotic.
    4. sphenotic.
    5. frontal.
    6. median ethmoid.
    7. parietal.
    8. lateral ethmoid.
    9. parasphenoid.
    10. vomer.
    11. exoccipital.
    12. opisthotic.
    13. alisphenoid.
    14. orbitosphenoid.
    16. foramen for passage of an artery.
    17. pro-otic.
    18. articular surface for hyomandibular.

II. VII. IX. X. foramina for the passage of cranial nerves.]

The greater part of the remainder of the dorsal surface is formed of
unossified cartilage which is pierced by three large vacuities or
=fontanelles=. The anterior fontanelle is unpaired, and lies far
forward near the anterior end of the long cartilaginous snout, the two
larger posterior ones lie just in front of the supra-occipital and
lead into the cranial cavity. In front of the orbit the skull widens
again, and is marked by two considerable =lateral ethmoid= (fig. 9, 8)
ossifications. In front of these are a pair of deep pits, the =nasal
fossae=, at the base of which are a pair of foramina through which the
olfactory nerves pass out; they communicate with a space, the =middle
narial cavity=, seen in a longitudinal section of the skull.

The long cartilaginous snout is more or less bifid in front,
especially in the male (fig. 9).

POSTERIOR END OF THE CRANIUM.

The =foramen magnum= forms a large round hole leading into the cranial
cavity, and is bounded laterally by the two =exoccipitals= and below
by them, and to a very slight extent by the =basi-occipital=, the
three bones together forming a concave =occipital condyle= by which
the vertebral column articulates with the skull.

The exoccipitals are connected laterally with a fourth pair of
auditory bones, the =opisthotics=, and just meet the epi-otics
dorsolaterally, while dorsally they are separated by a wide tract of
unossified cartilage from the supra-occipital.

The opisthotics are connected laterally with the pterotics.

SIDE OF THE CRANIUM.

At the posterior end is seen the =basi-occipital= in contact above
with the =exoccipital=, which is pierced by a prominent foramen for
the exit of the tenth nerve. In front of this lies a small foramen,
sometimes double, for the ninth nerve.

[Illustration FIG. 10. LATERAL VIEW OF THE CHONDROCRANIUM OF A SALMON
(_Salmo salar_) (after PARKER). A few membrane bones are also shown.
Cartilage is dotted.

    1. supra-occipital.
    2. epi-otic.
    3. pterotic.
    4. opisthotic.
    5. exoccipital.
    6. basi-occipital.
    7. parasphenoid.
    8. sphenotic.
    9. alisphenoid.
    10. orbitosphenoid.
    11. lateral- or ectethmoid.
    12. olfactory pit; the vomerine teeth are seen just below.
    14. pro-otic.
    15. basisphenoid.
    16. foramen for the passage of an artery.
    17. anterior fontanelle.
    18. posterior fontanelle.

I. II. V. VII. IX. X. foramina for the passage of cranial nerves.]

In front of the exoccipital is the large =pro-otic= pierced by two
prominent foramina. Through the more dorsal of these (fig. 10, VII.)
the facial nerve passes out, while the more ventral (fig. 10, 16) is
for the passage of an artery. Dorsal to the exoccipital are the
=opisthotic= and =pterotic=, and dorsal to the pro-otic is the
=sphenotic=. The =pterotic= is marked by a prominent groove often
lined by cartilage, which is continued forwards along a tract of
cartilage between the pro-otic and sphenotic. With this groove the
hyomandibular articulates.

There are considerable ossifications in the sphenoidal region of the
side of the cranium. The anterior boundary of the posterior fontanelle
is formed by the large =alisphenoid=, which is continuous behind with
the pro-otic and sphenotic, and below with a slender =basisphenoid=.
Both in front of and behind the basisphenoid there are considerable
vacuities in the walls of the cranium; through the posterior of these
openings (fig. 10, V.) the main part of the trigeminal nerve passes
out, and through the anterior one, the optic (fig. 10, II.). The
alisphenoid is continuous in front with the =orbitosphenoid= (fig. 10,
10), which is pierced by the foramen for the exit of the first nerve
(fig. 10, I.), and in front of the orbitosphenoid there is a large
vacuity. The =lateral ethmoid= is seen in the side view as well as in
the dorsal view. Further forwards are seen the olfactory pits, and the
long cartilaginous snout.

A =ventral view= of the cartilaginous cranium shows much the same
points as the side view. The basisphenoid appears on the surface
immediately in front of the basi-occipital.

THE SKULL WITH MEMBRANE BONES.

The =dorsal surface=. The greater part of the dorsal surface in front
of the supra-occipital is overlaid by a pair of large rough _frontals_
(figs. 9, A, 5, and 10, 5). They cover the posterior fontanelles and
stretch over from the sphenotic to the lateral ethmoid, forming a roof
for the orbit. They meet in the middle line behind, but in front are
separated by a narrow tract of unossified cartilage, and are
overlapped by the _median ethmoid_ (figs. 9, A, 6, and 11, 6). At the
sides of the supra-occipital behind the frontals are a pair of small
_parietals_ (figs. 9, A, 7, and 11, 7).

[Illustration FIG. 11. LATERAL VIEW OF THE SKULL OF A SALMON (_Salmo
salar_) (after PARKER). Cartilage is dotted.

    1. supra-occipital.
    2. epi-otic.
    3. pterotic.
    4. sphenotic.
    5. frontal.
    6. median ethmoid.
    7. parietal.
    8. nasal.
    9. lachrymal.
    10. suborbital.
    11. supra-orbital.
    12. cartilaginous sclerotic.
    13. ossification in sclerotic.
    14. meso-pterygoid.
    15. meta-pterygoid.
    16. palatine.
    17. jugal.
    18. quadrate.
    19. maxillae
    20. premaxillae.
    21. articular.
    22. angular.
    23. dentary.
    24. hyomandibular.
    25. symplectic.
    26. epi-hyal.
    27. cerato-hyal.
    28. hypo-hyal.
    29. glosso-hyal.
    30. opercular.
    31. sub-opercular.
    32. infra-opercular.
    33. pre-opercular.
    34. supratemporal.
    35. branchiostegal rays.
    36. basi-branchiostegal.]

In a =ventral view= the cranium is seen to be chiefly covered by two
large membrane bones, the _parasphenoid_ (fig. 9, B, 9) behind, the
_vomer_ in front. A view of the =posterior end= differs from that of
the cartilaginous cranium only in the fact that the end of the
_parasphenoid_ appears lying ventral to the basi-occipital.

The =lateral view= differs very markedly from that of the
cartilaginous cranium, there being a great development of membrane
bone in connection with the jaws and branchial apparatus. Lying
dorsally are seen the _median ethmoid_, _frontal_, _parietal_, and
=supra-occipital= as before. Lying external to the middle of the
_median ethmoid_ is seen the small _nasal_ (fig. 11, 8), and below the
hinder part is the _lachrymal_. The _lachrymal_ (fig. 11, 9) forms the
first of a series of seven small bones which surround the orbit
forming the =orbital ring=. Of these the one lying immediately in the
mid-ventral line of the orbit is the _suborbital_, while the one lying
in the mid-dorsal line and attached to the frontal is the
_supra-orbital_ (fig. 11, 11). The orbit has a cartilaginous
_sclerotic_ in which are two small ossifications (fig. 11, 13)
laterally placed.

BONES OF THE UPPER JAW.

The =palato-pterygo-quadrate bar= is in a very different condition
from that of the dogfish, it is partially cartilaginous, partially
converted into cartilage bone, partially overlapped by membrane bone.
It is narrow in front but becomes much broader and deeper when
followed back. Its anterior end forms the =palatine= which bears
teeth, and in front is completely ossified, while behind the cartilage
is only sheathed by bone.

Just behind the palatine the outer part of the cartilage is ossified,
forming two small bones, the =pterygoid= and =meso-pterygoid=, while
behind them is a larger, somewhat square bone, the =meta-pterygoid=
(fig. 11, 15).

Below the meta-pterygoid is a tract of unossified cartilage, and then
comes the =quadrate= (fig. 11, 18).

The lower angle of the quadrate bears a cartilaginous =condyle= with
which the mandible articulates. In front of the palatine the
cartilaginous snout is overlapped by three membrane bones, the
_jugal_, _maxilla_ and _premaxillae_.

The _premaxillae_ (fig. 11, 20), the largest of these, overlaps the
maxilla behind; both bones bear teeth. The _jugal_ (fig. 11, 17) lies
above the maxilla and overlaps it in front.

THE LOWER JAW.

The =lower jaw= is a strong bar and is like the upper jaw, partly
cartilaginous, forming =Meckel's cartilage=, partly ossified, and
sheathed to a considerable extent in membrane bone.

The outer side and posterior end is ossified, forming the large
=articular= (fig. 11, 21), but the condyle is cartilaginous and the
anterior part of the articular forms merely a splint on the outer side
of Meckel's cartilage, which extends beyond it for a considerable
distance. The angle of the jaw just below the condyle is formed by a
small _angular_ (fig. 11, 22), and the anterior two-thirds of the jaw
is sheathed in the large tooth-bearing _dentary_ (fig. 11, 23).

THE HYOID ARCH.

The =hyoid arch= has a number of ossifications in it and is closely
connected with the mandibular arch.

The =hyomandibular= (fig. 11, 24) is a large bone which articulates
with a shallow groove lined by cartilage and formed partly in the
pterotic, partly in front of it. The hyomandibular is overlapped in
front by the meta-pterygoid, while below it tapers and is succeeded by
a small area of unossified cartilage followed by the forwardly
directed =symplectic= which fits into a groove in the quadrate.

The unossified tract between the hyomandibular and symplectic is
continuous in front with a strong bar, which remains partly
cartilaginous and is partly converted into cartilage bone. The
proximal part is ossified, forming the =epi-hyal=, the middle part
forms the =cerato-hyal= (fig. 11, 27), in front of which is the small
=hypo-hyal=. The hyoid arches of the two sides are united by the large
tooth-bearing =glosso-hyal= (fig. 11, 29). Attached to the lower
surface of the hyoid arch are a series of twelve flat _branchiostegal
rays_ (fig. 11, 35). Each overlaps the one in front of it, the
posterior one being the largest. The branchiostegal rays of the two
sides are united in front by an unpaired membrane bone, the
_basi-branchiostegal_ (fig. 11, 36).

=Opercular bones.= Behind the hyomandibular there is a large bony
plate, the =operculum=, formed of four large membrane bones. The
anterior of these, the _pre-opercular_ (fig. 11, 33), is crescentic in
shape, and with its upper end a small _supratemporal_ (fig. 11, 34) is
connected.

Behind the upper part of the pre-opercular is the largest of the
opercular bones, the _opercular proper_. Its lower edge overlaps the
sub-opercular, and both opercular and sub-opercular are overlapped by
the _infra-opercular_ (fig. 11, 32) in front. The infra-opercular is
in its turn overlapped by the _pre-opercular_.

BRANCHIAL ARCHES.

There are five branchial arches, the first four of which bear gill
rays. Each of the first three consists of a shorter upper portion
directed obliquely backwards and outwards, and a longer lower portion
forming a right angle with the upper and directed obliquely forwards
and inwards. The greater part of each arch is ossified.

The upper part of either of the first two consists of a short tapering
=pharyngo-branchial= directed inwards, and of a long =epi-branchial=
tipped with cartilage at both ends. The junction of the upper and
lower parts is formed by a cartilaginous hinge-joint between the
epi-branchial and cerato-branchial. The =cerato-branchial= is a long
bony rod separated by a short area of cartilage from the
=hypo-branchial=, which is succeeded by the =basibranchial= meeting
its fellow in the middle line. The =fourth arch= has a short
epi-branchial and no ossified pharyngo-branchial, while the fifth is
reduced to little more than the cerato-branchial, which bears a few
teeth on its inner edge. All the branchial arches have projecting from
their surfaces a number of little processes which act as strainers.
The first and fourth arches have one series of these, the second and
third have two.


THE SKULL OF THE CODFISH[37].

A full description having been already given of the Salmon's skull,
that of the Codfish will be described in a briefer manner. The skull
is very fully ossified, and the great number of plate-like bones
render it a very complicated structure.

THE CRANIUM.

At the posterior end of the dorsal surface is the large
=supra-occipital=, which is drawn out behind into the large blade-like
=occipital spine=. On each side of the supra-occipital are the small
irregular _parietals_, while in front of it the roof of the skull is
mainly formed by the very large unpaired _frontal_.

A complicated series of bones are developed in connection with the
=auditory capsule=, which forms a large projecting mass united with
the side of the cranium and drawn out behind into a pair of strong
processes, the =epi-otic= and =parotic= processes. Both these
processes are connected behind with a large V-shaped bone, the
_post-temporal_ (fig. 13, 1), which will be described when dealing
with the pectoral girdle. The =epi-otic process= is formed by the
=epi-otic=, which is continuous in front with the parietal. The
=parotic process= is formed by two larger bones, a more dorsal one,
the =pterotic=, and a more ventral and internal one, the =opisthotic=,
which is continuous in front with the large =pro-otic=. Intervening
between the pterotic and frontal is another rather large bone, the
=sphenotic=, this articulates below with the pro-otic. The pterotic
and sphenotic together give rise to a large concave surface by which
the hyomandibular articulates with the cranium. Several of the cranial
nerves pass out through the bones of the auditory capsule. The ninth
leaves by a foramen near the posterior border of the opisthotic, the
fifth and seventh by a notch in the anterior border of the pro-otic.

A number of bones are likewise developed in connection with the orbit
forming the =orbital ring=. Of these the most anterior, the
_lachrymal_, is much the largest, the others are five to seven in
number, the most ventral being the _suborbital_. The sclerotic coat of
the eye is cartilaginous.

Two pairs of bones and one unpaired bone are developed in connection
with the =olfactory capsules=, of these, the _nasals_ are narrow bones
lying next the lachrymals, but nearer the middle line; they overlap
the second pair of bones, the irregular =lateral ethmoids=. These meet
one another in the middle line, and are overlapped behind by the
frontal. They articulate laterally with the lachrymal and palatine,
and ventrally with the parasphenoid.

In a =posterior view= the foramen magnum and the four bones which
surround it and together form the occipital segment are well seen. On
the ventral side is the =basi-occipital=, terminated posteriorly by a
slightly concave surface which articulates with the centrum of the
first vertebra. The sides of the foramen magnum are formed by the
=exoccipitals=, a pair of very irregular bones, pierced by a pair of
prominent foramina for the exit of the tenth nerves. The exoccipitals
also bear a pair of surfaces for articulation with corresponding ones
on the neural arch of the first vertebra. The most dorsal of the four
bones is the supra-occipital.

On the ventral surface of the cranium in front of the basi-occipital
is seen the _parasphenoid_, a very long narrow bone which underlies
the greater part of the cranium. Behind, it articulates dorsally with
the basi-occipital and dorsolaterally with the pro-otics and
opisthotics, in front it articulates dorsally with the lateral ethmoid
and ventrally with the vomer. At the sides of the parasphenoid are the
small =alisphenoids= articulating above with the postfrontals, in
front with the frontals, and behind with the pro-otics.

The _vomer_ is an unpaired bone lying immediately in front of the
parasphenoid. In front it terminates with a thickened curved margin
bearing several rows of small teeth; behind it tapers out into a long
process which underlies the anterior part of the parasphenoid.
Immediately dorsal to the vomer is another median bone, the _median
ethmoid_; this is truncated in front and tapers out behind into a
process which fits into a groove on the ventral side of the frontal.

BONES IN CONNECTION WITH THE UPPER JAW.

These bear a close resemblance to those of the Salmon. The most
anterior bone is the _premaxillae_, a thick curved bone meeting its
fellow in the middle line. The point of junction of the two is drawn
out into a short process, and the oral surface is thickly covered with
small teeth. The dorsal ends of the premaxillae are seen in the fresh
skull to meet a large patch of cartilage. Behind the premaxillae is the
_maxilla_, a long rod-like toothless bone, somewhat expanded at the
upper end where it articulates with the premaxillae and vomer.

Articulating in front with the anterior end of the maxilla and with
the =lateral ethmoid= is a very irregular bone, the =palatine= (fig.
12, 1); it articulates behind with two flat bones, the =pterygoid= and
=meso-pterygoid=. The pterygoid is united behind with two more bones,
the =quadrate= (fig. 12, 4) and =meta-pterygoid=. The =quadrate= is a
rather stout irregular bone, bearing on its lower surface a prominent
saddle-shaped articulating surface for the mandible. The palatine,
pterygoid and quadrate bones are the ossified representatives of the
palato-pterygo-quadrate bar of the Dogfish.

[Illustration FIG. 12. MANDIBULAR AND HYOID ARCHES OF A COD (_Gadus
morrhua_) × 1/2 (Brit. Mus.).

    1. palatine.
    2. meso-pterygoid.
    3. pterygoid.
    4. quadrate.
    5. symplectic.
    6. meta-pterygoid.
    7. hyomandibular.
    8. angular.
    9. articular.
    10. dentary.
    11. inter-hyal.
    12. epi-hyal.
    13. cerato-hyal.
    14. hypo-hyal.
    15. uro-hyal.
    16. branchiostegal rays.]

The quadrate is united behind with the =symplectic= (fig. 12, 5), and
the meta-pterygoid with the symplectic and =hyomandibular=, both of
which bones will be described immediately in connection with the hyoid
arch.


THE LOWER JAW.

The =lower jaw or mandible= like that of the Salmon is partly
cartilaginous, forming =Meckel's cartilage=, partly formed of
cartilage bone, partly of membrane bone. Meckel's cartilage is of
course not seen in the dried skull.

The lower jaw includes one cartilage bone, the =articular= (fig. 12,
9), this is a large bone connected by a saddle-shaped surface with the
quadrate. Meckel's cartilage lies in a groove on its under surface,
and projects beyond it in front. The _angular_ is a small thick bone
united to the lower surface of the articular at its posterior end. The
_dentary_ (fig. 12, 10) is a large tooth-bearing bone meeting its
fellow in the middle line in front, while the articular fits into a
deep notch at its posterior end.


THE HYOID ARCH.

The =hyomandibular= (fig. 12, 7) is a large irregular bone,
articulating by a prominent rounded head with the sphenotic and
pterotic. It is united in front with the meta-pterygoid and
symplectic, and sends off behind a strong process which articulates
with the opercular. The =symplectic= is a long somewhat triangular
bone drawn out in front into a process which fits into a groove on the
inner surface of the quadrate. The distal portion of the hyoid arch is
strongly developed and consists of first the =inter-hyal= (fig. 12,
11), a short bony rod, which articulates dorsally with a patch of
cartilage intervening between the posterior part of the hyomandibular
and the symplectic. Below it is united with the apex of the triangular
=epi-hyal=, a bone suturally connected with the large =cerato-hyal=
(fig. 12, 13) which unites distally with two small =hypo-hyals=. To
the cerato-hyal are attached a series of seven strong curved
cylindrical rods, the _branchiostegal rays_. The first of these is the
smallest and they increase in size up to the last. The four dorsal
ones are attached to the outer surface of the cerato-hyal, the three
ventral ones to its inner surface. Interposed between the hypo-hyals
of the two sides is an unpaired somewhat triangular plate, the
uro-hyal or _basi-branchiostegal_ (fig. 12, 15).


THE BRANCHIAL ARCHES.

The =branchial arches= are five in number and consist of the following
parts on each side. The dorsal end is formed of the =supra-pharyngeal=
bone, a large irregular bone covered ventrally with teeth of a fair
size, and representing the fused =pharyngo-branchials= of the four
anterior arches. Its external surface is continuous with four small
=epi-branchials= which pass horizontally backwards and outwards. Their
distal ends meet four long =cerato-branchials= which are directed
forwards and inwards and form the principal part of the arches.

Each of the first three cerato-branchials articulates ventrally with a
=hypo-branchial=, and the hypo-branchials of the two sides are united
in the middle line by an unpaired =basibranchial=. The third
hypo-branchial is much flattened. The fourth cerato-branchial is
united by cartilage with the posterior surface of the third
hypo-branchial, which it meets near the middle line.

The fifth arch consists only of the cerato-branchial, a wide structure
covered with teeth and generally called the =inferior pharyngeal
bone=.

The skeleton of the =operculum= consists of the same four bones as in
the Salmon, namely the _opercular_, the _infra-opercular_, the
_pre-opercular_ and the _sub-opercular_. Of these the anterior bone,
the _pre-opercular_, is the largest, while the _infra-opercular_ is
the smallest. The _opercular_ has a facet for articulation with the
hyomandibular.


2. THE APPENDICULAR SKELETON.

THE PECTORAL GIRDLE.

This is of a highly specialised type. Membrane bones are greatly
developed, and the cartilage bones, the =scapula= and =coracoid=, are
much reduced in size and importance.

[Illustration FIG. 13. THE RIGHT HALF OF THE PECTORAL GIRDLE AND RIGHT
PECTORAL FIN OF A COD (_Gadus morrhua_) × 1/2 (Brit. Mus.).

    1. post-temporal.
    2. supra-clavicle.
    3. clavicle.
    4. coracoid.
    5. scapula.
    6. post-clavicle.
    7. brachial ossicles.
    8. dermal fin-rays.]

The largest bone in the shoulder girdle is the _clavicle_ (fig. 13,
3), which is irregularly crescent shaped, thick in front and tapering
off behind. To the outer side of its upper part is attached a thick
cylindrical bone, the _supra-clavicle_, which passes upwards and is
connected with a strong =V= shaped bone, the _post-temporal_. The apex
of the =V= meets the supra-clavicle, the inner limb articulates with
the epi-otic process, the outer with the parotic process. Projecting
downwards from the upper part of the clavicle is a long bony rod,
flattened proximally and cylindrical and pointed distally, this is
the _post-clavicle_ (fig. 13, 6).

The =scapula= (fig. 13, 5) is a small irregular plate of bone attached
to the inner side of the middle of the _clavicle_. The =coracoid=[38]
is a larger plate of similar character, irregularly triangular in
shape, attached to the inner side of the clavicle immediately below
the scapula. The scapula and coracoid bear the pectoral fin.

THE PECTORAL FINS.

Each of these consists of four small irregular bones, the =brachial
ossicles= (fig. 13, 7), bearing a series of about nineteen dermal
_fin-rays_. The brachial ossicles represent the reduced and modified
radiale and basalia of cartilaginous fish such as the dogfish. The
fin-rays (fig. 13, 8) which form the whole external portion of the fin
are long slender rods having essentially the same character as those
of the unpaired fins.

THE PELVIC GIRDLE.

The =pelvic girdle= in the Cod as in other Teleosteans is entirely
absent, its place being taken by the enlarged basi-pterygia of the
fins.

THE PELVIC FINS.

These have a very anomalous position in the Cod, being attached to the
throat in front of the pectoral girdle. Each consists of a basal
portion, the =basi-pterygium=, and of a number of dermal rays. The
basi-pterygium consists of an expanded ventral portion which meets its
fellow below in the middle line, and to which the rays are attached,
and of an inwardly-directed dorsal portion which also meets its fellow
and is imbedded in the flesh. The rays are six in number and are long
slender structures similar to those of the other fins.


FOOTNOTES:

[35] See T.J. Parker's _Zootomy_, London, 1884, p. 86.

[36] See W.K. Parker and G.T. Bettany, _The Morphology of the Skull_,
London, 1877, chap. 3.

[37] T.J. Parker, _Zootomy_, London, 1884, p. 91.

[38] According to G. Swirski, _Schultergurtel des Hechtes_, Dorpat,
1880, the true coracoid is aborted, and the so-called coracoid of
Teleosteans is really the precoracoid.




CHAPTER VIII.

GENERAL ACCOUNT OF THE SKELETON IN FISHES[39].


EXOSKELETON.

The most primitive type of exoskeleton is that found in Elasmobranchs
and formed of =placoid= scales; these are tooth-like structures
consisting of dentine and bone capped with enamel, and have been
already described (p. 4). In most Elasmobranchs they are small and
their distribution is fairly uniform, but in the Thornback skate,
_Raia clavata_, they have the form of larger, more scattered spines.
In adult Holocephali and in _Polyodon_ and _Torpedo_ there is no
exoskeleton, in young Holocephali, however, there are a few small
dorsal ossifications.

The plates or scales of many Ganoids may have been formed by the
gradual fusion of elements similar to these placoid scales, and often
bear a number of little tooth-like processes. In _Lepidosteus_,
_Polypterus_, and many extinct species, these _ganoid_ scales, which
are rhomboidal in form and united to one another by a peg and socket
articulation, enclose the body in a complete armour. In _Trissolepis_
part of the tail is covered by rhomboidal scales, while rounded scales
cover the trunk and remainder of the tail. _Acipenser_ and
_Scaphirhynchus_ have large dermal bony plates which are not
rhomboidal in shape and do not cover the whole body. In _Acipenser_ a
single row extends along the middle of the back and two along each
side.

The majority of Teleosteans have thin flattened scales which differ
from those of Ganoids in being entirely mesodermal in origin,
containing no enamel. There are two principal types of Teleostean
scales, the cycloid and ctenoid. A =cycloid= scale is a flat thin
scale with concentric markings and an entire posterior margin. A
=ctenoid= scale differs in having its posterior margin pectinate. The
Dipnoi have overlapping cycloid scales. The rounded scales of _Amia_
and of many fossil ganoids such as _Holoptychius_ are shaped like
cycloid scales, but differ from them in being more or less coated with
enamel. In Eels and some other Teleosteans the scales are completely
degenerate and have almost disappeared. Some Teleosteans, like _Diodon
hystrix_, have scales with triradiate roots from which arise long
sharp spines directed backwards. These scales, which resemble teeth,
contain no enamel; they become erect when the fish inflates its body
into a globular form. Many Siluroids have dermal armour in the form of
large bony plates which are confined to the anterior part of the body.
In _Ostracion_ the whole body is covered by hexagonal plates, closely
united together.

The =fin-rays= are structures of dermal origin which entirely or
partially support the unpaired fins, and assist the bony or
cartilaginous endoskeleton in the support of the paired fins.

In Elasmobranchs, Dipnoi, and Chondrosteous ganoids the skeletons of
the fins are, as a rule, about half of exoskeletal, half of
endoskeletal origin, the proximal and inner portion being
cartilaginous and endoskeletal, the distal and outer portion being
exoskeletal, and consisting of horny or of more or less calcified
fin-rays. In bony Ganoids and Teleosteans the endoskeletal parts are
greatly reduced and the fins come to consist mainly of the fin-rays,
which are ossified and frequently become flattened at their distal
ends.

The fin-rays of the ventral part of the caudal fin are carried by the
haemal arches; those of the dorsal and anal fins and of the dorsal
part of the caudal fin generally by interspinous bones, which in adult
Teleosteans alternate with the neural and haemal spines. In Dipnoi
these interspinous bones articulate with the neural and haemal spines.
In many Siluroids the anterior rays of the dorsal and pectoral fins
are developed into large spines which often articulate with the
endoskeleton, or are sometimes fused with the dermal armour plates.
Similar spines may occur in Ganoids in front of both the dorsal and
anal fins. _Polypterus_ has a small spine or _fulcrum_ in front of
each segment of the dorsal fin. Such spines are often found
fossilised, and are known as _ichthyodorulites_.

Similar spines are found in many Elasmobranchs, but they are simply
inserted in the flesh, not articulated to the endoskeleton. They also
differ from the spines of Teleosteans and Ganoids in the fact that
they are covered with enamel, and often have their edges serrated like
teeth. In the extinct Acanthodii they generally occur in front of all
the fins, paired and unpaired.

In _Trygon_, the Sting-ray, the tail bears a serrated spine which is
used for purposes of offence and defence. Many ichthyodorulites may
have been spines of this nature fixed to the tail, rather than spines
situated in front of the fins. The spines, which are always found in
front of the dorsal fin in Holocephali, agree with those of
Elasmobranchs in containing enamel, and with those of Teleosteans in
being articulated to the endoskeleton.

TEETH.

The teeth of fish[40] are subject to a very large amount of variation,
perhaps to more variation than are those of any other class of
animals. Sometimes, as in adult Sturgeons, they are entirely absent,
sometimes they are found on all the bones of the mouth, and also on
the hyoid and branchial arches. The teeth are all originally developed
in the mucous membrane of the mouth, but they afterwards generally
become attached to firmer structures, especially to the jaws. In
Elasmobranchs, however, they are generally simply imbedded in the
tough fibrous integument of the mouth. Their attachment to the jaws
may take place in three different ways.

[Illustration FIG. 14. DIAGRAM OF A SECTION THROUGH THE JAW OF A SHARK
(_Odontaspis americanus_) showing the succession of teeth (Brit. Mus.
from specimen and diagram).

    1. teeth in use.
    2. teeth in reserve.
    3. skin.
    4. cartilage of the jaw.
    5. encrusting calcification of cartilage.
    6. connective tissue.
    7. mucous membrane of the mouth.]

(1) By an elastic hinge-joint, as in the Angler (_Lophius_), and the
Pike (_Esox lucius_). In the Angler the tooth is held by a fibrous
band attaching its posterior end to the subjacent bone, in the Pike by
uncalcified elastic rods in the pulp cavity.

(2) By ankylosis, i.e. by the complete union of the calcified tooth
substance with the subjacent bone. This is the commonest method among
fish.

(3) By implantation in sockets. This method is not very common among
fish. The teeth are sometimes, as in _Lepidosteus_, ankylosed to the
base of the socket. In this genus there is along each ramus of the
mandible a median row of large teeth placed in perfect sockets, and
two irregular lateral rows of small teeth ankylosed to the jaw.

Dentine, enamel and cement are all represented in the teeth of fishes,
but the enamel is generally very thin, and cement is but rarely
developed. Dentine forms the main bulk of the teeth; it is sometimes
of the normal type, but generally differs from that in higher
vertebrates in being vascular, and is known as _vasodentine_. A third
type occurs, known as _osteodentine_; it is traversed by canals
occupied by marrow, and is closely allied to bone.

[Illustration FIG. 15. PART OF THE LOWER JAW OF A SHARK (_Galeus_)
(from OWEN after ANDRÉ).

1. teeth in use.

2. reserve teeth folded back.

3. part of the caudal spine of a Sting-ray (_Trygon_) which has
pierced the jaw and affected the growth of the teeth.]

The teeth are generally continually renewed throughout life, but
sometimes one set persists.

The teeth of Selachii are fundamentally identical with placoid
scales. They are developed from a layer of dental germs which occurs
all over the surface of the skin, except in the region of the lips. At
this point the layer of tooth-producing germs extends back into the
mouth, being projected by a fold of the mucous membrane (fig. 14, 7).
Here new teeth are successively formed, and as they grow each is
gradually brought into a position to take the place of its predecessor
by the shifting outwards of the gum over the jaw. Owing to this
arrangement sharks have practically an unlimited supply of teeth
(figs. 14 and 15).

Two principal types of teeth are found in ELASMOBRANCHS. In Sharks and
Dogfish, on the one hand, the teeth are very numerous, simple, and
sharp-pointed, and are with or without serrations and lateral cusps.
Many Rays and fossil Elasmobranchs, on the other hand, have broad
flattened teeth adapted for crushing shells. Intermediate conditions
occur between these two extremes. Thus in _Cestracion_ and many
extinct sharks, such as _Acrodus_, while the median teeth are sharp,
the lateral teeth are more or less flattened and adapted for crushing.
In various species belonging to the genus _Raia_ the teeth of the male
are sharp, while those of the female are blunt. A very specialised
dentition is met with in the Eagle-rays (Myliobatidae), in which the
jaws are armed with flattened angular tooth-plates, arranged in seven
rows, forming a compact pavement; the plates of the middle row are
very wide and rectangular, those of the other rows are much smaller
and hexagonal. Lastly, in _Cochliodus_ the individual crushing teeth
are fused, forming two pairs of spirally-coiled dental plates on each
side of each jaw. _Pristis_, the Saw-fish, has a long flat
cartilaginous snout, bearing a double row of persistently-growing
teeth planted in sockets along its sides. Each tooth consists of a
number of parallel dentinal columns, united at the base, but elsewhere
distinct.

In the HOLOCEPHALI--_Chimaera_, _Hariotta_ and _Callorhynchus_--only
three pairs of teeth or dental plates occur, two pairs in the upper
jaw, one in the lower. These structures persist throughout life and
grow continuously. The upper tooth structures are attached
respectively to the ethmoid or vomerine region of the skull, and to
the palato-pterygoids. The vomerine teeth are small, while those
attached to the mandible and the palato-pterygoid region are large and
bear several roughened ridges adapted for grinding food. The teeth of
the two opposite sides of the jaw meet in a median symphysis. The
teeth of _Chimaera_ are more adapted for cutting, those of
_Callorhynchus_ for crushing. Many extinct forms are known, some of
whose teeth are intermediate in structure between those of _Chimaera_
and _Callorhynchus_.

The teeth of GANOIDS are also extremely variable. Among living forms,
the Holostei are more richly provided with teeth than are any other
fishes, as they may occur on the premaxillae, maxillae, palatines,
pterygoids, parasphenoid, vomers, dentaries, and splenials. Among the
Chondrostei, on the other hand, the adult Acipenseridae are toothless;
small teeth however occur in the larval sturgeon, and in _Polyodon_
many small teeth are found attached merely to the mucous membrane of
the jaws. Many fossil Ganoids have numerous flattened or knob-like
teeth, borne on the maxillae, palatines, vomers and dentaries. Others
have a distinctly heterodont dentition. Thus in _Lepidotus_ the
premaxillae bear chisel-like teeth, while knob-like teeth occur on the
maxillae, palatines and vomers. In _Rhizodus_ all the teeth are
pointed, but while the majority are small a few very large ones are
interspersed.

In TELEOSTEANS, too, the teeth are eminently variable both in form and
mode of arrangement. They may be simple and isolated, or compound, and
may be borne on almost any of the bones bounding the mouth cavity, and
also as in the Pike, on the hyoid and branchial arches. The splenial
however never bears teeth and the pterygoid and parasphenoid only
rarely, thus differing from the arrangement in the Holostei.

The isolated teeth are generally conical in form and are ankylosed to
the bone that bears them. Such teeth are, with a few exceptions such
as _Balistes_, not imbedded in sockets nor replaced vertically.

In some fish beak-like structures occur, formed partly of teeth,
partly of the underlying jaw bones. These beaks are of two kinds: (1)
In _Scarus_, the parrot fish, the premaxillae and dentaries bear
numerous small, separately developed teeth, which are closely packed
together and attached by their proximal ends to the bone, while their
distal ends form a mosaic. Not only the teeth but the jaws which bear
them are gradually worn away at the margins, while both grow
continuously along their attached edge. (2) In Gymnodonts, e.g.
_Diodon_, the beaks are formed by the coalescence of broad calcified
horizontal plates, which when young are free and separated from one
another by a considerable interval.

In some Teleosteans the differentiation of the teeth into biting teeth
and crushing teeth is as complete as in _Lepidosteus_. Thus in the
Wrasse (_Labrus_), the jaws bear conical slightly recurved teeth
arranged in one or two rows, with some of the anterior ones much
larger than the rest. The bones of the palate are toothless, while
both upper and lower pharyngeal bones are paved with knob-like
crushing teeth; such pharyngeal teeth occur also in the Carp but are
attached only to the lower pharyngeal bone, the jaw bones proper being
toothless.

In DIPNOI the arrangement of the teeth is very similar to that in
Holocephali. The mandible bears a single pair of grinding teeth
attached to the splenials, and a corresponding pair occur on the
palato-pterygoids. In front of these there are a pair of small conical
vomerine teeth loosely attached to the ethmoid cartilage. The
palato-pterygoid teeth of _Ceratodus_ are roughly semicircular in
shape with a smooth convex inner border, and an outer border bearing a
number of strongly marked ridges. The teeth of the extinct Dipteridae
resemble those of _Ceratodus_ but are more complicated.


ENDOSKELETON.

SPINAL COLUMN[41].

The spinal column of fishes is divisible into only two regions, a
caudal region in which the haemal arches or ribs meet one another
ventrally, and a precaudal region in which they do not meet.

The various modifications of the spinal column in fishes can be best
understood by comparing them with the arrangement in the simplest type
known, namely _Amphioxus_. In _Amphioxus_ the notochord is immediately
surrounded by a structureless cuticular layer, the _chordal sheath_.
Outside this is the _skeletogenous layer_, which in addition to
surrounding the notochord and chordal sheath embraces the nerve cord
dorsally, and laterally sends out septa forming the _myomeres_.

The CARTILAGINOUS GANOIDS[42] _Acipenser_, _Polyodon_ and
_Scaphirhynchus_ are the simplest fishes as regards their spinal
column. The notochord remains permanently unconstricted and is
enclosed in a chordal sheath, external to which is the skeletogenous
layer. In this layer the development of cartilaginous elements has
taken place. In connection with each _neuromere_, or segment as
determined by the points of exit of the spinal nerves, there are
developed two pairs of ventral cartilages, the ventral arches
(basiventralia) and intercalary pieces (interventralia); and at least
two pairs of dorsal pieces, the neural arches (basidorsalia) and
intercalary pieces (interdorsalia). The lateral parts of the
skeletogenous layer do not become converted into cartilage, so there
are no traces of vertebral centra. The ventral or haemal arches meet
one another ventrally and send out processes to protect the ventral
vessels. The neural arches do not meet, but are united by a
longitudinal elastic band.

In Cartilaginous ganoids the only indications of metameric
segmentation are found in the neural and haemal arches. The case is
somewhat similar with the Holocephali and Dipnoi.

In the HOLOCEPHALI the notochord grows persistently throughout life,
and is of uniform diameter throughout the whole body except in the
cervical region and in the gradually tapering tail. The chordal sheath
is very thick and includes a well-marked zone of calcification which
separates an outer zone of hyaline cartilage from an inner zone. There
are also a number of cartilaginous pieces derived from the
skeletogenous layer which are arranged in two series, a dorsal series
forming the neural arches and a ventral series forming the haemal
arches. These do not, except in the cervical region, meet one another
laterally round the notochord and form centra. To each neuromere there
occur a pair of basidorsals, a pair of interdorsals, and one or two
supradorsals. In the tail the arrangement is irregular.

In the DIPNOI as in the Holocephali the notochord grows persistently
and uniformly, and the chordal sheath is thick and cartilaginous
though there are no metamerically arranged centra. The neural and
haemal arches and spines are cartilaginous and interbasalia
(intercalary pieces) are present. The basidorsalia and basiventralia
do not in _Ceratodus_ meet round the notochord and enclose it except
in the anterior part of the cervical and posterior part of the caudal
region.

In ELASMOBRANCHII the chordal sheath is weak and the skeletogenous
layer strong. Biconcave cartilaginous vertebrae are developed, and as
is the case in most fishes, constrict the notochord _vertebrally_.

Two distinct types of vertebral column can be distinguished in
Elasmobranchs[43]:

1. In many extinct forms and in the living Notidanidae, _Cestracion_,
and _Squatina_, the dorsal and ventral arches do not meet one another
laterally round the centrum, and consequently readily come away from
it.

2. In most living Elasmobranchs the arches meet laterally round the
centrum.

The vertebrae are never ossified but endochondral calcification nearly
always takes place, though it very rarely reaches the outer surface of
the vertebrae. Elasmobranchs are sometimes subdivided into three
groups according to the method in which this calcification takes
place:

1. =Cyclospondyli= (_Scymnus_, _Acanthias_), in which the calcified
matter is deposited as one ring in each vertebra.

2. =Tectospondyli= (_Squatina_, _Raia_, _Trygon_), in which there are
several concentric rings of calcification.

3. =Asterospondyli= (Notidanidae, _Scyllium_, _Cestracion_), in which
the calcified material instead of forming one simple ring, extends out
in a more or less star-shaped manner.

In _Heptanchus_ the length of the vertebral centra in the middle of
the trunk is double that in the anterior and posterior portions, and
as the length of the arches does not vary, the long centra carry more
of them than do the short centra.

In many Rays the skull articulates with the vertebral column by
distinct occipital condyles.

In BONY GANOIDS the skeletogenous layer becomes calcified
ectochondrally in such a way that the notochord is pinched in at
intervals, and distinct vertebrae are produced. Ossification of the
calcified cartilage rapidly follows. In _Amia_ the vertebrae are
biconcave, in _Lepidosteus_ they are opisthocoelous, cup and ball
joints being developed between the vertebrae in a manner unique among
fishes. The notochord entirely disappears in the adult _Lepidosteus_,
but at one stage in larval life it is expanded vertebrally and
constricted intervertebrally in the manner usual in the higher
vertebrata, but unknown elsewhere among fishes.

The tail of _Amia_ is remarkable from the fact that as a rule to each
neuromere, as determined by the exit of the spinal nerves, there are
two centra, a posterior one which bears nothing, and an anterior one
which bears the neural and haemal arches, these being throughout the
vertebral column connected with the centra by cartilaginous discs.

In most TELEOSTEANS but not in the Plectognathi the neural arches are
continuous with the centra, which are nearly always deeply biconcave.

In some cases many of the anterior vertebrae are ankylosed together
and to the skull. The vertebrae often articulate with one another by
means of obliquely placed flattened surfaces, the zygapophyses. The
centrum in early stages of development is partially cartilaginous, but
the neural arches and spines in the trunk at any rate, pass directly
from the membranous to the osseous condition.


FINS.

The most primitive fins are undoubtedly the unpaired ones, which
probably originally arose as ridges or folds of skin along the
mid-dorsal line of the body, and passed thence round the posterior end
on to the ventral surface, partially corresponding in position and
function to the keel of a ship.

In long 'fish' which pass through the water with an undulating motion
such simple continuous fins may be the only ones found, as in
_Myxine_. To support these median fins skeletal structures came to be
developed; these show two very distinct forms, viz. cartilaginous
endoskeletal pieces, the _radiale_, and horny exoskeletal fibres, the
_fin-rays_. Mechanical reasons caused the fin to become concentrated
at certain points and reduced at intervening regions. Thus a terminal
caudal fin arose and became the chief organ of propulsion, and the
dorsal and ventral fins became specialised to act as balancing organs.

In some of the earlier Elasmobranchs, the Pleuracanthidae, the
endoskeletal cartilaginous radiale are directly continuous with
outgrowths from the dorsal and ventral arches of the vertebrae, and
form the main part of the fin. In later types of Elasmobranchs the
horny exoskeletal fin-rays have comparatively greater prominence. In
bony fish, as has been already stated, the horny fibres are replaced
by bony rays of dermal origin, and at the same time complete reduction
and disappearance of the cartilaginous radiale takes place.

THE CAUDAL FIN.

The caudal region of the spinal column in fishes is of special
importance. It is distinctly marked off from the rest of the spinal
column by the fact that the ventral or haemal arches meet one another
and are commonly prolonged into spines, while in the trunk region they
do not meet but commonly diverge from one another.

In some fish the terminal part of the caudal region of the spinal
column retains the same direction as the rest of the spinal column.
The blade of the caudal fin is then divided into two nearly equal
portions, and is said to be =diphycercal=. This condition is generally
regarded as the most primitive one; it occurs in the Ichthyotomi,
Holocephali, all living Dipnoi, _Polypterus_ and some extinct
Crossopterygii, and a few Selachii and Teleostei. It occurs also in
deep-sea fish belonging to almost every group, and under these
conditions obviously cannot be regarded as primitive, but must be
looked on as a feature induced by the peculiar conditions of life.

In the great majority of fish the terminal part of the caudal region
of the spinal column is bent dorsalwards, and the part of the blade of
the caudal fin which arises on the dorsal surface is much smaller than
is that arising on the ventral surface. Such a fin is said to be
=heterocercal=.

Strictly speaking all fish whose tails are not diphycercal have
heterocercal tails, but the term is commonly applied to two-bladed
tails in which the spinal column forms a definite axis running through
the dorsal blade, while the ventral blade is enlarged and generally
forms the functional part of the tail. Such heterocercal tails are
found in nearly all Elasmobranchii, together with the living
cartilaginous Ganoidei, and many extinct forms belonging to the same
order; _Lepidosteus_, _Amia_, and the Dipteridae among Dipnoi, have
tails which, though obviously heterocercal, are not two-bladed.

The vast majority of the Teleostei and some extinct Ganoidei have
heterocercal tails of the modified type to which the term =homocercal=
is applied. The hypural bones which support the lower half of the tail
fin become much enlarged, and frequently unite to form a wedge-shaped
bone which becomes ankylosed to the last ossified vertebral centrum.
The fin-rays then become arranged in such a way as to produce a
secondary appearance of symmetry. Some homocercal fish such as the
Perch have the end of the notochord protected by a calcified or
completely ossified sheath, the =urostyle=, to which several neural
and haemal arches may be attached, and which becomes united with the
centrum of the last vertebra; in others such as the Salmon the end of
the notochord is protected only by laterally placed bony plates.

THE SKULL.

It is often impossible to draw a hard and fast line between the
cranium and the vertebral column. This is the case for instance in
_Acipenser_ (fig. 18, 16) among Chondrostei, in _Amia_ among Holostei,
and in _Ceratodus_ and _Protopterus_ among Dipnoi. The occipital
region of the skull in _Amia_ is clearly formed of three cervical
vertebrae whose centra have become absorbed into the cranium, while
the neural arches and spines are still distinguishable.

The simplest type of cranium is that found in ELASMOBRANCHS: it
consists of a simple cartilaginous box, which is generally immovably
fixed to the vertebral column, though in some forms, like _Scymnus_
and _Galeus_, a joint is indicated, and in others, such as the
Rays, one is fairly well developed. The cranium in Elasmobranchs
is never bony, though the cartilage is sometimes calcified. It is
drawn out laterally into an antorbital process in front of the eye,
and a postorbital process behind it. The nasal capsules are always
cartilaginous, and the eye, as a general rule, has a cartilaginous
sclerotic investment. The cranium is often prolonged in front into
a rostrum which is enormously developed in _Pristis_ and some Rays.
The cartilaginous roof of the cranium is rendered incomplete by the
presence of a large hole, the anterior fontanelle.

[Illustration FIG. 16. A. SKULL OF _Notidanus_ × 1/2 (Brit. Mus.). B.
SKULL OF _Cestracion_ × 1/3 (after GEGENBAUR). In neither case are the
branchial arches shown.

    1. rostrum.
    2. olfactory capsule.
    3. ethmo-palatine process.
    4. palatine portion of palato-pterygo-quadrate bar.
    5. quadrate portion of bar.
    6. Meckel's cartilage.
    7. teeth.
    8. labial cartilage.
    9. hyomandibular.
    10. postorbital process.
    II. optic foramen.]

Two pairs of labial cartilages (fig. 16, B, 8) are often present. They
lie imbedded in the cheeks outside the anterior region of the jaws,
and are specially large in _Squatina_.

As regards the visceral arches[44] the simplest and most primitive
condition of the jaws is that of the Notidanidae, in which the
mandibular and hyoid arches are entirely separate. In these primitive
fishes the palato-pterygo-quadrate bar articulates with the
postorbital process (fig. 16, 10), while further forwards it is united
to the cranium by the ethmo-palatine ligament. The hyoid arch is small
and is broadly overlapped by the mandibular arch. The term
=autostylic= is used to describe this condition of the suspensorium.
From this condition we pass in the one direction to that of
_Cestracion_ (fig. 16, B), in which the whole of the palato-pterygo
quadrate bar has become bound to the cranium, and in the other to that
of _Scyllium_. In _Scyllium_ (fig. 6), while the ethmo-palatine
ligament is retained, the postorbital articulation of the palato
pterygo-quadrate has been given up, so that the palato-pterygo
quadrate comes to abut on the hyomandibular and is attached to it by
ligaments. The pre-spiracular ligament (fig. 16, 20) running from the
auditory capsule also assists in supporting the jaws.

Lastly we come to the purely =hyostylic= condition met with in Rays,
in which the mandibular arch is entirely supported by the
hyomandibular. In some Rays the hyoid is attached to the posterior
face of the hyomandibular near its proximal end, and may even come to
articulate with the cranium.

The =visceral arches of Elasmobranchs= may be summarised as follows:--

1. The =mandibular arch=, consisting of a much reduced dorsal portion,
the pre-spiracular ligament, and a greatly developed ventral portion
from which both upper and lower jaws are derived. The mandible
(Meckel's cartilage) is the original lower member of the mandibular
arch, and from it arises an outgrowth which forms the upper jaw or
palato-pterygo-quadrate bar. In _Scymnus_ this bears a few
branchiostegal rays.

2. The =hyoid arch=, which consists of the hyomandibular and the
hyoid, and bears branchiostegal rays on its posterior face.

[Illustration FIG. 17. DORSAL VIEW OF THE BRANCHIAL ARCHES OF
_Heptanchus_. (From GEGENBAUR).

    1. basi-hyal.
    2. cerato-hyal.
    3. second hypo-branchial.
    4. first cerato-branchial.
    5. first epi-branchial.
    6. first pharyngo-branchial.
    7. pharyngo-branchial, common to the sixth and seventh arches.
    8. basibranchial of second arch.
    9. basibranchial, common to the sixth and seventh arches.]

3. The =branchial arches=, generally five in number, all of which
except the last bear branchiostegal rays. In the Notidanidae the
number of branchial arches is increased beyond the normal series,
thus in _Hexanchus_ there are six, and in _Heptanchus_ seven. There
are six also in _Chlamydoselache_ and _Protopterus_.

4. The so-called external branchial arches which are cartilaginous
rods attached to all the visceral arches. They are especially large in
_Cestracion_.

The skull in HOLOCEPHALI is entirely cartilaginous. The
palato-pterygo-quadrate bar is fixed to the cranium, and to it the
mandible articulates. There is a well-marked joint between the skull
and the spinal column.

In living Cartilaginous GANOIDS the primitive cartilaginous cranium is
very massive, and is greatly prolonged anteriorly, while posteriorly
it merges into the spinal column. Although it is mainly cartilaginous
a number of ossifications take place in the skull, and membrane bones
are now found definitely developed, especially in connection with the
roof of the cranium. In _Acipenser_ (fig. 18) the ossifications in the
cartilage include the pro-otic, which is pierced by the foramen for
the fifth nerve, the alisphenoid, orbitosphenoid, ectethmoid,
palatine, pterygoid, meso-pterygoid, hyomandibular (fig. 18, 11),
cerato-hyal, all the cerato-branchials, and the first two
epi-branchials. Most of these structures are, however, partly
cartilaginous, though they include an ossified area. The membrane
bones too of _Acipenser_ are very well developed, they include a bone
occupying the position of the supra-occipital, and form a complete
dorsal cephalic shield. Resting on the ventral surface are a vomer and
a very large parasphenoid (fig. 18, 3). There is a bony operculum
attached to the hyomandibular, and membrane bones representing
respectively the maxilla and dentary are attached to the jaws. The
suspensorium is most markedly hyostylic. The palato-pterygo-quadrate
bar has a very curious shape and is quite separate from the cranium.
It is connected to the hyomandibular by a thick symplectic ligament
containing a small bone homologous with the symplectic of
Teleosteans.

_Polyodon_ differs much from _Acipenser_, the membrane bones not being
so well developed though they cover the great cartilaginous snout.

[Illustration FIG. 18. LATERAL VIEW OF THE SKULL OF A STURGEON
(_Acipenser sturio_). Nearly all the membrane bones have been removed
(Brit. Mus.).

    1. nasal cavity.
    2. orbit.
    3. parasphenoid.
    4. vomer.
    5. pterygoid.
    6. maxilla. (The dotted line running from 6 passes into the mouth
        cavity.)
    7. dentary.
    8. symplectic.
    10. palatine.
    11. hyomandibular.
    12. pharyngo-branchial.
    13. epi-branchial.
    14. cerato-branchial.
    15. hypo-branchial.
    16. coalesced anterior vertebrae.
    17. inter-hyal.
    18. cerato-hyal.
    19. rib.]

The skull in _Polypterus_ (Crossopterygii) shows a great advance
towards the condition met with in Teleostei. The cranium remains to a
great extent unossified, and large dorsal and ventral fontanelles
pierce its walls. It is covered by a great development of membrane
bones, paired nasals, frontals, parietals, supra- and post-temporals,
and dermo-supra-occipitals among others being present. The
palato-pterygo-quadrate bar is fused to the cranium, and in connection
with it the following paired membrane bones appear, palatine, ecto-,
meso- and meta-pterygoid, and further forwards jugal, vomer, maxilla
and premaxillae. The membrane bones developed in connection with each
ramus of the mandible are the dentary, angular, and splenial, in
addition to the cartilage bone the articular. Several large opercular
bones occur. There are also a pair of large jugular or gular plates,
and several large opercular bones.

In Bony Ganoids both cartilage bone and membrane bone is well
developed. The pro-otics and exoccipitals are well ossified, but the
supra-occipital and pterotics are not. Lateral ethmoids are developed,
and there are ossifications in the sphenoidal region which vary in
different forms. The place of the cartilaginous palato-pterygo
quadrate is taken by a series of bones, the quadrate behind and the
palatine, ecto-, meso- and meta-pterygoids in front. In _Lepidosteus_,
however, the palatine and pterygoid are membrane bones, as they are in
_Polypterus_ and the Frog. Paired maxillae, premaxillae, vomers and a
parasphenoid occur forming the upper jaw and roof of the mouth, and a
series of membrane bones are found investing the mandible and forming
the operculum.

In _Amia_[45] membrane bones are as freely developed as they are in
Teleosteans; they include on each side a squamosal, four opercular
bones, a lachrymal, a pre-orbital, one or two suborbitals, two large
postorbitals and a supratemporal; while investing the mandible,
besides the dentary, splenial, angular, and supra-angular, there is an
unpaired jugular. The articular too is double and a mento-meckelian
occurs. In _Amia_ teeth are borne on the premaxillae, maxillae, vomers,
palatines and pterygoids.

Bony Ganoids are the lowest animals in which squamosal bones are
found, and they do not occur in Teleosteans.

The suspensorium in bony Ganoids, as in the Chondrostei, is hyostylic,
and there are two ossifications in the hyomandibular cartilage, viz.
the hyomandibular, and the symplectic.

The skull of TELEOSTEI is very similar to those of _Lepidosteus_ and
_Amia_. Although the bony skull is greatly developed and very
complicated, much of the original cartilaginous cranium often
persists. Membrane bones are specially developed on the roof of the
skull where they include the parietal, frontal, and nasal bones. The
same bones are developed in connection with the upper jaw and roof of
the mouth as in bony Ganoids, but only two membrane bones occur in the
lower jaw, viz. the angular and dentary. A number of large
ossifications take place in the cartilage of the auditory capsules. In
some forms parts of the last pair of branchial arches are broadened
out and form the pharyngeal bones which bear teeth. The opercular
bones and those of the upper and lower jaws are quite comparable to
those of bony Ganoids.

A full account of the Teleostean skull has been given in the case of
the Salmon (pp. 87-96) and the Cod (pp. 96-101).

In DIPNOI the skull is mainly cartilaginous, but both cartilage- and
membrane-bone occur also. Cartilage-bone is found in the ossified
exoccipitals, while of membrane-bones _Protopterus_ has among unpaired
bones a fronto-parietal, a median ethmoid, and a parasphenoid, and
among paired bones nasals and large supra-orbitals. The skull of
_Ceratodus_ (fig. 19) has an almost complete roof of membrane bones,
including some whose homology is doubtful. The ethmo-vomerine region
is always cartilaginous, but bears small teeth. The palato-pterygo
quadrate bar is ossified and firmly united to the cranium, and the
mandible articulates directly with it (autostylic). Membrane bones are
freely developed in connection with the mandible, dentary, splenial,
and angular bones being all present. There are two opercular bones.

In the extinct Dipteridae the cranium is very completely covered with
plates of dermal bone, and the skeleton in general is more ossified
than is the case in recent Dipnoi.

Six pairs of branchial arches occur in _Protopterus_; _Ceratodus_ and
_Lepidosiren_ have five, like most other fish. The branchial arches
bear gill rakers.

[Illustration FIG. 19. DORSAL (TO THE LEFT) AND VENTRAL (TO THE RIGHT)
VIEWS OF THE CRANIUM OF _Ceratodus miolepis_ (after GÜNTHER).

    1. cartilaginous part of the quadrate with which the mandible
        articulates.
    2. scleroparietal.
    3. frontal.
    4. ethmoid.
    5. nares.
    6. orbit.
    7. pre-opercular (squamosal).
    8. second rib.
    9. first rib.
    10. vomerine tooth.
    11. palato-pterygoid tooth.
    12. palato-pterygoid.
    13. parasphenoid.
    14. interopercular.]

RIBS.

As has been already mentioned (p. 24), although ribs commonly appear
to be the cut-off ends of the transverse processes, they are really
elements derived from the ventral or haemal arch.

In Elasmobranchii and other cartilaginous fish they have the form of
small cartilaginous structures imperfectly separated from the
diverging halves of the ventral arch, and are often absent.

In Teleostei and bony Ganoids they often have different attachments in
different parts of the body. In the tail region they are not
differentiated from the two halves of the ventral arch, which meet in
the middle line, and are prolonged into a haemal spine. In the
posterior trunk region they sometimes form distinct processes
diverging from the two halves of the ventral arch; while further
forward they may shift their attachment so as to arise from the dorsal
side of the two halves of the ventral arch and at some distance from
their ends, which now diverge as ventri-lateral processes.

APPENDICULAR SKELETON.

PECTORAL GIRDLE.

The simplest type of pectoral girdle is found in Elasmobranchs. It is
entirely cartilaginous and consists of a curved ventrally-placed rod,
ending dorsally in two horn-like scapular processes which are
sometimes attached to the cranium or vertebral column. In Rays the
shoulder girdle is very large, and has a distinct suprascapular
portion forming a broad plate attached to the neural spines of the
vertebrae. There is often a cup-like glenoid cavity for the
articulation of the limb; this cavity is specially large in Rays and
is much pierced by holes. In Dipnoi the cartilaginous girdle still
occurs, but on it there is a deposit of membrane bone forming the
clavicle, infraclavicle, and supra-clavicle. These bones, which with
the exception of the clavicle, are unknown in higher vertebrates, are
better developed in Ganoids, and best of all in Teleosteans. They are
connected by the supratemporal with the epi-otic and opisthotic
regions of the cranium. Owing to this development of dermal bone, the
original cartilaginous arch becomes much reduced, but ossifications
representing the scapula and coracoid occur in bony Ganoids and
Teleosteans.

PELVIC GIRDLE.

In Elasmobranchs the pelvic girdle consists of a short ventral rod of
cartilage representing the ischium and pubis, which does not send up
dorsal iliac processes. In _Chimaera_ the pelvic girdle has a
flattened pointed iliac portion, and ventrally an unpaired movable
cartilaginous plate which bears hooks and is supposed to be copulatory
in function. Claspers of the usual type are present as well. The
Dipnoi have a primitive kind of pelvis in the form of a cartilaginous
plate lying in the mid-ventral line and drawn out into three horns
anteriorly. In Ganoids the pelvis has almost entirely disappeared,
though small cartilaginous vestiges of it remain in _Polypterus_. In
Teleosteans even these vestiges are gone, and in these fish and
Ganoids the place of the pelvis is taken by the enlarged basi-pterygia
(meta-pterygia) of the fins.

PAIRED FINS[46].

As regards the origin of the limbs or paired fins of fishes there are
two principal views. One view, that of Gegenbaur, considers that limbs
and their girdles are derived from visceral arches which have migrated
backwards. The other view, which probably now has the greater number
of supporters, considers that the paired fins of fishes are of
essentially the same nature as the median fins.

According to Gegenbaur's view[47] the =archipterygium= of _Ceratodus_
(fig. 20) represents the lowest type of fin; it consists of a central
cartilaginous axis bearing a large number of radiale. The dorsal or
pre-axial radiale are more numerous than the ventral or postaxial, and
at the margin of the fin[48] the cartilaginous endoskeletal radiale
are replaced by horny exoskeletal fin-rays.

[Illustration FIG. 20. LATERAL VIEW OF THE SKELETON OF _Ceratodus
miolepis_ (after GÜNTHER).

    1. ethmoid.
    2. scleroparietal.
    3. frontal.
    4. cartilaginous posterior part of cranium.
    5. pre-opercular (squamosal).
    6. opercular.
    7. suborbital.
    8. orbit.
    9. pectoral girdle.
    10. proximal cartilage of pectoral fin.
    11. pectoral fin.
    12. pelvic girdle.
    13. pelvic fin.
    14. spinal column.
    15. caudal fin (diphycercal).]

It is impossible here to give a full discussion of the rival views,
but some of the points which support Gegenbaur's view may be
mentioned. The fact that migration of visceral arches has to be
assumed is no difficulty, as it is obvious that migration in the
opposite direction has taken place in many Teleosteans such as the
Cod, whose pelvic fins are attached to the throat in front of the
pectorals. If migration did take place, the pelvic fins being older
than the pectoral should be the more modified, and this is the case.
Again, if the pectoral girdle is a modified branchial arch, it must at
some period have carried a gill, and in _Protopterus_ it does bear a
vestigial gill.

According to the view more prevalent at the present time, the paired
fins have been derived from two continuous folds of skin and their
skeletal supports running forward from the anal region along the sides
of the body, their character being similar to the fold that gave rise
to the median fins. In support of this view it may be argued that the
paired and unpaired fins are often identical in structure, and that
some Elasmobranch embryos do show a ridge running between the pectoral
and pelvic fins. Then from this continuous fold two pairs of smaller
folds may have been specialised off, and in each a number of
cartilaginous radiale may have been developed. The fin of
_Cladoselache_ from the Carboniferous of Ohio apparently illustrates
this condition. It consists of certain basal pieces which do not
project beyond the body wall and bear a number of unsegmented
cartilaginous radiale, which show crowding together and are sometimes
bifurcated distally; they extend throughout the whole fin from the
body wall to the margin. From this fin the archipterygium might be
easily derived by the enlargement of one of the middle radiale and the
segmentation and partial fusion of them all.

Whether the archipterygium be a primitive or secondary type of fin,
when it is once reached it is easy to derive all the other types from
it. The fins of the other living Dipnoi,--_Protopterus_ and
_Lepidosiren_--are simply archipterygia from which the radiale have
almost or completely disappeared, leaving only the segmented axes.
Archipterygia too are found in the pectoral fins of the Ichthyotomi,
but the postaxial radiale are much reduced.

The =ichthyopterygium=, or type of fin, characteristic of many modern
Elasmobranchs such as _Scyllium_, may have been derived from the
archipterygium by the gradual reduction of the rays on the postaxial
side of the axis and their condensation on the pre-axial side. The
Ichthyotomi such as _Xenacanthus_ show one stage in the reduction of
the postaxial rays, and a further stage is seen in the Notidanidae and
some other sharks like _Scymnus_ and _Acanthias_, in which a few
postaxial rays still remain. The condensation of the pre-axial rays
when further continued leads to one of the rays getting an attachment
to the girdle. Thus the fin comes to articulate with the girdle by two
basalia or basal pieces; a third attachment is formed in the same way
and the three basalia are called respectively pro-, meso-, and
meta-pterygia. By some authors the meta-pterygium and by others the
meso-pterygium is regarded as homologous with the axis of the
archipterygium.

The pectoral fins of Elasmobranchs vary very much in their mode of
attachment. In some of the sharks, including the Notidanidae and
_Scyllium_, all three basalia articulate with the pectoral girdle,
while in others such as _Cestracion_ the meta-pterygium is excluded.
In Rays the propterygium and the meta-pterygium are long and narrow
and diverge much from one another; other basalia work their way in
between the meso-pterygium and meta-pterygium, and come to articulate
with the pectoral girdle. Sometimes they fuse and form a second
meso-pterygium. The radiale are greatly elongated and are segmented.

In _Chimaera_ all three basalia are present, but the meso-pterygium
is shifted and does not articulate with the pectoral girdle[49].

In _Acipenser_ and _Polyodon_ the pectoral fin is built on the same
type as in Elasmobranchs, but becomes modified from the fact that the
propterygium is replaced by dermal bone which forms a large =marginal
ray=. Extra meso-pterygia are formed in the same way as in Rays.

In _Polypterus_ the pro-and meta-pterygia have ossified while the
meso-pterygium remains chiefly cartilaginous; the fin-rays are also
chiefly ossified.

In _Amia_, _Lepidosteus_, and certain Teleosteans like _Salmo_, not
only the propterygium but the meso-pterygium is almost suppressed by
the marginal ray.

In the great majority of Teleosteans a still further stage is reached,
the endoskeletal elements, the basalia and radiale are almost entirely
suppressed and the fin comes to consist entirely of ossified fin-rays
of dermal origin.

In some Teleosteans--_Exocaetus_, a herring, and _Dactylopterus_, a
gurnard--the pectoral fins are so enormously developed that by means
of them the fish is able to fly through the air for considerable
distances. The skeleton of these great fins is almost entirely
composed of dermal bone.

PELVIC FIN.

The pelvic fin is almost always further removed from the
archipterygial condition, and is in general more modified than is the
pectoral. Thus in the Ichthyotomi, while the pectoral fins are
archipterygia similar to those of _Ceratodus_, the pelvic fins consist
of an axis bearing rays on the postaxial side only, and prolonged
distally into a clasper. In Dipnoi however the pelvic fins are very
similar to the pectoral. In Elasmobranchs the meso-pterygium is
missing, the propterygium is small or absent, and the fin is mainly
composed of the meta-pterygium (generally called basi-pterygium) and
its radiale. The males in Elasmobranchii and Holocephali have the
distal end of the meta-pterygium prolonged into a clasper.

In Ganoids and in Teleosteans the loss of the pelvic girdle causes the
pelvic fin to be still further removed from the primitive state. There
is always a large basi-pterygium which lies imbedded in the muscles
and meets its fellow at its proximal end. In Cartilaginous Ganoids it
has a secondary segmentation. Its relation to its fellow is subject to
much variation in Teleosteans, sometimes as in the Perch the two are
in contact throughout, sometimes as in the Salmon they meet distally
as well as proximally, but are elsewhere separated by a space,
sometimes as in the Pike and Bony Ganoids they diverge widely. The
radiale are articulated to the basi-pterygium. In Cartilaginous
Ganoids and _Polypterus_ they are well developed, in other Ganoids and
in Teleosteans they are in the main replaced by dermal fin-rays.

In some Teleosteans such as the Cod the pelvic fins have migrated from
their usual position and come to be attached to the throat in front of
the pectoral fins. Fish with this arrangement are grouped together as
=jugulares=.


FOOTNOTES:

[39] The following general works on fishes may be referred to:
Bashford Dean, _Fishes, Living and Fossil_, New York, 1895. A.
Günther, _An Introduction to the Study of Fishes_, Edinburgh, 1880.
A.A.W. Hubrecht and M. Sagemehl, _Fische_ in Bronn's _Classen und
Ordnungen des Thierreichs_, Band VI. Leipzig, 1876.

[40] See W.G. Ridewood, _Nat. Sci._ vol. VIII. 1896, p. 380. Full
references are there given to the literature of the subject.

[41] See H. Gadow and E.C. Abbott, _Phil. Trans._ vol. 186 (1895) B.
pp. 163-221.

[42] C. Hasse, _Zeitschr. wiss. Zool._ LVII. 1893, p. 76.

[43] C. Hasse, _Das natürliche System der Elasmobranchier auf
Grundlage des Baues und der Entwickelung ihrer Wirbelsäule_, Jena,
1879 and 1885, and "Die fossilen Wirbel, Morph. Studien I.-IV.,"
_Morphol. Jahrb. Bd._ II., III. and IV. 1876-78.

[44] See H.B. Pollard, _Anat. Anz._ X. 1894.

[45] T.W. Bridge, "The Cranial Osteology of _Amia calva_," _J. Anat.
Physiol. norm. path._ 1876, vol. XI. p. 605. R. Shufeldt, "The
Osteology of _Amia calva_," _Ann. Rep. of the Commissioner for Fish
and Fisheries_, Washington, 1885.

[46] A. Smith Woodward, _Nat. Sci._ vol. I. 1892, p. 28. Further
references are here given on the literature of the subject.

[47] C. Gegenbaur, Ueber das Archipterygium, _Jena Zeitschr. der
Wirbelthiere_, 2^e Heft, 1873, vol. 7, and _Morphol. Jahrb._ XXII.
1894, p. 119.

[48] The fins of _Ceratodus_ are very variable, no two being exactly
alike. Sometimes even the main axis bifurcates. See W.A. Haswell,
_Linn. Soc. N. S. Wales_, vol. VII. 1882.

[49] Some of these views with regard to the homologies of the parts of
the fins are not accepted by all anatomists.




CHAPTER IX.

CLASS II. AMPHIBIA[50].


AMPHIBIA differ markedly from Pisces in the fact that in the more
abundant and familiar forms the skin is naked, and that when the
integument is prolonged into median fins they are devoid of fin-rays.
The notochord may persist, but bony vertebral centra are always
developed. These are sometimes biconcave, sometimes procoelous,
sometimes opisthocoelous. There is only one sacral vertebra, except in
rare cases. The cartilaginous cranium persists to a considerable
extent but is more or less replaced by cartilage bone, and overlain by
membrane bone. The basi-occipital is not completely ossified, and the
skull articulates with the vertebral column by means of two occipital
condyles formed by the exoccipitals.

There is a large parasphenoid, but there are no ossifications in the
basisphenoidal, presphenoidal, and alisphenoidal regions. In most
cases the epi-otics and opisthotics are ossified continuously with the
exoccipitals.

The palato-pterygo-quadrate bar is firmly united to the cranium, so
the skull is autostylic. The palatines and pterygoids are membrane
bones. Teeth are nearly always borne on the vomers and commonly on the
maxillae and premaxillae. There are no sternal ribs, and the sternum is
very intimately related to the pectoral girdle. There are no obturator
foramina. The limbs are as in the higher vertebrata, divisible into
upper arm, fore-arm, and manus (wrist and hand), and into thigh, shin,
and pes (ankle and foot) respectively. The posterior limb is, as a
rule, pentedactylate, but in nearly every case the pollex is vestigial
or absent.


_Order 1._ URODELA[51].

The Urodela are elongated animals with a naked skin, a persistent
tail, and generally four short limbs.

The vertebral centra are opisthocoelous or biconcave, and there are
numerous precaudal vertebrae. Portions of the notochord commonly
persist in the intervertebral spaces. In the skull there is no
sphenethmoid forming a ring encircling the anterior end of the brain,
its place being in many cases partly taken by a pair of
orbitosphenoids. There is no quadratojugal, and the quadrate is more
or less ossified. The mandible has a distinct splenial, and the
articular is ossified.

There is no definite tympanic cavity. The hyoid apparatus is
throughout life connected to the quadrate by ligament, and a large
basilingual plate does not occur. The ribs are short structures with
bifurcated proximal ends. In the pelvis the pubis remains
cartilaginous, and there is a bifid cartilaginous epipubis. The bones
of the fore-arm and shin remain distinct, and the manus never has more
than four digits.

_Suborder_ (1). ICHTHYOIDEA.

The vertebrae are amphicoelous, but the notochord remains but little
constricted throughout the whole length of the vertebral column. Three
or four branchial arches nearly always persist in the adult. The
cartilages of the carpus and tarsus remain unossified.

The Ichthyoidea may be subdivided again into two groups:--

A. _Perennibranchiata_, whose chief distinguishing skeletal characters
are that the skull is elongated, the premaxillae are not ankylosed, the
maxillae are vestigial or absent; there are sometimes no nasals, and
the palatines bear teeth;

    e.g. _Siren_, _Proteus_, _Menobranchus_.

B. _Derotremata_, whose chief distinguishing skeletal characters are
that there are large maxillae and nasals; teeth are borne by both
maxillae and premaxillae; there are no palatines; and both pectoral and
pelvic limbs are always present;

    e.g. _Amphiuma_, _Megalobatrachus_, _Cryptobranchus_.

_Suborder_ (2). SALAMANDRINA.

The vertebrae are opisthocoelous. The skull is broad, and teeth are
borne by both premaxillae and dentaries. Nasal bones are present. The
remains of only two branchial arches are found in the adult. The
carpus and tarsus are more or less ossified.

This suborder includes the Newts (_Molge_), Salamanders
(_Salamandra_), and _Amblystoma_.


_Order_ 2. LABYRINTHODONTIA[52].

These are extinct Amphibia with a greatly developed dermal
exoskeleton, which is generally limited to the ventral surface. The
body and tail are long and in some cases limbs are absent. The teeth
are pointed and often have the dentine remarkably folded. The
vertebrae are amphicoelous, and are generally well ossified. The skull
is very solid, and has a greatly-developed secondary roof which hides
the true cranium and is very little broken up by fossae. Paired
dermal supra-occipitals are found, and there is an interparietal
foramen. The epi-otics and opisthotics form a pair of bones distinct
from the exoccipitals. Four simple limbs of moderate length are
generally present, and in some cases all four limbs are
pentedactylate. Among the better known genera of Labyrinthodonts are
_Mastodonsaurus_, _Nyrania_, and _Archegosaurus_.


_Order 3._ GYMNOPHIONA[53].

These animals form a group of abnormal worm-like Amphibia having an
exoskeleton in the form of subcutaneous scales arranged in rings. The
vertebrae are biconcave and are very numerous; very few however belong
to the tail. The skull has a complete secondary bony roof, the
mandible bears teeth and has an enormous backward projection of the
angular. The hyoid arch has very slender cornua and no distinct body,
it is attached neither to the cranium nor to the suspensorium. The
ribs are very long and there are no limbs or limb girdles.


_Order 4._ ANURA.

These are tailless Amphibia, which except in a few instances, are
devoid of an exoskeleton. The vertebrae are as a rule procoelous, and
are very few in number. The post-sacral part of the spinal column
ossifies continuously, forming an unsegmented cylindrical rod, the
urostyle. Remains of the notochord persist, lying _vertebrally_, i.e.
enclosed within the centra of the several vertebrae, and not as in
Urodela lying between one vertebra and the next. The skull is very
short and wide. The mandible is almost always, if not invariably,
toothless.

The frontals and parietals on each side are united so as to form a
pair of fronto-parietals, and a girdle-like sphenethmoid is present.

The quadrate is not generally ossified. A predentary or
mento-meckelian bone is commonly present in the mandible, and a single
bone represents the angular and splenial. The branchial arches are
much reduced in the adult, and the distal ends of the cornua unite to
form a flat basilingual plate of a comparatively large size.

Ribs are very little developed. Clavicles are present. The ilia are
very greatly elongated. The anterior limb has four well-developed
digits and a vestigial pollex, and is of moderate length; the radius
and ulna have fused. The posterior limb is greatly elongated and is
pentedactylate; the tibia and fibula are fused, while the calcaneum
and astragalus are greatly elongated, and it is largely owing to them
that the length of the limb is so great. The group includes the Frogs
and Toads, the predominant Amphibia of the present time.


FOOTNOTES:

[50] T.H. Huxley, _Amphibia_ (_Encyclopaedia Britannica_).

[51] See R. Wiedersheim, _Morphol. Jahrb._ Bd. III. 1877, p. 459.

[52] See A. Fritsch, _Fauna der Gaskohle_, Prague, 1883-85-86, also
writings of Cope, Credner, Huxley, H. v. Meyer, Miall.

[53] See R. Wiedersheim, _Anatomie der Gymnophionen_, Jena, 1879.




CHAPTER X.

THE SKELETON OF THE NEWT (_Molge cristata_).


I. EXOSKELETON.

The skin of the Newt is quite devoid of any exoskeletal structures.
The only exoskeletal structures that the animal possesses are the
teeth, and these are most conveniently described with the
endoskeleton.


II. ENDOSKELETON.

The endoskeleton of the Newt, though ossified to a considerable
extent, is more cartilaginous than is that of the frog. It is
divisible into an =axial portion= including the vertebral column,
skull, ribs, and sternum, and an =appendicular portion= including the
skeleton of the limbs and their girdles.

1. THE AXIAL SKELETON.

A. THE VERTEBRAL COLUMN.

This consists of about fifty vertebrae arranged in a regular
continuous series. The first vertebra differs a good deal from any of
the others; the seventeenth or sacral vertebra and the eighteenth or
first caudal also present peculiarities of their own. The remaining
vertebrae are divided by the sacrum into an anterior series of =trunk=
vertebrae which bear fairly large ribs, and a posterior series of
=caudal= vertebrae, all of which except the first few are ribless.

THE TRUNK VERTEBRAE.

Any vertebra from the second to the sixteenth may be taken as a type
of the trunk vertebrae.

The general form is elongated and somewhat hour-glass shaped, and the
=centra= are convex in front and concave behind; an opisthocoelous
condition such as this is quite exceptional in Anura. The =notochord=
may persist intervertebrally[54], but in the centre of each vertebra
it becomes greatly constricted or altogether obliterated, and replaced
by marrow. The superficial portion of the centrum is ossified, while
the articular surfaces are cartilaginous. The =neural arches= are low
and articulate together by means of =zygapophyses= borne on short
diverging processes. The anterior zygapophyses look upwards, the
posterior downwards. Each neural arch is drawn out dorsally into a
very slight cartilaginous =neural spine=.

On each centrum, at a little behind the middle line, there arise a
pair of short backwardly-directed =transverse processes=; each of
which becomes divided into two slightly divergent portions, a dorsal
portion which meets the tubercular process of the rib and is derived
from the neural arch, and a ventral portion which meets the capitular
process of the rib and is derived from the ventral or haemal arch. The
division between these two parts of the transverse processes can be
traced back as far as the sacrum.

The =first vertebra= as already mentioned differs much from all the
others. It has no ribs, and presents anteriorly two slightly divergent
concave surfaces which articulate with the occipital condyles of the
skull. Between these surfaces the dorsal portion of the anterior face
of the centrum is drawn out into a prominent =odontoid process=, the
occurrence of which renders it probable that the first vertebra of
the newt is really the axis, and that the atlas with the exception of
the odontoid process has become fused with the skull. The sacral
vertebra or =sacrum= differs from the vertebrae immediately in front
of it only in the fact that its transverse processes are stouter and
more obviously divided into dorsal and ventral portions.

THE CAUDAL VERTEBRAE.

The =caudal vertebrae= are about twenty-four in number. The anterior
ones have hour-glass shaped centra, and short backwardly-directed
transverse processes. The middle and posterior ones have rather
shorter centra, and are without transverse processes. The neural
arches resemble those of the trunk vertebrae, but each is drawn out
into a rather high cartilaginous neural spine abruptly truncated
anteriorly. All the caudal vertebrae except the first have also a
haemal arch, which is very similar to the neural arch, and is drawn
out into a haemal spine quite similar to the neural spine. Both neural
and haemal arches are ossified continuously with the centra.

B. THE SKULL.

The skull of the newt is divisible into three principal parts:--

(1) an axial part, the =cranium proper=, which encloses the brain and
to which

(2) the =capsules= of the =auditory and olfactory sense organs= are
fused;

(3) the skeleton of the =jaws and hyoid apparatus=. The skull is much
flattened and expanded, though not so much as in the frog.

(1) THE CRANIUM PROPER.

The =cranium proper= or =brain case= is an unsegmented tube which
remains partly cartilaginous, and is partly converted into cartilage
bone, partly sheathed by membrane bone. The roof and floor of the
cartilaginous cranium are, as is the case also in the frog, pierced
by holes or fontanelles, and these are so large that the main part of
the roof and floor comes to be formed by membrane bone.

Two pairs of large ossifications take place in the cranial walls. Of
these the more posterior on each side represents the =exoccipital= and
all three =periotic= bones. It bears a small convex patch of cartilage
for articulation with the atlas, and with its fellow forms the
boundary of the foramen magnum.

Two foramina pierce the exoccipital just in front of the occipital
condyle and transmit respectively the glossopharyngeal and
pneumogastric (fig. 21, X) nerves. Lying laterally to these nerve
openings is seen a patch of cartilage, the =stapes=, which is
homologous with the stapes or proximal element of the columellar chain
in the frog. Further forward in front of the stapes is the small
opening for the exit of the facial nerve, and seen in a lateral view
close to the orbitosphenoid, that for the trigeminal (fig. 21, C, 5).

In front of these large bones the lateral parts of the cranial walls
remain cartilaginous for a short distance, and then there follow two
elongated bones, the =orbitosphenoids= (fig. 21, B and C, 11), pierced
by the foramina for the exit of the optic nerves. These bones partly
correspond to the sphenethmoid of the frog.

The _membrane bones_ connected with the cranium are the _parietals_,
_frontals_ and _prefronto-lachrymals_ on the dorsal surface, and the
_parasphenoid_ on the ventral surface.

The _parietals_ (fig. 21, A and C, 6) roof over the posterior part of
the great dorsal fontanelle and overlap the exoccipito-periotics. They
meet one another along a sinuous suture in the middle line, as do also
the _frontals_ which overlap them in front. The _frontals_ and
_parietals_ both extend for a short distance down the sides of the
cranium and meet the orbitosphenoids. The _prefronto-lachrymals_ (fig.
21, A and C, 7) connect the frontals with the maxillae.

[Illustration FIG. 21. A DORSAL, B VENTRAL, AND C LATERAL VIEWS OF THE
SKULL OF A NEWT (_Molge cristata_) × 2-1/2 (after PARKER).

The cartilage is dotted, the cartilage bones are marked with dots and
dashes, the membrane bones are left white.

    1. premaxillae.
    2. anterior nares.
    3. posterior nares.
    4. nasal.
    5. frontal.
    6. parietal.
    7. prefronto-lachrymal.
    8. maxillae.
    9. vomero-palatine.
    10. parasphenoid.
    11. orbitosphenoid.
    12. pterygoid.
    13. squamosal.
    14. pro-otic region of exoccipito-periotic.
    15. quadrate.
    16. quadrate cartilage.
    17. exoccipital region of exoccipito-periotic.
    18. articular.
    19. articular cartilage.
    20. dentary.
    21. splenial.
    22. middle narial passage.
    II. V. VII. IX. X. foramina for the exit of cranial nerves.]

On the ventral surface is the large _parasphenoid_ (fig. 21, B, 10),
which is widest behind and overlapped anteriorly by the
vomero-palatines.

(2) THE SENSE CAPSULES.

The =auditory capsules= become almost completely ossified continuously
with the exoccipitals; they have been already described.

The =nasal capsules= are large and quite unossified though they are
overlain by membrane bone. They appear on the dorsal surface between
the anterior nares and the nasal process of the premaxillae. They
enclose the nasal organs, bound the inner side of the anterior narial
opening, and are connected with one another posteriorly by a
cartilaginous area.

Developed in connection with the nasal capsules are a pair of rather
large _nasals_ (fig. 21, A and C, 4), which lie on the dorsal surface
immediately in front of the frontals. Each forms part of the posterior
boundary of one of the anterior nares, and the two are separated from
one another in the middle line by the nasal process of the premaxillae
(fig. 21, A, 1), and the opening of the =middle narial passage= (fig.
21, A and B, 22), which passes right through the skull.

On the ventral surface of the skull and forming the greater part of
the boundary of the posterior nares are two large bones, the
_vomero-palatines_ (fig. 21, B and C, 9). Each consists of a wide
anterior portion, partly separated from its fellow in the middle line
by the ventral opening of the middle narial passage, and of a long
pointed posterior portion which is separated from its fellow by the
_parasphenoid_, and bears a row of small pointed teeth formed of
dentine capped with enamel.

(3) THE JAWS.

The =upper jaw= of the newt is a discontinuous structure divided into
two parts, an anterior part which consists of membrane bones, the
_maxillae_ and _premaxillae_, and a posterior part which remains mainly
cartilaginous.

The _premaxillae_ are united, forming a single bone, which in a
ventral view is seen to meet the maxillae and vomero-palatines, and in
a dorsal view to send back a nasal process (fig. 21, A, 1) between the
nasals.

The _maxillae_ are large bones, each terminating in a point
posteriorly. A single row of teeth similar to those on the
vomero-palatines runs along the outer margin of the maxillae and
premaxillae.

The posterior part of the upper jaw forms a mass of cartilage which
extends forwards towards the maxillae as a long pointed process whose
ventral surface and sides are overlapped by a membrane bone, the
_pterygoid_ (fig. 21, 12).

The suspensorial bones include the =quadrate= and _squamosal_. The
=quadrate= (fig. 21, 15) which forms the true =suspensorium= is
directed forwards and outwards, and is terminated by a patch of
cartilage with which the mandible articulates.

The lower jaw or mandible remains partly cartilaginous, while its
ossifications include two membrane bones and one cartilage bone. The
cartilage bone is the =articular= (fig. 21, C, 18), it forms the
posterior part of the ramus, extends forwards for some distance along
its inner side, and is terminated posteriorly by a patch of cartilage
which articulates with the quadrate. The _dentary_ (fig. 21, C, 20) is
a large bone which forms the anterior part and nearly all the outer
half of each ramus, and bears teeth similar to those of the upper jaw.
Attached to its inner face is a long slender _splenial_ (fig. 21, C,
21).

THE HYOID APPARATUS.

This consists of the hyoid arch and part of the first two branchial
arches.

The =hyoid arch= (fig. 29, A, 2) consists of a pair of =cornua=, each
of which is divided into two halves. The dorsal half forming the
=cerato-hyal= is mainly ossified though tipped with cartilage, and is
connected by ligament with the suspensorium. The ventral half
(=hypo-hyal=) is cartilaginous, and is connected with the
basibranchial.

The =branchial arches= consist of a median piece, the =basibranchial=,
which is ossified in the centre and cartilaginous at either end, and
of two pairs of =cerato-branchials= which are attached to the
cartilaginous part (fig. 29, A, 8) of the basibranchial. The first
cerato-branchial is chiefly ossified, the second (fig. 29, A, 4) is a
good deal smaller and is cartilaginous. Both are united dorsally to a
single =epi-branchial=, which is terminated by a small cartilaginous
area at the free end but is elsewhere well ossified.

C. THE RIBS.

The ribs are short imperfectly ossified structures, bifid at their
proximal end where they articulate with the transverse processes, and
tipped both proximally and distally with cartilage. The dorsal portion
of the proximal end corresponds to the =tuberculum= of the ribs of
higher animals, and the ventral portion to the =capitulum=. Some of
the anterior ribs have a step-like notch on their dorsal surfaces.

The second to twelfth ribs are fairly equal in size, but further back
they decrease slightly. The ribs which connect the sacral vertebrae
with the ilia are however large. The short ribs borne on the anterior
caudal vertebrae are cartilaginous.

D. THE STERNUM.

The sternum (fig. 22, A, 6) is a rather broad plate of cartilage,
drawn out posteriorly into a median process marked by a prominent
ridge. On its antero-lateral margins it bears surfaces for
articulation with the pectoral girdle.

2. THE APPENDICULAR SKELETON.

A. THE PECTORAL GIRDLE.

This is of a very simple character, and remains throughout life in an
imperfectly ossified condition. It consists of a dorsal =scapular
portion=, and a ventral =coracoid portion= partially divided into an
anterior part, the precoracoid, and a posterior part, the =coracoid=.

[Illustration FIG. 22. A VENTRAL, AND B LATERAL VIEW OF THE SHOULDER
GIRDLE AND STERNUM OF AN OLD MALE CRESTED NEWT (_Molge cristata_) × 3
(after PARKER).

    1. scapula.
    2. suprascapula.
    3. coracoid.
    4. glenoid cavity.
    5. precoracoid.
    6. sternum.]

The =scapular portion= is a slightly curved oblong plate; its proximal
third the =scapula= (fig. 22, 1) is ossified and bounds part of the
well-marked =glenoid cavity= (fig. 22, 4); its distal portion forms a
large oblong cartilaginous plate, the =suprascapula= (fig. 22, 2).

The =precoracoid= (fig. 22, 5) forms a small forwardly-directed
cartilaginous plate. The =coracoid= (fig. 22, 3) forms a much larger
plate, the greater part of which is unossified and overlaps its fellow
in the middle line, the two being overlapped by the sternum. Around
the glenoid cavity is an area which is mainly ossified and is
continuous with the scapula.

B. THE ANTERIOR LIMB.

This is divisible into three parts, the =upper arm= or =brachium=, the
=fore-arm= or =antibrachium=, and the =manus=.

The =upper arm= includes a single bone, the =humerus=.

The =humerus= is a slender bone cylindrical in the middle and expanded
at either end, the proximal part forms a rounded =head= which
articulates with the glenoid cavity. Along the proximal part of the
anterior or pre-axial surface runs a strong =deltoid ridge=. The
proximal part of the postaxial surface also bears a small outgrowth.

The =fore-arm= contains two bones, the =radius= and =ulna=, both of
which are small and imperfectly ossified at their terminations.

The =radius= (fig. 23, B, 11) or pre-axial bone is rather the larger
of the two, and is considerably expanded at its proximal end. The
=ulna= or postaxial bone is somewhat expanded distally, but is not
drawn out proximally into an olecranon process.

The =manus= consists of two parts, a group of small bones forming the
=carpus= or =wrist=, and the =hand=.

The =carpus= is in a very simple unmodified condition as compared with
that of the Frog. It consists of a proximal row of two bones and a
distal row of four, with one, the =centrale=, interposed between. All
these bones are small and polygonal and are imbedded in a plate of
cartilage.

The bones of the proximal row are a smaller pre-axial bone, the
=radiale= (fig. 23, B, 13), and a larger postaxial bone, which
represents the fused =ulnare= and =intermedium= of the very simple
carpus described on pp. 26 and 27.

The four bones of the distal row are respectively =carpalia= 2, 3, 4
and 5.

The =hand= consists of four digits, that corresponding to the thumb of
the human hand, judging from the analogy of the frog probably being
the one that is absent.

[Illustration FIG. 23. A RIGHT POSTERIOR, AND B RIGHT ANTERIOR LIMB OF
A NEWT × 1-1/2 (_Molge cristata_).

    1. femur.
    2. tibia.
    3. fibula.
    4. tibiale.
    5. intermedium.
    6. fibulare.
    7. centrale of tarsus.
    8. tarsale 1.
    9. tarsalia 4 and 5 fused.
    10. humerus.
    11. radius.
    12. ulna.
    13. radiale.
    14. intermedium and ulnare fused.
    15. centrale of carpus, the pointing line passes across carpale 2.
    16. carpale 3.
    17. carpale 5.
    I. II. III. IV. V. digits.]

Each digit consists of a somewhat elongated =metacarpal= and of two or
three phalanges. The metacarpals are contracted in the middle and
expanded at either end. They are connected with the carpus by
cartilage, and the articulations between the several phalanges, and
between the metacarpals and phalanges are also cartilaginous. The
second, third, and fifth digits have two phalanges apiece, the fourth,
which is the longest, has three. The second metacarpal in the
specimen examined and figured articulates partly with carpale 2,
partly with carpale 3.

C. THE PELVIC GIRDLE.

The pelvic girdle of the Newt is in a much less modified condition
than is that of the Frog (see p. 165). It consists of a dorsal
element, the =ilium=, a posterior ventral element, the =ischium=, and
an anterior ventral element, the =pubis=, to which is attached an
=epipubis=.

The =ilium= is a somewhat cylindrical bone which at its ventral end
meets the ischium, and forms part of the =acetabulum=. It is then
directed upwards and slightly backwards, and is attached to the ribs
of the sacral vertebra.

The =ischia= are a pair of somewhat square bones which meet one
another in the middle line; they form part of the acetabulum, and are
united to the ilia above.

In front of the ischia is a narrow cartilaginous area which represents
the =pubes=. Projecting forwards from it is a bifid cartilaginous
=epipubis=.

D. THE POSTERIOR LIMB.

This is divisible into a proximal portion, the =thigh=, a middle
portion, the =crus= or =shin=, and a distal portion, the =pes=.

The =thigh= consists of a single bone, the =femur= (fig. 23, A, 1),
which has a thin shaft and expanded ends. The anterior part of the
pre-axial border and posterior part of the postaxial border bear
slight outgrowths.

The =crus= or =shin= includes two short bones, the =tibia= and
=fibula=, which are nearly equal in length. The pre-axial bone or
tibia is a straight bone thickest at its proximal end, the postaxial
bone or =fibula= (fig. 23, A, 3) is a rather stouter curved bone of
nearly equal diameter throughout.

The =pes= includes the =tarsus= or =ankle=, and the =foot=.

The =tarsus= consists of eight small bones arranged in a proximal row
of three, the =tibiale=, =intermedium= and =fibulare=, and a distal
row of four =tarsalia=, with one bone, the =centrale= (fig. 23, A, 7),
interposed between the two rows. In the specimen examined, the
=tibiale=, is a small bone articulating with the tibia, the
=intermedium= (fig. 23, A, 5) is larger and articulates with both
tibia and fibula, the =fibulare= is the largest of the three and
articulates with the fibula.

The bones of the distal row are =tarsalia 1=, =2=, =3=, and a bone
representing =4= and =5= fused. In the specimen examined tarsale 1 is
pushed away dorsally (fig. 23, A, 8), so as to lie between the tibiale
and tarsale 2. All the tarsal bones are small and somewhat polygonal,
and are connected with one another, and with the tibia and fibula on
the one hand, and with the metatarsals on the other by a thin layer of
cartilage.

The five =digits= of the foot each consist of a =metatarsal= and of a
certain number of =phalanges=. In the specimen examined, owing to the
shifting of tarsale 1, the first metatarsal as well as the second
articulates with tarsale 2, while the fifth metatarsal articulates
partially with the bone representing the fused tarsalia 4 and 5,
partially with the fibulare. All the bones of the digits except the
distal phalanges are terminated at each end by cartilaginous
epiphyses, the distal phalanx of each digit has a cartilaginous
epiphysis only on its proximal end.

The first, second, and fifth digits have two phalanges apiece, the
third and fourth have three.

Figure 31 B, showing a Newt's tarsus copied from Gegenbaur, has
precisely the arrangement generally regarded as primitive for the
higher vertebrates, except that tarsalia 4 and 5 are fused.


FOOTNOTES:

[54] i.e. between one vertebra and the next.




CHAPTER XI.

THE SKELETON OF THE FROG[55] (_Rana temporaria_).


I. EXOSKELETON.

The skin of the frog is smooth and quite devoid of scales or other
exoskeletal structures. The only exoskeletal structures met with in
the frog are:--

1. The =teeth=, which are most conveniently described with the
endoskeleton.

2. The horny covering of the calcar or prehallux (see p. 167).


II. ENDOSKELETON.

The endoskeleton of the adult frog consists partly of cartilage,
partly of bone and each of these types of tissue occurs in two forms.
The cartilage may be hyaline, as in the omosternum and xiphisternum,
or may be more or less calcified as in part of the suprascapula and
the epiphyses of the limb bones. The bone may be cartilage bone, or
membrane bone.

The skeleton is divisible into an =axial portion= consisting of the
skull, vertebral column, and sternum, and an =appendicular portion=
consisting of the skeleton of the limbs and their girdles.

1. THE AXIAL SKELETON.

A. THE VERTEBRAL COLUMN.

The vertebral column is a tube, formed of a series of ten bones which
surround and protect the spinal cord. Of these ten bones nine are
vertebrae, while the tenth is a straight rod, the =urostyle=, and is
almost as long as all the vertebrae put together. The second to eighth
vertebrae inclusive have a very similar structure, but the first and
ninth differ from the others.

Any one of the second to eighth vertebrae forms a bony ring with a
somewhat thickened floor, the =centrum= or body, which articulates
with the centra of the immediately preceding and succeeding vertebrae.
The articulating surfaces are covered with cartilage and are
procoelous, or convex in front and concave behind. The eighth vertebra
is however amphicoelous or biconcave. The centrum of each vertebra
encloses an isolated vestige of the notochord. The =neural arch= forms
the roof and sides of the neural canal, which is very spacious in the
anterior vertebrae, but becomes more depressed in the posterior ones.
The arch bears the =neural spine=, a low median ridge of variable
character, and is drawn out in front and behind, forming the two pairs
of articulating surfaces or =zygapophyses= by means of which the
vertebrae are attached together. Of these the anterior articulating
surfaces or =prezygapophyses= look upwards and slightly inwards, while
the posterior articulating surfaces or =postzygapophyses= look
downwards and slightly outwards. The sides of the neural arches are
drawn out into a pair of prominent =transverse processes=. Those of
the second vertebra look somewhat forwards, those of the third look
directly outwards or somewhat forwards, while those of the fourth,
fifth, and sixth are directed slightly backwards, and those of the
seventh and eighth nearly straight outwards. All the transverse
processes are terminated by very small cartilaginous =ribs=.

SPECIAL VERTEBRAE.

The =first vertebra= is a ring-like structure with a much depressed
centrum. It bears in front two oval concave surfaces for articulation
with the condyles of the skull, while the centrum is terminated behind
by a prominent convex surface. There are as a rule no transverse
processes, and the postzygapophyses look downwards and outwards.
Occasionally however transverse processes do occur. Projecting
forwards from the centrum is a minute process better developed in the
Newt. This resembles an odontoid process, and it has hence been
supposed that the first vertebra is homologous with the axis of
mammalia, and that the atlas of the frog is fused with the skull.

The =ninth vertebra= has very stout transverse processes directed
backwards and somewhat upwards. They articulate with the pelvic girdle
and hence this vertebra is regarded as the =sacrum=. The neural arch
is much depressed, the centrum is convex in front and bears on its
posterior surface two short rounded processes for articulation with
the urostyle.

The =urostyle= is a long rod-like bone forming the posterior
unsegmented continuation of the vertebral column. It is probably
equivalent to three vertebrae, the tenth, eleventh, and twelfth fused
together, and to an unsegmented rod of cartilage which lies ventral to
the notochord. The anterior end is expanded and bears two concave
articular surfaces by means of which it articulates with the sacrum. A
prominent ridge runs along the dorsal surface, but gradually
diminishes when traced back. The anterior portion contains a canal
which is a continuation of the neural canal. At a point not far from
the anterior end, this canal communicates with the exterior by a pair
of minute holes which correspond with the intervertebral foramina.

B. THE SKULL[56].

The skull of the Frog consists of three principal parts:--

(1) an axial part, the =cranium proper=, which encloses the brain. To
it are firmly fused

(2) the =capsules of the olfactory and auditory sense organs=,

(3) lastly there is the =hyoid apparatus= and the =skeleton of the
jaws=.

The skull is by no means so completely ossified as is the vertebral
column, but in addition to the cartilage bone, there is a great
development of membrane bone in connection with it.

The skull has a peculiarly flattened and expanded form depending on
the wide lateral separation of the jaws from the cranium.


(1) THE CRANIUM PROPER or Brain case.

This is an unsegmented tube, which is widest behind. It remains to a
considerable extent cartilaginous, but is partly converted into
cartilage bone, partly sheathed in membrane bone. Its roof is
imperfect, being pierced by three holes or =fontanelles=, one large
anterior fontanelle (fig. 25, A, 9), and two smaller posterior
fontanelles (fig. 25, A, 10).

The cartilage bones of the cranium proper are the two =exoccipitals=
and the =sphenethmoid=.

The =exoccipitals= (figs. 24, 25, and 26, 6) are a pair of irregular
bones bounding the foramen magnum at the posterior end of the skull.
They almost completely surround the foramen magnum, and bear a pair of
oval convex surfaces, the =occipital condyles=, with which the first
vertebra articulates. The bones generally called the exoccipitals of
the frog include the =epi-otic= and =opisthotic= elements of many
skulls, in addition to the exoccipitals.

[Illustration FIG. 24. A DORSAL, AND B VENTRAL VIEWS OF THE CRANIUM OF
A COMMON FROG (_Rana temporaria_) × 2 (after PARKER).

In this and the next two figs. cartilage is dotted, cartilage bones
are marked with dots and dashes, membrane bones are left white.

    1. sphenethmoid.
    2. fronto-parietal.
    3. pterygoid.
    4. squamosal.
    6. exoccipital.
    7. parasphenoid.
    8. pro-otic.
    9. quadratojugal.
    10. maxillae.
    11. nasal.
    12. premaxillae.
    13. anterior nares.
    14. vomer.
    15. posterior nares.
    16. palatine.
    18. columella.
    19. quadrate.
    20. occipital condyle.
    II. optic foramen.
    V. VII. foramen for exit of trigeminal and facial nerves.
    IX. X. foramina for exit of glossopharyngeal and pneumogastric
           nerves.]

The patch of unossified cartilage immediately external to the
occipital condyle is pierced by two small foramina, through which the
ninth and tenth nerves leave the cranial cavity. The ninth nerve
passes through the more external of these foramina, the tenth through
the one nearer the condyle. The foramina lie however very close
together and are sometimes confluent. The cranial walls for a
considerable distance in front of the occipitals are unossified, but
the anterior end of the cranial cavity is encircled by another
cartilage bone, the =sphenethmoid= (figs. 24 and 25, 1) or girdle
bone. This partly corresponds to the orbitosphenoids of the Newt's
skull. Anteriorly it is pierced by a pair of small foramina through
which the ophthalmic branches of the trigeminal nerve pass out.

The anterior part of the cranial cavity is divided into two halves by
a vertical plate, the =mesethmoid=. Some little distance behind the
sphenethmoid the ventro-lateral walls of the cartilaginous cranium are
pierced by a pair of rather prominent holes, the =optic foramina=
(figs. 24 and 25, B, II), and at a similar distance further back,
occupying a kind of notch in the pro-otic are the large =trigeminal
foramina=, through which the fifth and seventh nerves leave the
cranium. Between the trigeminal and optic foramina are the very small
foramina for the sixth nerves (fig. 25, B, VI).

The _membrane bones_ of the cranium proper include the
_fronto-parietals_ and the _parasphenoid_.

The _fronto-parietals_ (figs. 24 and 26, A, 2) form a pair of long
flat bones closely applied to one another in the middle line, the line
of junction being the =sagittal suture=. They cover over the
fontanelles and overlap the sphenethmoid in front.

The _parasphenoid_ (figs. 24 and 26, B, 7) is a bone shaped like a
dagger with a very short handle. It lies on the ventral surface of the
cranium, the blade being directed forwards and underlying the
sphenethmoid; its lateral processes underlie the auditory capsules.


(2) THE SENSE CAPSULES.

The sense capsules are cartilaginous or bony structures which surround
the olfactory and auditory organs and are closely united to the
cranium.

[Illustration FIG. 25. A DORSAL AND B VENTRAL VIEW OF THE CRANIUM OF A
COMMON FROG (_Rana temporaria_) from which the membrane bones have
mostly been removed. × 2 (after PARKER).

    1. sphenethmoid.
    2. palatine.
    3. pterygoid.
    4. quadrate.
    5. columella.
    6. exoccipital.
    7. ventral cartilaginous wall of cranium.
    8. pro-otic.
    9. anterior fontanelle.
    10. right posterior fontanelle.
    11. quadratojugal.
    12. nasal capsule.
    II. V. VI. IX. X. foramina for exit of cranial nerves.]

The =auditory capsules= are fused with the sides of the posterior
end of the cranium just in front of the exoccipitals. They are
largely cartilaginous, but include in their anterior walls a pair
of irregular cartilage bones, the =pro-otics= (figs. 24 and 25, 8).
The cartilaginous area lying ventral to the pro-otic and external
to the exoccipital is pierced by a rather prominent hole, the
=fenestra ovalis=, which forms a communication between the internal
ear cavity, and a space the tympanic cavity, which lies at the side
of the head, and is bounded externally by the tympanic membrane. The
fenestra ovalis is occupied by a minute cartilaginous structure, the
=stapes=, and articulated partly to this and partly to a slight recess
in the pro-otic is the =columella= (fig. 25, B, 5), a rod in part
bony and in part cartilaginous, whose outer end is attached to the
tympanic membrane. The columella and stapes are together homologous
with the mammalian auditory ossicles and with the hyomandibular of
Elasmobranchs. Sometimes the term columella is used to include the
whole ossicular chain,--the columella together with the stapes.

The =olfactory= or =nasal capsules= (fig. 25, B, 12) are fused with
the anterior end of the cranium and differ from the auditory capsules
in being to a great extent unossified. There are however two pairs of
membrane bones developed in connection with them, the _vomers_ and the
_nasals_. They are drawn out into three pairs of cartilaginous
processes, on the dorsal surface into the =prenasal= and =alinasal=
processes which bound the external nares, and on the ventral surface
towards the middle line into the forwardly-projecting =rhinal=
processes.

The _nasals_ (figs. 24 and 26, 11) form a pair of triangular bones
lying dorsolaterally in front of the fronto-parietals. Their bases are
turned towards one another and their apices are directed outwards and
backwards. They correspond in position with the prefrontals of the
reptilian skull as well as with the nasals.

The _vomers_ are a pair of irregular bones lying on the ventral
surface of the olfactory capsules. Each bears on its inner and
posterior angle a group of minute pointed teeth, while its outer
border is drawn out into three or four small slightly diverging
processes, the two posterior of which form the inner boundary of the
=posterior nares= (fig. 24, B, 15).

(3) THE JAWS.

The =upper jaw= consists of a rod of cartilage connected with the
cranium near its two ends, but widely separated from it in the middle.
It is almost completely overlain by membrane bone. With its posterior
end the lower jaw articulates.

The membrane bones of the upper jaw include first the _premaxillae_, a
small bone meeting its fellow in the middle line, and forming the
extreme anterior end of the upper jaw. It gives off on its dorsal
surface a backwardly-projecting process. It is connected behind with
the _maxillae_ (figs. 24 and 26, 10), a long flattened bone which forms
the greater part of the margin of the upper jaw, and gives off near
its anterior end a short process which projects upwards and meets the
nasal.

[Illustration FIG. 26. A, LATERAL VIEW OF THE SKULL, B, POSTERIOR VIEW
OF THE CRANIUM OF A COMMON FROG (_Rana temporaria_) × 2 (after
PARKER).

    1. sphenethmoid.
    2. fronto-parietal.
    3. pterygoid.
    4. squamosal.
    5. tympanic membrane.
    6. exoccipital.
    7. parasphenoid.
    8. pro-otic.
    9. quadratojugal.
    10. maxillae.
    11. nasal.
    12. premaxillae.
    13. anterior nares.
    14. mento-meckelian.
    15. dentary.
    16. angulo-splenial.
    17. basilingual plate.
    19. quadrate.
    20. columella.
    21. occipital condyle.
    22. anterior cornu of the hyoid (cerato-hyal).
    23. foramen magnum.

    II. IX. X. foramina for the exit of cranial nerves.]

Both maxillae and premaxillae are grooved ventrally, and bear, attached
to the outer more prominent margin of the groove, a row of minute
conical teeth. These teeth are =pleurodont=, that is, are ankylosed by
their bases and outer sides to the margin of the jaw. Each tooth is a
hollow cone, the basal part of which is formed of bone, the apical
part of dentine, capped by a very weak development of enamel.

The posterior end of the maxillae is overlapped by a small bone, the
_quadratojugal_ (figs. 24 and 26, 9), whose posterior end forms part
of the articular surface for the lower jaw. Just behind the
quadratojugal there is a small unossified area which lies at the angle
of the mouth, and is connected by a narrow bar of cartilage with the
cranium; this forms the =quadrate= (figs. 24 and 26, 19). A
backwardly-directed outgrowth from the cartilaginous bar more or less
completely surrounds the tympanic membrane, forming the tympanic ring.
When followed back the maxillae and quadratojugal diverge further and
further from the cranium, till the angle of the mouth comes to be
separated from the foramen magnum by a space nearly double the width
of the cranium. This space is bridged over to a considerable extent by
two triradiate bones, the _pterygoid_ and _squamosal_.

The _pterygoid_ (figs. 24 and 26, 3) is a large bone, whose anterior
limb runs forwards meeting the maxillae and palatine; while its inner
limb meets the auditory capsule and parasphenoid, and its outer limb
runs backwards and outwards to the angle of the mouth. The _palatine_
is a small transversely-placed bone, which connects the pterygoid with
the anterior part of the sphenethmoid. The _squamosal_ (figs. 24 and
26, 4) is a T-shaped bone whose anterior arm is pointed and passes
forwards to meet the pterygoid. The posterior upper arm is closely
applied to the pro-otic, while the posterior lower arm meets the
pterygoid and quadratojugal at the angle of the jaw, and surrounds the
narrow cartilaginous bar of the quadrate which goes to join the
cranium. The squamosal is probably homologous with the squamosal
together with the pre-opercular of Bony Ganoids.

The quadrate and squamosal form the =suspensorium= by which the lower
jaw is connected with the cranium.

The =lower jaw= or =mandible= consists of a pair of cartilaginous rods
(=Meckel's cartilages=) in connection with each of which there are
developed two membrane bones and one cartilage bone. The cartilage
bone is the =mento-meckelian= (fig. 26, A, 14), a very small
ossification at the extreme anterior end. The membrane bones are the
_angulo-splenial_ and the _dentary_. The _angulo-splenial_ is a strong
flat bone which forms the inner and lower part of the mandible for the
greater part of its length. Its dorsal surface is produced into a
slight =coronoid process=. The _dentary_ (fig. 26, A, 15) is a flat
plate which covers the outer surface of the anterior half of the
mandible, as far forwards as the mento-meckelian. The lower jaw is
devoid of teeth. The part of Meckel's cartilage which in most
vertebrates ossifies, forming the articular bone, remains unossified
in the Frog.

THE HYOID APPARATUS.

The =hyoid= of the adult Frog is formed of the modified hyoid and
branchial arches of the tadpole. It consists of a broad thin plate of
cartilage, the =basilingual plate= (fig. 29, B, 1), drawn out into two
pairs of long processes, the =cornua=. The basilingual plate is
broader in front than behind, and is formed from the fused ventral
ends of the hyoid and branchial arches of the tadpole.

The =anterior cornua= (fig. 29, B, 2) form a pair of long slender
cartilaginous rods which project from the body of the hyoid at first
forwards, then backwards, and finally upwards and somewhat forwards
again, to be united to the auditory capsules just below the fenestrae
ovales. They are formed from the dorsal portion of the hyoid arch of
the tadpole and are homologous with the cerato-hyals of the Dogfish.

The =posterior cornua= form a pair of straight bony rods diverging
outwards from the posterior end of the basilingual plate. They are
formed from the fourth branchial arches of the tadpole, and differ
from the rest of the hyoid apparatus in being well ossified.

The =columellar chain=, which has been already described (p. 157),
should be mentioned with the hyoid as it is homologous to the
hyomandibular of fishes.

The =sternum= of the =Frog=, though regarded as part of the axial
skeleton, is so intimately connected with the pectoral girdle, that it
will be described with the appendicular skeleton.


2. THE APPENDICULAR SKELETON.

This consists of the skeleton of the two pairs of limbs and their
respective girdles. It is at first entirely cartilaginous but the
cartilage becomes later on mainly replaced by bone. The only bone
developed in connection with the appendicular skeleton, which has no
cartilaginous predecessor, is the _clavicle_.


A. THE PECTORAL GIRDLE.

This consists originally of two half rings of cartilage encircling the
sides of the body a short way behind the head. These two halves meet
one another in the ventral middle line, and separate the anterior
elements of the sternum from the posterior ones.

Each half-ring bears on the middle of its outer and posterior surface
a prominent cup, the =glenoid cavity=, with which the proximal
arm-bone articulates. This cup divides the half-arch into a dorsal
=scapular= and a ventral =coracoid= portion.

The =scapular portion= consists of two parts, the =suprascapula= and
the =scapula=.

The =suprascapula= (fig. 30, A, 2) is a wide, thin plate attached by
its ventral and narrowest border to the scapula. Its proximal and
anterior half is imperfectly ossified, its whole border or sometimes
only its dorsal and posterior borders consist of unaltered hyaline
cartilage, while the rest of it is composed of calcified cartilage.
The =scapula= (fig. 30, A, 3) is a fairly stout rod of bone
constricted in the middle, and forming the dorsal half of the glenoid
cavity.

The =coracoid portion= consists of three parts, the =coracoid=,
=precoracoid= and _clavicle_.

The largest and most posterior of these is the _coracoid_ (fig. 30, A,
4) which like the scapula, is contracted in the middle and expanded at
the ends, especially at the ventral end. It forms a large part of the
glenoid cavity. The ventral ends of the coracoids which meet one
another in the middle line are unossified, and form narrow strips of
calcified cartilage, the =epicoracoids= (fig. 30, A, 5); these are
often regarded as sternal elements.

The =precoracoid= forms a narrow strip of cartilage lying in front of
the coracoid, from which it is separated by the wide =coracoid
foramen= (fig. 30, A, 9). The dorsal end is continuous with an area of
unossified cartilage which separates the coracoid and scapula and
forms part of the glenoid cavity.

The _clavicle_ is a narrow membrane bone closely attached to the
anterior surface of the precoracoid, its dorsal end is expanded.


THE STERNUM.

The sternum consists of four parts arranged in two groups; two parts
to each group. The anterior members are the episternum and omosternum.

The =episternum= (fig. 30, A, 10) is a thin almost circular plate of
cartilage much of which remains hyaline.

The =omosternum= (fig. 30, A, 11) is a slender bony rod widest at its
posterior end; it connects the episternum with the ventral ends of the
precoracoids.

The =sternum proper= is a short rod of cartilage sheathed in bone; it
is contracted in the middle and expanded at each end. It bears
attached to its posterior end a broad somewhat bilobed plate of
partially calcified cartilage, the =xiphisternum= (fig. 30, A, 13).

B. THE ANTERIOR LIMB.

This is divisible into three parts, the =upper arm= or =brachium=, the
=fore-arm= or =antibrachium=, and the =manus=.

All the larger bones have their ends formed by prominent epiphyses
which do not unite with the shaft till late in life. Their
articulating surfaces are covered by hyaline cartilage.

In the =upper arm= there is a single bone, the =humerus=.

This has a more or less cylindrical shaft and articulates by a
prominent rounded =head= with the glenoid cavity. The distal end shows
a large rounded swelling on either side of which is a =condylar
ridge=, the inner or postaxial one being the larger. A prominent
=deltoid ridge= runs along the proximal half of the anterior surface,
and in the male frog a second equally prominent ridge runs along the
distal half of the posterior surface.

The =fore-arm= consists of two bones, the =radius= and =ulna=, united
together and forming the =radio-ulna=. The two bones are quite fused
at their proximal ends where they form a deep cup which articulates
with the distal end of the humerus, and is drawn out into a rather
prominent backwardly-projecting =olecranon process=, which ossifies
from a centre distinct from that of the shaft. The distal end is
distinctly divided by a groove into an anterior radial and a posterior
ulnar portion.

The =manus= consists of two parts, the =wrist= or =carpus= and the
=hand=.

The =carpus=[57] consists of six small bones arranged in two rows. The
three bones of the proximal row are the =ulnare=, =radiale= and
=centrale=. The =ulnare= and =radiale= are about equal in size and
articulate regularly with the radio-ulna. The =centrale= is pushed out
of its normal position and lies partly on the pre-axial side, partly
in front of the radiale. Of the three bones of the distal row the two
pre-axial ones, =carpalia 1= and =2=, are small; carpale 2 articulates
with the second metacarpal, carpale 1 with both the first and second.
The third bone is large and articulates with the third, fourth and
fifth metacarpals, it represents =carpalia 3-5=, with probably in
addition the representative of a second centrale.

The =hand= consists of four complete digits, and a vestigial =pollex=
reduced to a short metacarpal.

Each of the four complete digits consists of a =metacarpal= and a
variable number of =phalanges=. The first digit, as just mentioned,
has no phalanges, the second and third have two, and the fourth and
fifth have three.

C. THE PELVIC GIRDLE.

The pelvic girdle of the Frog is much modified from the simple or
general type found in the Newt (p. 149).

It is a V-shaped structure consisting of two halves which are fused
together in the middle line posteriorly, while in front they are
attached to the ends of the transverse processes of the sacral
vertebra. Each half bears at its posterior end a deep cup, the
=acetabulum=, with which the head of the femur articulates.

Each half of the pelvis ossifies from two centres. The anterior and
upper half of the acetabulum, and the long laterally compressed bar
extending forwards to meet the sacral vertebra ossify from a single
centre and are generally called the =ilium=; it is probable however
that they represent both the =ilium= and =pubis= of mammals[58]. The
posterior part of this bone meets its fellow in a median symphysis.

The posterior third of the acetabulum is formed by a small bone, the
=ischium=, which likewise meets its fellow in a median symphysis.

The ventral portion of the pelvic girdle never ossifies, even in old
animals being formed only of calcified cartilage. This is generally
regarded as the pubis, but it perhaps corresponds to the =acetabular
bone= of mammals.


D. THE POSTERIOR LIMB.

This corresponds closely to the anterior limb and, like it, is
divisible into three parts, the =thigh=, the =shin= or =crus= and the
=pes=.

As was the case with the anterior limb, all the long bones have their
ends formed by prominent epiphyses which do not unite with the shaft
till late in life.

In the =thigh= there is only a single bone, the =femur=.

The =femur= is a moderately long, slender bone with a well-ossified
hollow shaft slightly curved in a sigmoid manner. Both ends are
expanded, the proximal end is hemispherical and articulates with the
acetabulum, the distal end is larger and more laterally expanded.

The =shin= likewise includes a single bone, the =tibio-fibula=, but
this, as can be readily seen by the grooves at the proximal and distal
ends of the shaft, is formed by the fusion of two distinct bones, the
=tibia= and =fibula=. The tibio-fibula is longer and straighter than
the femur.

The =pes= consists of two parts, the =ankle= or =tarsus= and the
=foot=.

The =tarsus= consists of two rows of structures, very different in
size. The proximal row consists of two long bones, the =tibiale= and
=fibulare=, which are united by common epiphyses at the two ends,
while in the middle they are widely separated. The tibiale lies on the
tibial or pre-axial side, and the fibulare which is the larger of the
two bones on the fibular or postaxial side. The distal row of tarsals
consists of three very small pieces of calcified cartilage. The
postaxial of these is the largest, it articulates with the second and
third metatarsals and is probably homologous with tarsalia 2 and 3
fused. The middle one is very small, it articulates with the first
metatarsal and is probably tarsale 1. The pre-axial one articulates
with the metatarsal of the calcar, a structure to be described
immediately, and has been regarded as a =centrale=.

The =foot= includes five complete digits and a supplemental toe as
well. Each of the five digits consists of a long =metatarsal= with
epiphyses at both ends, and of a variable number of phalanges. The
first digit or =hallux= and the second have two phalanges, the third
three, the fourth, which is the largest, four, and the fifth, three.
The distal phalanges have epiphyses only at their proximal ends, the
others at both ends.

On the pre-axial side of the hallux is the supplemental digit, the
=prehallux= or =calcar=. It consists of a short metatarsal and one or
two phalanges, and is terminated distally by a horny covering of
epidermal origin.


FOOTNOTES:

[55] See A. Ecker, _Die anatomie des Frosches_, Braunschweig 1864,
translated by G. Haslam, Oxford, 1889, also A.M. Marshall, _The Frog_,
5th edition, Manchester and London, 1894.

[56] W.K. Parker, _Phil. Trans._ 161, 1871, p. 137, and W.K. Parker
and G.T. Bettany, _The Morphology of the Skull_, London, 1877, p. 136.

[57] See G.B. Howes and W. Ridewood, _P.Z.S._, 1888, p. 141.

[58] See bottom of p. 187.




CHAPTER XII.

GENERAL ACCOUNT OF THE SKELETON IN AMPHIBIA.


EXOSKELETON.

The exoskeleton, at any rate in most living forms, is very slightly
developed in Amphibia. The only representatives of the epidermal
exoskeleton are (1) the minute horny beaks found coating the
premaxillae and dentaries in _Siren_ and the tadpoles of most Anura,
(2) the nails borne by the first three digits of the pes in _Xenopus_
and by the Japanese Salamander _Onychodactylus_, (3) the horny
covering of the calcar or prehallux of frogs. The Urodela and nearly
all the Anura, which form the vast majority of living Amphibia, have
naked skins. A few Anura belonging to the genera _Ceratophrys_ and
_Brachycephalus_ have bony dermal plates developed in the skin of the
back, and these plates become united with some of the underlying
vertebrae.

In the Gymnophiona the integument bears small cycloid scales arranged
in rings which are equal in number to the vertebrae. These scales
contain calcareous concretions. Scales also occur between the
successive rings.

In the Labyrinthodontia the dermal exoskeleton is in many genera
greatly developed. It is generally limited to the ventral surface and
consists principally of a buckler formed of three bony plates, one
median and two lateral. These plates protect the anterior part of the
thorax, and are closely connected with the adjacent endoskeleton. They
probably represent the interclavicle and clavicles. Behind this
buckler numerous scutes are generally developed, which often cover the
whole ventral surface, and may cover the whole body.


TEETH[59].

In Amphibia teeth are generally present on the maxillae, premaxillae and
vomers, and except in Anura on the dentaries; sometimes they occur on
the palatines as in many Urodela, most Labyrinthodontia, and the
Gymnophiona; less commonly on the pterygoids as in _Menobranchus_,
_Siredon_, some Labyrinthodontia, and _Pelobates cultripes_[60], or on
the splenials as in _Siren_ and _Menobranchus_, or parasphenoid as in
_Pelobates cultripes_, _Spelerpes belli_ and _Batrachoseps_. In some
Anura such as _Bufo_ and _Pipa_ the jaws are toothless.

In Gymnophiona, _Menobranchus_, and _Siredon_, the teeth are arranged
in two concentric curved rows. The teeth of the outer row are borne on
the premaxillae and maxillae if present, (the maxillae are absent in
_Menobranchus_), the teeth of the second row on the vomers and
pterygoids in _Menobranchus_ and _Siredon_, and on the vomers and
palatines in Gymnophiona. In some Gymnophiona there is a double row of
mandibular teeth. The vomerine, palatine and parasphenoid teeth of all
forms are numerous and are not arranged in rows.

The teeth of all living Amphibia are simple conical structures
ankylosed to the bone, and consisting of dentine, coated or capped
with a thin layer of enamel. In the Labyrinthodontia teeth of more
than one size are sometimes present. The dentine of the basal part of
the larger teeth is in some genera very greatly folded, causing the
structure to be highly complicated. These folds, the intervals between
which are filled with cement, radiate inwards from the exterior and
outwards from the large pulp cavity. The basal part of the teeth of
_Ceratophrys_ (Anura) has a similar structure.


ENDOSKELETON.

VERTEBRAL COLUMN.

Four regions of the vertebral column can generally be recognised in
Amphibia, viz. the cervical, the trunk or thoraco-lumbar, the sacral
and the caudal regions. In the limbless Gymnophiona, however, only
three regions, the cervical, thoracic, and post-thoracic can be made
out. The cervical region is limited to a single vertebra which
generally differs from the others in having no transverse processes or
indication of ribs. It is generally called the atlas, but it commonly
bears a small process arising from the anterior face of the centrum
which resembles the odontoid process of higher animals, and renders it
probable that the first vertebra of Amphibia corresponds to the axis,
not to the atlas. Amphibia generally have a single sacral vertebra.

Three elements go to make up the vertebral column in Amphibia, viz.

1. the notochord,

2. the long vertebral centra,

3. intervertebral cartilage which forms the joints between successive
centra.

The relations which these three elements bear to one another are
subject to much variation. The successive stages can be well traced in
the Urodela.

1. The first stage is found in larval Urodeles in general and in adult
Ichthyoidea, and some Salamandrina. In these forms the notochord
persists and retains approximately the same diameter throughout the
whole length of the vertebral column. Bony biconcave centra are
present and constrict it to a certain extent vertebrally, while
intervertebrally there is a development of cartilage. The connection
between the bony vertebrae is effected mainly by the expanded
notochord.

2. In the next stage, as seen in _Gyrinophilus porphyriticus_, the
growth of intervertebral cartilage has caused the almost complete
obliteration of the notochord intervertebrally, and its entire
disappearance vertebrally, i.e. in the centre of each vertebra. The
intervertebral cartilage now forms the main connection between
successive vertebrae, and sometimes cases are found whose condition
approaches that of definite articulations. Readily recognisable
remains of the notochord are still found at each end of the
intervertebral constriction.

3. In the third stage differentiation and absorption of the
intervertebral cartilage has given rise to definitely articulating
opisthocoelous vertebrae. These are found in most adult Salamandrina.

       *       *       *       *       *

The transverse processes of the earlier trunk vertebrae are divided
into two parts, a dorsal part which meets the tubercular process of
the rib and is derived from the neural arch, and a ventral part which
meets the capitular process of the rib, and is derived from the
ventral or haemal arch. In the caudal vertebrae and often also in the
posterior trunk vertebrae the two processes are fused.

_Siren_ and _Proteus_, although they possess minute posterior limbs,
have no sacral vertebrae, while _Cryptobranchus lateralis_ has two.
The caudal vertebrae, except the first, have haemal arches very
similar to the neural arches.

In Labyrinthodontia the centra of the vertebrae are generally well
ossified biconcave discs. In some forms however, like _Euchirosaurus_,
the centra are imperfectly ossified, and consist of bony rings
traversed by a wide notochordal canal. Each ring is formed of four
pieces, a large well-ossified neural arch, a basal piece, and a pair
of lateral pieces. Vertebrae of this type are called _rachitomous_.

In the tail region of other forms each vertebra consists of an
anterior centrum bearing the neural arch, and a posterior
intercentrum[61] bearing chevron bones. Vertebrae of this type are
called _embolomerous_. Haemal arches similar to the neural arches are
often found as in Urodela. The transverse processes are sometimes well
developed and are divided into tubercular and capitular portions.

In Gymnophiona the vertebrae are biconcave and are very numerous, they
sometimes number about two hundred and thirty. Only quite the last few
are ribless and so can be regarded as post-thoracic vertebrae. The
first vertebra has nothing of the nature of an odontoid process.

In Anura the number of vertebrae is very greatly reduced, only nine
and the urostyle being present. Of these, eight are presacral and one
sacral. The urostyle is post-sacral and corresponds to three or more
modified vertebrae. The first vertebra is without transverse
processes, the remaining presacral vertebrae have the transverse
processes fairly large, while the sacral vertebra has them very large,
forming in some genera widely expanded plates. The urostyle is a long
cylindrical rod which articulates with the sacrum generally by two
facets. Ankylosed to its anterior end are the remains of two neural
arches.

In Anura remains of the notochord are found in the centre of each
vertebra, i.e. vertebrally, while in the Urodela they only occur
intervertebrally.

The vertebrae in Anura are, as a rule, procoelous. The eighth vertebra
is however generally amphicoelous, while the ninth commonly has one
convexity in front, and two behind.

In some forms such as _Bombinator_, _Pipa_, _Discoglossus_ and
_Alytes_ they are opisthocoelous; in others like _Pelobates_ they are
variable.


THE SKULL[62].

CRANIUM AND MANDIBLE.

In the Amphibian skull there are as a rule far fewer bones than in the
skull of bony fish. The primordial cartilaginous cranium often
persists to a great extent. Only quite a few ossifications take place
in it; namely in the occipital region--the exoccipitals, further
forwards--the pro-otics, and at the boundary of the orbital and
ethmoidal regions--the sphenethmoid. The basi-occipital and
basisphenoid are never ossified. As in Mammalia there are two
occipital condyles formed by the exoccipitals.

Large vacuities commonly occur in the cartilage of both floor and roof
of the primordial cranium. These are roofed over to a greater or less
extent by the development of membrane bone. Thus on the roof of the
cranium there are paired parietals, frontals, and nasals, and on its
floor are paired vomers, and a median unpaired parasphenoid.

In all living forms the parietals meet and there is no interparietal
foramen, though this exists in Labyrinthodonts.

The palato-pterygo-quadrate bar is united at each end with the
cranium, but elsewhere in most cases forms a wide arch standing away
from it. The suspensorium is, as in Dipnoi and Holocephali,
autostylic. The palato-pterygo-quadrate bar sometimes remains entirely
cartilaginous, sometimes its posterior half is ossified forming the
quadrate. In connection with it a number of membrane bones are
generally developed, viz. the maxillae, premaxillae, palatines,
pterygoids, quadratojugals, and squamosals. The pterygoids are,
however, sometimes partially formed by the ossification of cartilage.
The cartilage of the lower jaw and its investing membrane bones
generally have much the same relations as in bony fishes.

URODELA. The skulls of the various Urodeles show an interesting series
of modifications and differ much from one another, but all agree in
the absence of the quadratojugals, in the fact that the palatines lie
parallel to the axis of the cranium, and in the large size of the
parasphenoid.

The lower types _Menobranchus_, _Siren_, _Proteus_, and _Amphiuma_
have longer and narrower skulls than do the higher types.

_Menobranchus_ has a very low type of skull which remains throughout
life in much the same condition as that of a young tadpole or larval
salamander. The roof and floor of the cranium internal to the membrane
bones are formed of fibrous tissue, not of well-developed cartilage.
The epi-otic regions of the skull are ossified, forming a pair of
large bones which lie external to, and distinct from, the
exoccipitals. _Proteus_ and the Labyrinthodonts are the only other
Amphibia which have these elements separately ossified. The parietals
send a pair of long processes forwards along the sides of the
frontals. Nasals and maxillae are absent, as is likewise the case in
_Proteus_. Teeth are borne on the vomers, premaxillae, pterygoids,
dentaries and angulo-splenials. The suspensorium is forwardly
directed.

The skull of _Siren_ resembles that of _Menobranchus_ in several
respects, as in the forward direction of the suspensorium and in the
absence of maxillae, but differs in the possession of nasals, in the
toothless condition of the premaxillae and dentaries, and in the fusion
and dentigerous condition of the vomers and palatines.

_Amphiuma_ has a skull which, though narrow and elongated, differs
from those of _Menobranchus_, _Proteus_, and _Siren_, and resembles
those of higher types in the following respects:--

(1) the suspensorium projects nearly at right angles to the cranium
instead of being directed forwards, (2) the maxillae are well
developed, and the premaxillae are completely ankylosed together, (3)
there are no palatines.

The skulls of _Megalobatrachus_, _Cryptobranchus_ and _Siredon_
resemble those of the highest Urodeles the Salamanders in their wide
form, in having the pro-otics distinct from the exoccipitals which are
ossified continuously with the epi-otics and opisthotics, and in
having no palatines, but differ in having the two premaxillae
separate, and in the arrangement of the vomerine teeth which in
_Megalobatrachus_ and _Cryptobranchus_ are placed along the anterior
boundaries of the bones, these meeting in the middle line. In
_Siredon_ the vomers are separated by the very large parasphenoid.

The suspensorium in _Megalobatrachus_ and _Cryptobranchus_ projects at
right angles to the cranium; in _Siredon_ it projects somewhat
downwards and forwards as in the Salamandrina.

Modifications of the vomers, pterygoids and palatines accompany the
changes of the larval ichthyoid _Siredon_ into the adult salamandroid
_Amblystoma_, the vomers especially come to resemble to a much greater
extent those of the Salamandrina.

The ossification of the skull in the Salamandrina is carried further
than in the Ichthyoidea, though the supra-occipital and basi-occipital
are not ossified. The skull differs from that in the Ichthyoidea in
the size of the part of the vomero-palatines which lies in front of
the teeth, in the frequent union of the two premaxillae and in the
ossification of all the periotic bones continuously with the
exoccipital.

The skull differs from that of Anura in the following respects:--

(1) the bones of the upper jaw do not form a complete arch standing
away from the cranium, and the maxillae are not united to the quadrates
by quadratojugals, (2) the long axis of the suspensorium passes
obliquely downwards and forwards instead of downwards and backwards,
(3) there is no sphenethmoid encircling the anterior end of the
brain, its place being partly taken by a pair of orbitosphenoids, (4)
there is no definite tympanic cavity.

[Illustration FIG. 27. DORSAL VIEW OF THE SKULL OF A LABYRINTHODONT
(_Capitosaurus nasutus_) × 1/9 (from VON ZITTEL).

    1. premaxillae.
    2. nasal.
    3. maxillae.
    4. anterior nares.
    5. frontal.
    6. prefrontal.
    7. lachrymal.
    8. jugal.
    9. orbit.
    10. parietal.
    11. postfrontal.
    12. postorbital.
    13. interparietal foramen.
    14. squamosal.
    15. supratemporal.
    16. quadratojugal.
    17. quadrate.
    18. epi-otic.
    19. dermo-supra-occipital.
    20. exoccipital.
    21. foramen magnum.]

LABYRINTHODONTIA. The skull in Labyrinthodontia is remarkable for its
extreme solidity, the large number of bones which are present, and the
extent to which the roofing over of the temporal and other fossae has
taken place. In many forms the surface of the bones is as in
Crocodiles, strongly sculptured (fig. 27, right half) with ridges and
grooves which probably lodged sensory organs. The bones forming the
roof of the skull are generally very uniform in size, perhaps the most
noticeable of them being the paired dermo-supra-occipitals (fig. 27,
19). Paired dermo-supra-occipitals occur also in certain Ganoids. The
Labyrinthodont skull also bears resemblance to that of many fish in
the development of a pair of long pointed epi-otics (fig. 27, 18),
which remain permanently distinct from the surrounding bones. The
parietals are small and enclose between them the interparietal foramen
(fig. 27, 13). In some forms in which the head is protected with an
armour of scutes, these do not roof over the interparietal foramen,
and from this fact it has been inferred that the Labyrinthodonts had a
functional pineal eye. Both supra- and infra-temporal fossae are
partially or completely roofed over by the postorbitals and large
supra-temporals (fig. 27, 15).

There is generally a ring of bones in the sclerotic coat of the eye.
The pterygoids do not meet in the middle line, being separated by the
parasphenoid. The palatines bear teeth, and in some genera
(_Archegosaurus_) form long splints lying along the inner side of the
maxillae and more or less surrounding the posterior nares. In others
(_Nyrania_) they lie in the normal position near the middle line, one
on each side of the parasphenoid. The vomers bear teeth and sometimes
meet in the middle line; they are sometimes confluent with the
parasphenoid. On the ventral surface of the cranium there are
generally large palatal vacuities.

In the mandible there is often a well-marked postglenoid process, and
the articular is generally completely ossified.

[Illustration FIG. 28. A, VENTRAL VIEW OF THE CRANIUM; B, LATERAL VIEW
OF THE CRANIUM AND MANDIBLE OF _Siphonops annulatus_ (after
WIEDERSHEIM).

    1. anterior nares.
    2. naso-premaxillae.
    3. frontal.
    4. parietal.
    5. maxillae.
    6. vomer.
    7. orbit.
    8. quadrate united with the pterygoid in front.
    9. squamosal.
    10. exoccipital.
    11. dentary.
    12. angular.
    13. basi-occipital and basisphenoid fused.
    14. posterior narial opening surrounded by the palatine.
    X. pneumogastric foramen.]

GYMNOPHIONA. The skull bears a considerable resemblance to that of
Labyrinthodonts, especially in the arrangement of the bones which
bound the mouth cavity. The cranium is very hard, and is covered by
a complete bony roof formed mainly of the exoccipitals, parietals,
frontals, prefrontals, nasals and premaxillae. The nasals and
premaxillae are sometimes ossified continuously. There is a median
unpaired ethmoid whose dorsal end appears at the surface wedged in
between the frontals and parietals. The bone generally regarded as the
squamosal[63] is very large, and it and the maxillae generally together
surround the orbit, which, in _Epicrium_, has in it a ring of bones.
The palatines form long tooth-bearing bones fused with the inner sides
of the maxillae; they nearly surround the posterior nares.

The quadrate bears the knob, and the angular the cup for the
articulation of the mandible,--a very primitive feature. The mandible
is also noticeable for the enormous backward projection of the
angular.

       *       *       *       *       *

ANURA. In Anura the skull is very short and wide owing to the
transverse position of the suspensorium. There is often a small
ossification representing the quadrate. Sometimes as in _Hyla_ and
_Alytes_ there is a fronto-parietal fontanelle.

As compared with the skull in Urodela the chief characteristics of the
skull of Anura are:--

1. the presence of a sphenethmoid,

2. the union of the frontals and parietals on each side,

3. the occasional occurrence of small supra- and basi-occipitals,

4. the backward growth of the maxillae and its connection with the
suspensorium by means of the quadratojugal,

5. the dagger-like shape of the parasphenoid,

6. the occurrence of a definite tympanic cavity,

7. the frequent occurrence of a predentary or mento-meckelian
ossification in the mandible.

The skull of _Pipa_ is abnormal, being greatly flattened and
containing little cartilage. The fronto-parietals are fused, and there
is no sphenethmoid. The quadrates are well developed and the
squamosals and parasphenoid differ much from those of other Anura.


HYOID AND BRANCHIAL ARCHES.

In larval Amphibia the hyoid and four branchial arches are generally
present, and in adult Ichthyoidea they are frequently almost as well
represented as in the larva, and are of use in strengthening the
swallowing apparatus. They are very well seen in _Siredon_, and
consist of a hyoid attached by ligaments to the suspensorium, followed
by four branchial arches of which the first and second are united by a
copula (fig. 29, D, 8), while the third and fourth are not. The hyoid
is not always the largest and best preserved of the arches, for
sometimes as in _Spelerpes_ one of the branchials is far larger than
the hyoid. Four branchial arches occur in _Siren_ as in _Siredon_, but
in _Proteus_ there are only three.

In some larval Labyrinthodontia (_Branchiosaurus_) four branchial
arches are known to occur, and their arrangement is almost precisely
similar to that in _Siredon_.

In Gymnophiona the remains of only three branchial arches occur in
addition to the hyoid. The four arches are all very similar to one
another, each consists of a curved rod of uniform diameter throughout.
The hyoid is united with the first branchial arch, but has no
attachment to the cranium.

In larval Anura (fig. 29, C) the arrangement of the hyoid and
branchial arches is much as in Urodela. In the adult, however, the
ventral parts of all the arches unite, forming a compact structure,
the _basilingual plate_ (fig. 29, B, 1).

[Illustration FIG. 29. VISCERAL ARCHES OF AMPHIBIA.

    A. _Molge cristata_             (after PARKER).
    B. _Rana temporaria_      adult (after PARKER).
    C. Tadpole of _Rana_            (after MARTIN ST ANGE).
    D. _Siredon pisciformis_        (after CREDNER).

In each case the ossified portions are slightly shaded, while the
cartilaginous portions are left white.

    1. basilingual plate.
    2. hyoid arch.
    3. first branchial arch.
    4. second    do.
    5. third branchial arch.
    6. fourth    do.
    7. thyro-hyal.
    8. copula.]

The dorsal parts of the first three branchial arches disappear, but
those of the fourth become ossified and form the short, stout
thyro-hyals or posterior cornua. The dorsal parts of the hyoid arch in
the adult form a pair of long bars, the anterior cornua, which are
united to the periotic region of the skull in front of the fenestra
ovalis either by short ligaments or by fusion as in _Bufo_. In _Pipa_
and _Xenopus_ the first and second branchial arches persist as well as
the fourth (thyro-hyal), but in _Pipa_ the hyoid is wanting.


RIBS.

Ribs are generally very poorly developed in Amphibia. In Anura they
are in most cases absent; when present they generally form minute
unossified appendages attached to the transverse processes, but in
_Discoglossus_ and _Xenopus_ the anterior vertebrae are provided with
distinct ribs. In Urodela and Labyrinthodontia they are generally
short structures, each as a rule attached to the vertebra by a
bifurcated proximal end. The number of rib-bearing vertebrae varies,
but the first and the posterior caudal vertebrae are always ribless.
The anterior caudal vertebrae too are generally ribless, but sometimes
a few of them bear small ribs. In _Spelerpes_ the last two trunk
vertebrae are ribless, and hence may be regarded as lumbar vertebrae.

In Gymnophiona ribs are better developed than in any other Amphibia;
they occur on all the vertebrae except the first and last few, and are
attached to the transverse processes, sometimes by single, sometimes
by double heads.

Sternal ribs are almost unknown in Amphibia, but traces of them occur
in _Menobranchus._


STERNUM.

In Amphibia the sternum is not very well developed; sometimes as in
Gymnophiona and _Proteus_ no traces of it occur, and in the Urodela it
is never ossified. It is always very intimately related to the
pectoral girdle. In the Salamandrina it has the form of a broad thin
plate of cartilage, grooved and overlapped by the coracoid.

[Illustration FIG. 30. SHOULDER-GIRDLE AND STERNUM OF

    A. An old male common Frog (_Rana temporaria_).
    B. An adult female _Docidophryne gigantea_ (after PARKER).

In both A and B the left suprascapula is removed. The parts left
unshaded are ossified; those marked with small dots consist of hyaline
cartilage, those marked with large dots of calcified cartilage.

    1. calcified cartilage of suprascapula.
    2. ossified portion of suprascapula.
    3. scapula.
    4. coracoid.
    5. epicoracoid.
    6. precoracoid.
    7. clavicle.
    8. glenoid cavity.
    9. coracoid foramen.
    10. episternum.
    11. omosternum.
    12. sternum.
    13. xiphisternum.]

In most Anura the sternum consists of a number of parts arranged in
series. At the anterior end is a flat cartilaginous plate with a
bony basal stalk. This plate is called the episternum, and its stalk
the omosternum. The continuity of the sternum is now interrupted
by a pair of cartilaginous structures, the epicoracoids, which are
shoulder-girdle elements, and represent the unossified ventral ends of
the coracoids. In some cases cartilaginous epiprecoracoids can also
be distinguished. Further back is the long sternum proper, while last
comes the xiphisternum, a broad expanded plate of cartilage.

In some Anura such as _Pipa_ and _Hyla_ the number of sternal elements
is considerably reduced.


APPENDICULAR SKELETON.


PECTORAL GIRDLE.

The most primitive Amphibian shoulder-girdle is found in the Urodela.
It consists of a dorsal element, the scapula, a posterior ventral
element, the coracoid, and an anterior ventral element, the
precoracoid. The clavicle is not developed, and the two coracoids
overlap in the middle line. The shoulder-girdle remains largely
cartilaginous but the proximal end of the scapula is ossified, and the
ossification may extend through part of the coracoid and precoracoid.

In Labyrinthodontia there is an exoskeletal ventral buckler formed of
three plates, a median one, which probably represents an
interclavicle, and two lateral ones, which are probably clavicles.
Traces of endoskeletal structures, probably corresponding to the
precoracoid and scapula, are also known in some cases. The Gymnophiona
and some of the Labyrinthodontia have lost the pectoral girdle and
limbs.

The ossification of the shoulder-girdle has gone on much further in
Anura than it has in Urodela. Clavicles are present and the scapula
and coracoid of each side are ossified from separate centres. The
distal part of the scapula forms a large imperfectly ossified plate,
the suprascapula.

The shoulder-girdle of Anura is however subject to considerable
variations. In the Toads (Bufonidae) the epicoracoids or unossified
ventral ends of the coracoids and precoracoids overlap in the middle
line (fig. 30, B, 5). This arrangement is called _Arciferous_. In the
Frogs,--Ranidae, and other forms belonging to the group
_Firmisternia_,--the epicoracoids do not overlap but form a narrow
cartilaginous bar separating the ventral ends of the coracoids (fig.
30, A, 5).


ANTERIOR LIMB.

In many Amphibia and especially in the Urodela the anterior limb has a
very simple unmodified arrangement. The humerus is straight and of
moderate length, its ends are rounded for articulation on the one hand
with the shoulder-girdle, and on the other hand with the radius and
ulna. In the Urodela the radius and ulna are distinct. In the Anura
they have fused, though the line of junction of the two is not
obliterated. Their proximal ends are hollowed for articulation with
the convex end of the humerus.

The manus in all recent Amphibia agrees in never having more than four
complete digits, but is subject to considerable variation, this
statement applying especially to the carpus.

In the larva of Salamandra (fig. 31, A), except that the pollex is
absent[64], the manus retains completely the condition which is
generally regarded as primitive for the higher Vertebrata. It consists
of an anterior row of three elements, the ulnare, intermedium, and
radiale, and a posterior row of four, the carpalia 2, 3, 4, and 5.
Interposed between the two rows is a centrale. _Menobranchus_ has a
similar very simple carpus. In most other Amphibia this simplicity is
lost. This loss may be due to:--

(_a_) fusion of certain structures, e.g. in the adult _Salamandra_ the
intermedium and ulnare have fused,

(_b_) displacement of structures, e.g. in _Bufo viridis_, the centrale
has been pushed up till it comes to articulate with the radius,

(_c_) the development of supernumerary elements, especially of extra
centralia. In _Megalobatrachus_ two or even three centralia sometimes
occur.

[Illustration FIG. 31. A, RIGHT ANTIBRACHIUM AND MANUS OF A LARVAL
SALAMANDER (_Salamandra maculosa_) (after GEGENBAUR).

B, RIGHT TARSUS AND ADJOINING BONES OF _Molge sp._ (after GEGENBAUR).

    1. radius.
    2. ulna.
    3. radiale.
    4. intermedium.
    5. ulnare.
    6. centrale.
    7. carpale 2.
    8.    "    3.
    9.    "    4.
    10.   "    5.
    11. tibia.
    12. fibula.
    13. tibiale.
    14. intermedium.
    15. fibulare.
    16. centrale.
    17. tarsale 1.
    18. tarsalia 4 and 5 fused.
    I. II. III. IV. V. digits.]

In the great majority of Amphibia while one digit, probably the first,
is absent, the other four digits are well developed. In the forms
however with degenerate limbs like _Amphiuma_, _Siren_ and _Proteus_
the number of digits is still further reduced. In _Siren_ there are
three or four, in _Proteus_ three, and in _Amphiuma_ two or three
digits in the manus.

In Anura the pollex is represented only by a short metacarpal. There
are sometimes traces of a prepollex. The carpus often has two
centralia and the intermedium is absent.

In Labyrinthodontia the limbs are generally very simple and resemble
those of Urodela. In some forms, however, the manus differs from that
of all living Amphibia in possessing five well-developed digits.


PELVIC GIRDLE.

The simplest Amphibian pelvis is that of some of the Labyrinthodontia;
thus in _Mastodonsaurus_ it consists dorsally of a short broad ilium
placed vertically and attached to the sacrum, and ventrally of a small
pubis and of a large ischium meeting its fellow in the middle line. In
some Labyrinthodonts the pubes as well as the ischia meet in a ventral
symphysis, and in many there are no obturator foramina. In _Siren_,
Gymnophiona and some Labyrinthodontia the pelvic girdle and limbs are
absent.

In Urodela the ventral element of the pelvis on each side forms a flat
plate which meets its fellow of the opposite side. The anterior part
of the plate, representing the pubis, generally remains cartilaginous
throughout life; the posterior part representing the ischium is in
almost every case well ossified. Attached to the anterior end of the
pubes there is an unpaired bifid cartilaginous structure, the
epipubis. The ilia are vertically placed.

In most Anura the pelvis is peculiarly modified in correlation with
the habits of jumping. The long bone generally called the ilium is
placed horizontally and is attached at its extreme anterior end to the
sacrum. The ischium is ossified and distinct. Ventrally in front of
the ischium there is a tract of unossified cartilage which is often
regarded as the pubis. In _Xenopus_, however, the bone corresponding
to the ilium of the Frog is seen to ossify from two centres, one
forming the ilium, the other, which lies at the symphysis, being
apparently the pubis. This makes it probable that the so-called ilium
of the Frog is really to be regarded as an ilio-pubis, and renders the
homology of the cartilaginous part uncertain, but it probably
corresponds to the acetabular bone of mammals. In _Xenopus_ also there
is a minute epipubis similar to that of Urodeles.


POSTERIOR LIMB.

In Urodela the posterior limb (fig. 31, B) closely resembles the
anterior limb, but is even less removed from the primitive condition
of the higher vertebrates in the fact that all five digits are
commonly present. The tibia and fibula are short bones approximately
equal in size. In some cases the number of digits is reduced. Thus in
_Menobranchus_ the pes has four digits, in _Proteus_ it has two, and
in _Amphiuma_ two or three, while in _Siren_ the posterior limbs have
atrophied.

In correlation with their habits of jumping, the posterior limbs in
Anura are much lengthened and considerably modified. The tibia and
fibula are completely fused. The intermedium is absent, while the
tibiale and fibulare are greatly elongated. Tarsalia 4 and 5 are
absent. Five digits are always present, and there is a prehallux
formed of two or more segments.

In general the posterior limbs in Labyrinthodontia bear the closest
resemblance to the anterior limbs; in some cases three centralia are
found.

In Ichthyoidea, and in most Labyrinthodontia, the cartilages of the
carpus and tarsus remain unossified; in Salamandrina and in Anura they
are generally ossified.


FOOTNOTES:

[59] O. Hertwig. Ueber das Zahnsystem der Amphibien. _Arch. mikr.
Anat._ supplem. Bd. XI. 1875.

[60] G.A. Boulenger, _P.Z.S._ 1890, p. 664.

[61] See p. 14.

[62] See many papers by W.K. Parker published in the _Phil. Trans._ of
the Royal Soc.

[63] Perhaps this bone includes supra-orbital and postorbital
elements.

[64] The first digit present is sometimes regarded as the pollex, but
from analogy with Anura it is probable that the pollex is the missing
digit.




CHAPTER XIII.

SAUROPSIDA.


This great group includes the Reptiles and Birds and forms the second
of the three into which the Gnathostomata may be divided. There is
nearly always a strongly-developed epiblastic exoskeleton which has
the form of scales or feathers, and in some cases a dermal exoskeleton
is also well developed. In living forms the notochord never persists,
being replaced by vertebrae, but in some extinct forms the centra are
notochordal. The vertebral centra are ossified, and only in
exceptionally rare cases have terminal epiphyses. The skull is well
ossified and has membrane bones incorporated in its walls.

The occipital segment is completely ossified, and an interorbital
septum or bony partition separating the two orbits is usually
developed to a greater or less extent. The skull generally articulates
with the vertebral column by a single occipital condyle into the
composition of which the exoccipitals and basi-occipital enter in
varying proportions. The pro-otic ossifies, and either remains
distinct from the epi-otic[65] and opisthotic throughout life, or
unites with them only after they have fused with the adjacent bones.
The hyoid and branchial arches are much reduced; and the
representative of the hyomandibular is connected with the auditory
apparatus, forming the auditory ossicles[66]. Each ramus of the
mandible always consists of a cartilage bone, the articular, and
several membrane bones. The mandible articulates with the cranium by
means of a quadrate. The ribs in Birds and some Reptiles bear
_uncinate processes_, i.e. small, flat, bony or cartilaginous plates
projecting backwards from their posterior borders. The sternum is not
transversely segmented as in mammals, and there are commonly distinct
cervical ribs. The ankle joint is intertarsal, or situated between the
proximal and distal row of tarsal bones, not cruro-tarsal as in
Mammalia.


CLASS I. REPTILIA[67].

The axial skeleton is generally long, and that of the limbs frequently
comparatively short, or sometimes absent.

The exoskeleton generally has the form of epidermal scales, which are
often combined with underlying bony dermal plates or scutes and may
sometimes form a continuous armour. Neither feathers nor true hairs
are ever present. The vertebral column is generally divisible into the
five usual regions. The centra of the vertebrae vary enormously, and
may be amphicoelous, procoelous, opisthocoelous or flat, but they
never have saddle-shaped articulating surfaces. The quadrate is always
large, and is sometimes fixed, sometimes movable. A transpalatine bone
uniting the pterygoid and maxillae is generally present.

Free ribs are often borne along almost the whole length of the trunk
and tail, and often occur attached to the cervical vertebrae. The
sacrum is generally composed of two vertebrae which are united with
the ilia by means of expanded ribs. The sternum is rhomboidal, and may
either be cartilaginous or formed of cartilage bone, but never of
membrane bone; it differs from that of birds also in the fact that it
does not ossify from two or more centres. An interclavicle is
generally present. There are always more than three digits in the
manus, and never less than three in the pes. In all living reptiles
the ilia are prolonged further behind the acetabula than in front of
them, and the bones of the pelvis remain as a rule, distinct from one
another throughout life.

The pubes (pre-pubes) and ischia both commonly meet in ventral
symphysis, and the acetabula are wholly or almost wholly ossified. The
metatarsals are not ankylosed together.


_Order 1._ THEROMORPHA[68].

This order includes a number of mainly terrestrial, extinct reptiles,
which differ much from one another, and show remarkable points of
affinity on the one hand with the Labyrinthodont Amphibia, and on the
other with the Mammalia. The vertebrae are nearly always amphicoelous
and sometimes have notochordal centra. The skull is short and has the
quadrate immovably fixed. There is an interparietal foramen, and
generally large supratemporal fossae bounded by supratemporal[2]
arcades, but with no infratemporal[69] arcades; _Elginia_ however has
the whole of the temporal region completely roofed over.

The teeth are placed in distinct sockets and are very variable in
form, the dentition sometimes resembling the heterodont dentition of
mammals. The humerus has distinct condyles and an ent-epicondylar
foramen[70] as in many mammals.

The pubis is fused with the ischium, and both pectoral and pelvic
girdles are remarkably solid. The obturator foramen is remarkably
small or even absent. The anterior ribs have two articulating
surfaces, and each articulates by its tuberculum with the transverse
process, and by its capitulum with the centrum as in mammals.

These reptiles occur chiefly in deposits of Triassic and Permian age.
Some of the best known genera are _Dicynodon_, _Udenodon_, _Placodus_,
_Pariasaurus_ and _Galesaurus_. They will be noticed in the general
account of the skeleton in reptiles.


_Order 2._ SAUROPTERYGIA.

This order includes a number of extinct marine reptiles, devoid of an
exoskeleton. The tail is short, the trunk long, and the neck in the
most typical forms extremely long. The vertebrae have slightly
biconcave, or nearly flat centra. The skull is relatively small and
has large supratemporal fossae. The teeth are placed in distinct
sockets, and are generally confined to the margins of the jaws; they
are sharp and curved and are coated with grooved enamel. The
premaxillae are large, and there is an interparietal foramen. The
quadrate is firmly united to the cranium. The anterior nares are
separate and are placed somewhat close to the orbits. There is no
ossified sclerotic ring. The palatines and pterygoids meet the vomers,
and more or less completely close the palate, and in some forms, e.g.
_Plesiosaurus_, there is a distinct parasphenoid. Thoracic ribs are
strongly developed and each articulates with its vertebra by a single
head. The cervical vertebrae have well-marked ribs, which articulate
only with the centra, in this respect differing from those of
Crocodiles. The caudal vertebrae bear both ribs and chevron bones, and
abdominal splint-ribs are largely developed.

In the shoulder-girdle the coracoids are large and meet in a ventral
symphysis; precoracoids and a sternum are apparently absent, but parts
generally regarded[71] as the clavicles and interclavicle are well
developed. In the pelvis, the pubes and ischia meet in a long
symphysis. The limbs are pentedactylate, and in the best known forms,
the Plesiosauridae, form swimming paddles.

The Sauropterygia occur in beds of Secondary age, and some of the best
known genera are _Plesiosaurus_, _Pliosaurus_ and _Nothosaurus_.


_Order 3._ CHELONIA.

In the Tortoises and Turtles the body is enclosed in a bony box,
formed of the dorsal carapace, and a flat ventral buckler, the
plastron. Except in _Dermochelys_ the carapace is partly formed from
the vertebral column and ribs, partly from dermal bones. Both carapace
and plastron are, except in _Dermochelys_, _Trionyx_ and their allies,
covered with an epidermal exoskeleton of horny plates, which are
regularly arranged, though their outlines do not coincide with those
of the underlying bones. The thoracic vertebrae have no transverse
processes, and are quite immovably fixed, but the cervical and caudal
vertebrae are very freely movable. There are no lumbar vertebrae. The
skull is extremely solid, and frequently has a very complete false
roof. Teeth have been detected in embryos of _Trionyx_ but with this
exception the jaws are toothless, and are encased in horny beaks. The
quadrate is firmly fixed. The facial part of the skull is very short,
and the alisphenoidal and orbitosphenoidal regions are unossified. In
living forms there are no separate nasal bones, while large
prefrontals and postfrontals are developed. There is a comparatively
complete bony palate chiefly formed of the palatines and pterygoids.
The anterior nares are united and placed at the anterior end of the
skull, and the premaxillae are very small. There is no transpalatine
bone and the vomer is unpaired. The dentaries are generally fused
together.

There are ten pairs of ribs, and each rib has only a single head and
is partially attached to two vertebrae; there are no cervical or
sternal ribs. There is no true sternum.

The three anterior elements of the plastron are respectively
homologous with the interclavicle and two clavicles of other reptiles,
while the remaining elements of the plastron are probably homologous
with the abdominal ribs of Crocodiles. The pectoral girdle lies within
the ribs, and the precoracoids and coracoids do not meet in ventral
symphyses. The scapula and precoracoid are ossified continuously. The
pubis probably corresponds with the pre-pubis of Dinosaurs. There are
four limbs each with five digits.

The order includes three suborders:--


_Suborder (1)._ TRIONYCHIA.

The carapace and plastron have a rough granular surface covered with
skin and without any horny shields.

The plastron is imperfectly ossified, and marginal bones may be
absent, or if present are confined to the posterior portion of the
carapace. The pelvis is not united to the plastron. The cranium has
not a complete false roof and the head can be drawn back under the
carapace.

The first three digits of both manus and pes bear claws, and the
fourth digit in each case has more than three phalanges. The most
important genus is _Trionyx_.


_Suborder (2)._ CRYPTODIRA.

The carapace and plastron vary in the extent to which they are
ossified, and except in _Dermochelys_[72] and its allies are covered
by horny plates. Marginal bones are always present. The head can
generally be drawn back under the carapace. The pelvis is not firmly
united to the plastron. The cranium often has a complete false roof,
and in the mandibular articulation the cup is borne by the cranium,
and the knob by the mandible. Among the more important genera are
_Dermochelys_, _Chelone_, and _Testudo_.


_Suborder (3)._ PLEURODIRA.

The carapace and plastron are strongly ossified, and firmly united to
the pelvis. The head and neck can be folded laterally under the
carapace, but cannot be drawn back under it. The cranium has a more or
less complete false roof, and in the mandibular articulation the knob
is borne by the cranium, and the cup by the mandible. _Chelys_ is a
well-known genus.


_Order 4._ ICHTHYOSAURIA[73].

The order includes a number of large extinct marine reptiles whose
general shape is similar to that of the Cetacea. The skull is
enormously large, and the neck short. The tail is very long, and is
terminated by a large vertically-placed bilobed fin, the vertebral
column running along the lower lobe. The very numerous vertebrae are
short and deeply biconcave. The vertebral column can be divided into
caudal and precaudal regions only, as the ribs which begin at the
anterior part of the neck are continued to the posterior end of the
trunk without being connected with any sternum or sacrum. The precaudal
vertebrae bear two surfaces for the articulation of the ribs, while
in the caudal vertebrae the two surfaces have coalesced. The caudal
region is also distinguished by its chevron bones. The vertebrae have
no transverse processes, and the neural arches are not firmly united
to the centra, and have only traces of zygapophyses. The atlas and
axis are similar to the other vertebrae, but there is a wedge-shaped
intercentrum between the atlas and the skull, and another between
the atlas and the axis. The skull is greatly elongated (fig. 32) and
pointed, mainly owing to the length of the premaxillae. The orbits
are enormous, and there is a ring of bones in the sclerotic (fig. 32,
15). The anterior nares are very small; and are placed far back just
in front of the orbits. There is an interparietal foramen, and the
supratemporal fossae (fig. 32, 9) are very large, while there are no
infratemporal fossae. An epipterygoid occurs. The quadrate is firmly
fixed to the cranium, and there is a large parasphenoid. There are
large prefrontals, but the frontals are very small. The very numerous
teeth are large and conical, and are placed in continuous grooves
without being ankylosed to the bone. They are confined to the jaw-bones.

[Illustration FIG. 32. LATERAL (BELOW) AND DORSAL (ABOVE) VIEWS OF THE
SKULL OF AN _Ichthyosaurus_. (Modified from Deslongchamps.)

    1. premaxillae.
    2. maxillae.
    3. nasal.
    4. prefrontal[1].
    5. frontal.
    6. postfrontal[74].
    7. anterior nares.
    8. orbit.
    9. supratemporal fossa.
    10. interparietal foramen.
    11. parietal.
    12. squamosal.
    13. supratemporal.
    14. quadratojugal.
    15. sclerotic ring.
    16. postorbital.
    17. jugal.
    18. lachrymal.
    19. dentary.
    20. articular.
    21. angular.]

The ribs are long, and the anterior ones have capitula and tubercula.
There is no sternum, but the ventral body wall is strengthened by a
complex system of abdominal splint ribs.

The pectoral girdle is strongly developed, the scapulae are narrow,
the coracoids broad, and meet ventrally without overlapping. There are
probably no precoracoids, but clavicles and a T-shaped interclavicle
are well developed.

The limbs are very short, and completely modified into swimming
paddles. The humerus and femur are both short, while the radius and
ulna, tibia and fibula are generally still further reduced to the form
of short polygonal bones.

The digits are formed of longitudinal series of very numerous small
bones. The number of digits is five, but there sometimes appear to be
more owing to the bifurcation of certain of them, or to the addition
of marginal bones, either to the radial or ulnar side of the limb. The
humerus has no foramen, and both humerus and femur are unique in that
they are distally terminated by concave surfaces instead of by convex
condyles. The pelvic limb is much smaller than the pectoral. The
pelvis has no bony connection with the vertebral column, and all the
component bones are small and rod-like.

The Ichthyosauria are confined to beds of Secondary age and by far the
best known genus is _Ichthyosaurus_.


_Order 5._ RHYNCHOCEPHALIA.

This order includes the living _Sphenodon_ (_Hatteria_) and various
extinct forms. The general shape of these animals is lizard-like and
the tail is long.

The vertebrae are amphicoelous or sometimes nearly flat, and the
notochord sometimes persists to some extent. _Proterosaurus_ differs
from the other members of the order in having opisthocoelous cervical
vertebrae.

The sacrum is composed of two vertebrae. Ossified inter centra
(interdorsalia) generally occur in the cervical and caudal regions,
and sometimes throughout the whole vertebral column. In the skull the
quadrate is immovably fixed and united to the pterygoid. The palate is
well ossified, while the premaxillae which are often beak-like are
never ankylosed together. The jaws may be toothless or may be provided
with teeth which are usually acrodont (see p. 199). The palatines
frequently bear teeth, and in _Proterosaurus_ teeth occur also on the
pterygoids and vomers. The rami of the mandible are united by ligament
at the symphysis except in the Rhynchosauridae, in which the union is
bony. Superior and inferior temporal arcades occur.

The ribs have capitula and tubercula, and often uncinate processes
(see p. 190) as in birds. A pectoral girdle and sternum, with
clavicles and a T-shaped interclavicle are developed, and abdominal
ribs are always found. The precoracoid is however absent. The limbs
are pentedactylate.

_Sphenodon_[75] (Hatteria) now living in some of the islands of the
New Zealand group, is certainly the most generalised of all living
reptiles. Though lizard-like in form it differs from all living
lizards in the possession of two temporal arcades, abdominal ribs and
a fixed quadrate; and is often considered to be nearly allied in many
respects to the type of reptile from which all the others took their
origin.

Among the better known extinct forms are _Proterosaurus_ of Permian
and _Hyperodapedon_ of Triassic age.


_Order 6._ SQUAMATA.

This order includes the extinct Mosasaurians, and the lizards and
snakes which form the vast majority of living reptiles. The trunk may
be moderately elongated and provided with four short limbs as in
lizards, or it may be limbless, extremely elongated, and passing
imperceptibly into the tail. The surface is generally completely
covered with overlapping horny epidermal scales, below which bony
dermal scutes may be developed.

The vertebrae are procoelous, rarely amphicoelous. There are no inter
centra, and the neural arches are firmly united to the centra.
Additional articulating surfaces, the zygosphenes and zygantra, are
often developed[76]. The sacrum is formed of two or rarely three
vertebrae, or may be wanting as in Ophidia. In the skull an
infratemporal arcade forming the lower boundary of the infratemporal
fossa is absent, and the quadrate, except in the Chamaeleons, is
movably articulated to the squamosal. The palatal vacuities are large
and the nares are separate. There is often a distinct parasphenoid.
The teeth are either _acrodont_ (i.e. ankylosed to the summit of the
jaw), or _pleurodont_, i.e. ankylosed to the inner side of the jaw.
The thoracic ribs each have a single head which articulates with the
centrum of the vertebra; while uncinate processes and abdominal ribs
never occur.

A pectoral girdle and sternum may be present, or may be completely
absent as in snakes. Except in snakes there are generally four
pentedactylate limbs which may either form paddles or be adapted for
walking.


_Suborder (1)._ LACERTILIA[77].

The body is elongated, and as a rule four short pentedactylate limbs
are present, but sometimes limbs are vestigial or absent. The
exoskeleton generally has the form of horny plates, spines, or scales;
while sometimes as in the Chamaeleons and Amphisbaenians it is absent.
In other forms such as _Tiliqua_ and _Scincus_, the body has a
complete armour of bony scutes, whose shape corresponds with that of
the overlying horny scales.

The vertebrae are procoelous, rarely as in the Geckos amphicoelous;
they are usually without zygosphenes and zygantra, but these
structures occur in the Iguanidae. The sacral vertebrae of living
forms are not ankylosed together, and the caudal vertebrae usually
have well-developed chevron bones.

In the skull[78] the orbits are separated from one another, only by an
imperfectly developed interorbital septum, the cranial cavity not
extending forwards between them, while the alisphenoidal region is
unossified. The premaxillae may be paired or united (Amphisbaenidae),
and there is usually an interparietal foramen. There may be a complete
supratemporal[79] arcade bounding the lower margin of the
supratemporal fossa, or the supratemporal fossa may be open below. The
quadratojugal is not ossified, and the quadrate articulates with the
exoccipital. There is no infratemporal arcade. There is commonly a
rod-like epipterygoid[80] (fig. 33, 14) connecting the pterygoid and
parietal.

Teeth are always present, and may be confined to the jaws or may be
developed also on the pterygoids and rarely on the palatines; they are
either acrodont or pleurodont. The rami of the mandible are suturally
united.

A pectoral girdle is always present, and generally also a sternum.
Clavicles and a T-shaped interclavicle are commonly present, but are
absent in the Chamaeleons.

[Illustration FIG. 33. A, LATERAL VIEW, AND B, LONGITUDINAL SECTION OF
THE SKULL OF A LIZARD (_Varanus varius_). × 3/5. (Brit. Mus.)

    1. premaxillae.
    2. maxillae.
    3. nasal.
    4. lateral ethmoid.
    5. supra-orbital.
    6. lachrymal.
    7. frontal.
    8. postfrontal.
    9. prefrontal.
    10. basisphenoid.
    11. pro-otic.
    12. epi-otic.
    13. pterygoid.
    14. epipterygoid (columella cranii).
    15. jugal.
    16. transpalatine.
    17. parasphenoid.
    18. quadrate.
    19. parietal.
    20. squamosal.
    21. supratemporal.
    22. exoccipital.
    23. dentary.
    24. splenial.
    25. supra-angular.
    26. angular.
    27. coronoid.
    28. articular.
    29. vomer.
    30. basi-occipital.
    31. orbitosphenoid.]

There is no separate precoracoid but a precoracoidal process (fig. 34,
7) of the coracoid is generally prominent.

[Illustration FIG. 34. LATERAL VIEW OF THE SHOULDER-GIRDLE OF
_Varanus._ × 3/5.

(Brit. Mus.).

    1. suprascapula.
    2. scapula.
    3. glenoid cavity.
    4. coracoid.
    5. clavicle.
    6. interclavicle.
    7. precoracoidal process.]

Sternal ribs are present in chamaeleons and scinks. The limbs are in
the great majority of cases pentedactylate and the digits are clawed.
The phalanges articulate by means of condyles. Sometimes one or both
pairs of limbs are absent. When the posterior limbs are absent the
pelvis is also wanting, though the loss of the anterior limbs does not
lead to a corresponding loss of the pectoral girdle.

The pubis corresponds to the pre-pubis of Dinosaurs, and both pubes
and ischia meet in ventral symphyses.

The suborder includes the Lizards, Chamaeleons and Amphisbaenians.


_Suborder (2)._ OPHIDIA[81].

The Ophidia or snakes are characterised by their greatly elongated
body and want of limbs. The body is covered with overlapping horny
scales and bony dermal scutes are never present. The vertebrae are
procoelous, and are distinguishable into two groups only, precaudal or
rib-bearing, and caudal or ribless. The atlas vertebra is also
ribless. The neural arches are always provided with zygosphenes and
zygantra. Many of the vertebrae have strong hypapophyses, and the
caudal vertebrae are without chevron bones.

In the skull the cranial cavity extends forwards between the orbits,
and is closed in front by downgrowths from the frontals and parietals
which meet the well-ossified alisphenoids and orbitosphenoids[82]. The
cranium is strongly ossified, and there are no parotic processes or
interparietal foramen. There are no temporal arcades and no
epipterygoid. The premaxillae if present are very small (fig. 51, 1)
and usually toothless. The quadrates articulate with the squamosals,
and do not as in Lacertilia meet the exoccipitals. The palatines do
not unite directly with the vomers or with the base of the cranium,
and the whole palato-maxillary apparatus is more loosely connected
with the cranium than it is in Lacertilia. The pterygoids, and in most
cases also the palatines, bear teeth. The dentition is acrodont, and
the rami of the mandible are united only by an elastic ligament--an
important point serving to distinguish the Ophidia from the
Lacertilia. There is an imperfectly developed interorbital septum, the
ventral part of which is formed by the parasphenoid. The postfrontal
is generally well developed, while the jugals and quadratojugals are
absent. There are never any traces of the anterior limbs or pectoral
girdle, but occasionally there are vestiges of a pelvis and posterior
limbs.


_Suborder (3)._ PYTHONOMORPHA[83].

This suborder includes _Mosasaurus_ and its allies, a group of
enormous extinct marine reptiles found in beds of Cretaceous age.

The skin is in most forms at any rate unprovided with dermal scutes.
The vertebrae may be with or without zygosphenes and zygantra. The
skull resembles that of lizards, having an interparietal foramen, and
a cranial cavity open in front. The squamosal takes part in the
formation of the cranial wall, and the quadrate articulates with the
squamosal, not as in Lacertilia with the exoccipital. There are large
supratemporal fossae, bounded below by supratemporal arcades. The
teeth are large and acrodont, and occur on the pterygoids as well as
on the jaws. The two rami of the mandible are united by ligament only.
Pectoral and pelvic girdles are present, but clavicles are wanting,
and the pelvis is not as a rule united to any sacrum.

The limbs are pentedactylate, and are adapted for swimming, while all
the limb bones except the phalanges are relatively very short. The
number of phalanges is not increased beyond the normal, and they
articulate with one another by flat surfaces. The terminal phalanges
are without claws.


_Order 7._ DINOSAURIA[84].

The extinct reptiles comprising this order were all terrestrial, and
include the largest terrestrial animals known. They vary greatly in
size and in the structure of the limbs, some approach close to the
type of structure met with in birds, others are allied to crocodiles.

Passing to the more detailed characters:--there is sometimes a
well-developed exoskeleton having the form of bony plates or spines.
The vertebrae may be solid or their centra may be hollowed internally;
their surfaces may be flat, biconcave or opisthocoelous. The sacrum is
composed of from two to six vertebrae.

As regards the skull, the quadrate is large and fixed, and
supratemporal and infratemporal fossae bounded by bone occur. The
teeth are more or less laterally compressed, and often have serrated
edges; they may be placed in distinct sockets or in a continuous
groove. The ribs have capitula and tubercula, and sternal ribs often
occur. The scapula is very large, the coracoid small, and there is no
precoracoid, or T-shaped interclavicle. Clavicles are only known in a
few cases. In the pelvis the ilium is elongated both in front of, and
behind, the acetabulum, sometimes the pre-pubis, sometimes the
post-pubis is the better developed. The anterior limbs are shorter
than the posterior, and the long bones are sometimes solid, sometimes
hollow.

There are three well-marked suborders of the Dinosauria.


_Suborder (1)._ SAUROPODA[85].

The reptiles belonging to this group were probably quadrupedal and
herbivorous.

They have the cervical and anterior trunk vertebrae opisthocoelous,
while the posterior vertebrae are biconcave; all the presacral, and
sometimes the sacral vertebrae are hollowed internally. The teeth are
spatulate and without serrated edges, they are always planted in
distinct sockets, and some of them are borne by the premaxillae.

[Illustration FIG. 35. RESTORED SKELETON OF _Ceratosaurus nasicornis_.
× 1/30. (After Marsh.)

    1. anterior nares.
    2. prominence on the nasal bones which probably carried a horn.
    3. pre-orbital vacuity.
    4. orbit.
    5. scapula.
    6. coracoid.
    7. ilium.
    8. pubis (pre-pubis).
    9. ischium.]

The nares have the form of long slits and there are large pre-orbital
vacuities.

The limb bones are solid, and the anterior limbs are not much shorter
than the posterior ones. All the limbs are plantigrade and
pentedactylate, and the digits of the pes are clawed. There is a large
pre-pubis directed downwards and forwards, meeting its fellow in a
ventral symphysis, but there is no post-pubis.

The Sauropoda are found in the secondary rocks of Europe and N.
America and include the largest land animals that are known to have
existed. Many of the best known forms such as _Brontosaurus_ and
_Morosaurus_ are North American.


_Suborder (2)._ THEROPODA.

The members of this suborder were all carnivorous, and from the small
comparative size of the anterior limbs many of them were probably
bipedal.

The vertebrae are opisthocoelous or amphicoelous, their neural arches
are provided with zygosphenes and zygantra, and their centra are
frequently hollowed internally; the limb bones are also hollow, and in
fact the whole skeleton is extremely light. The tail is of great
length. The teeth are pointed and recurved, and have one or both
borders serrated; they are always planted in distinct sockets, and
some of them are borne by the premaxillae. There are large pre-orbital
vacuities. The digits of both manus and pes are terminated by pointed
ungual phalanges which must have borne claws. In the pelvis the
pre-pubes and ischia are slender bones, the former meeting in a
ventral symphysis. The ilia are very deep vertically and there are no
post-pubes. The astragalus is closely applied to the tibia, in front
of which it sends an ascending process, sometimes the two bones appear
to have been ankylosed together, as in birds. The metatarsals are
elongated and the feet digitigrade.

The Theropoda vary greatly in size, one of the best known genera
_Compsognathus_ was about as large as a cat, another, _Megalosaurus_,
perhaps as large as an elephant. _Ceratosaurus_ is the name of a
well-known North American form regarded by many authorities as
identical with _Megalosaurus_.


_Suborder (3)._ ORTHOPODA.

This suborder includes the most specialised of the Dinosaurs, certain
of which resemble the Theropoda in being bipedal. In some of them such
as _Stegosaurus_ the exoskeleton is strongly developed, in others such
as _Iguanodon_ it is absent.

The vertebrae are solid and may be opisthocoelous, biconcave, or flat.
The teeth are compressed and serrated, often irregularly, and are
frequently not set in distinct sockets. The anterior part of the
premaxillae is without teeth, and a toothless predentary or
mento-meckelian bone is present. The pre-orbital vacuities are small
or absent, and the nares are large and placed far forwards.

The most characteristic features of the group are found in the pelvis
which, except in the Ceratopsia, bears a striking resemblance to that
of birds. The ischium and post-pubis are long slender bones directed
backwards parallel to one another, and the pre-pubis is also well
developed. The ischium has an obturator process. The limb bones are
sometimes hollow, sometimes solid. The anterior limbs are much shorter
than the posterior, pointing to a bipedal method of progression. The
pes is digitigrade or plantigrade, and has three, rarely four,
digits.

The suborder Orthopoda may be further subdivided into three
sections:--


A. STEGOSAURIA.

A dermal exoskeleton is strongly developed. The vertebral centra are
flat or biconcave, and neither they nor the limb bones are hollowed
out by internal cavities. The limbs are plantigrade, the anterior ones
short, the posterior ones very large and strong. The post-pubis is
well developed;

e.g. _Stegosaurus_ from the Upper Jurassic of Colorado.


B. CERATOPSIA.

There is sometimes a well-developed dermal exoskeleton formed of small
granules and plates of bone. The bones are solid, and the vertebral
centra flat. The cranium bears a pair of enormous pointed frontal
horns, and the parietal is greatly expanded and elevated behind,
forming with the squamosals a shield which overhangs the anterior
cervical vertebrae. The premaxillae are united, and in front of them is
a pointed beak-like bone which bites upon a toothless predentary
ossification of the mandible. The teeth have two roots. The anterior
limbs are but little shorter than the posterior ones. There is no
post-pubis;

e.g. _Polyonax_ from the uppermost Cretaceous of Montana.


C. ORNITHOPODA[86].

There is no dermal exoskeleton. The cervical vertebrae are
opisthocoelous, and so are sometimes the thoracic. The limb bones are
hollow and the anterior limbs are much shorter than the posterior
ones. The feet are digitigrade and provided with long pointed claws.
The post-pubis is long and slender and directed back parallel to the
ischium;

e.g. _Iguanodon_ from the European Cretaceous.


_Order 8._ CROCODILIA[87].

This order includes the Crocodiles, Alligators and Garials and various
extinct forms, some of which are closely allied to the early
Dinosaurs.

There is always a more or less complete exoskeleton formed of bony
scutes overlain by epidermal scales; these bony scutes are specially
well developed on the dorsal surface but may occur also on the
ventral. The vertebral column is divisible into the five regions
commonly distinguishable. In all living forms the vertebrae, with the
exception of the atlas and axis, the two sacrals, and first caudal,
are procoelous, but in many extinct forms they are amphicoelous. The
atlas (fig. 71) is remarkable, consisting of four pieces, and the
first caudal is biconvex.

The teeth are, in the adult, planted in separate deep sockets. The
skull is very dense and solid, and all the component bones including
the quadrate are firmly united. The dorsal surface of the skull is
generally characteristically sculptured. There is an interorbital
septum, and the orbitosphenoidal and presphenoidal regions are
imperfectly ossified. Supratemporal, infratemporal, and post-temporal
fossae occur, but no interparietal foramen. In living genera there is
a long secondary palate formed by the meeting in the middle line of
the palatines, pterygoids and maxillae (fig. 43, A).

Cervical ribs (fig. 41, 8 and 9) are well developed, and articulate
with rather prominent surfaces borne on the neural arches and centra
respectively. The thoracic ribs articulate with the long transverse
processes, and sternal ribs and abdominal splint ribs (fig. 46, 4)
occur. The sternum is cartilaginous, and both it and the
shoulder-girdle are very simple. The precoracoid is represented by
merely a small process on the coracoid, while the clavicles are
absent, except in the Parasuchia. In the pelvis (fig. 49) there is a
large ilium, and an ischium meeting its fellow in a ventral symphysis;
these two bones form almost the whole of the acetabulum. In front of
the acetabulum, in the Eusuchia, projects a bone which is generally
called the pubis, but is in reality rather an epipubis (fig. 49, 4),
the true pubis being probably represented by a fourth element which
remains cartilaginous for some time, and later on ossifies and
attaches itself to the ischium. The limbs are small in proportion to
the size of the body, and are adapted for swimming or for shuffling
along the ground; they are plantigrade and the bones are all solid. In
living forms the anterior limbs have five digits and the posterior
four, the fifth being represented only by a short metatarsal. The
first three digits in each case are clawed. The calcaneum has a large
backwardly-projecting process.

The order Crocodilia may be subdivided into two suborders.


_Suborder (1)._ PARASUCHIA.

The vertebral centra are flat or biconcave. The premaxillae are very
large, and the nares are separated, and placed far back. The posterior
narial openings lie comparatively far forward between the anterior
extremities of the palatines.

The palatines and pterygoids do not form a secondary palate. The
supratemporal fossae are small, and open posteriorly, the lateral
temporal fossae are very large. The parietals and frontals are paired.
Clavicles are present. The best known and most important genus of
these extinct crocodiles is _Belodon_.


_Suborder_ (2). EUSUCHIA.

The vertebrae are either biconcave or procoelous. The premaxillae are
small, and the anterior nares are united and placed far forwards. The
posterior nares lie far back, the palatines and in living genera the
pterygoids, meeting in the middle line, and giving rise to a closed
palate. The supratemporal fossae are surrounded by bone on all sides,
and the parietals, and often also the frontals are united. There are
no clavicles. The suborder includes the genera _Crocodilus_,
_Alligator_, _Garialis_ and others living and extinct.


_Order 9._ PTEROSAURIA[88].

These animals, called also the pterodactyles or Ornithosauria, are a
group of extinct reptiles, whose structure has been greatly modified
from the ordinary reptilian type for the purpose of flight.

The skin was naked and they vary greatly in size and in the length of
the tail. The vertebrae and limb bones are pneumatic just as in birds.
The presacral vertebrae are procoelous and have their neural arches
firmly united to the centra. The neck is long, the caudal vertebrae
are amphicoelous, and from three to five vertebrae are fused together
in the sacral region. The skull is large and somewhat bird-like, the
facial portion being much drawn out anteriorly, and the sutures being
obliterated. It resembles that of other reptiles in having large
supratemporal fossae; large pre-orbital vacuities also occur. The jaws
may be toothed or toothless, and the teeth, when present, are imbedded
in separate sockets. The premaxillae are large, and the quadrate is
firmly attached to the skull. The rami of the mandible are united at
the symphysis, and there is an ossified ring in the sclerotic. The
occurrence of a postfrontal and its union with the jugal behind the
orbit, are characteristic reptilian features.

The ribs have capitula and tubercula, and sternal and abdominal ribs
occur. The sternum has a well-developed keel, and the scapula and
coracoid are large and bird-like. There are no clavicles or
interclavicle.

The anterior limbs are modified to form wings by the great elongation
of the fifth digit, to which a membrane was attached. The second,
third and fourth digits are clawed and are not elongated in the way
that they are in bats. The pollex, if present at all, is quite
vestigial.

The pelvis is weak and small, and though the ilia are produced both in
front of and behind the acetabula, in other features the pelvis is not
bird-like. The ischia are short and wide, and the pubes are
represented only by the pre-pubes. The posterior limbs are small and
the fibula is much reduced. The pes is quite reptilian in type, and
has five separate slender metatarsals. The two best known genera are
_Pterodactylus_, in which the tail is short, and _Rhamphorhynchus_, in
which it is long. The Pterosauria are found throughout the Jurassic
and Cretaceous formations in both Europe and North America.


FOOTNOTES:

[65] According to Baur a distinct epi-otic is not recognisable in the
reptilian skull.

[66] H. Gadow, _Phil. Trans._, vol. 179, 1888.

[67] See G. Baur, _J. Morph._, vol. I., 1887. R. Lydekker, _Catalogue
of the Fossil Reptilia and Amphibia in the British Museum_, Parts I. &
II. C.K. Hoffmann, _Reptilien_, in Bronn's _Classen und Ordnungen des
Thierreichs_, Bd. VI., 3 abth. 1879-90.

[68] T.H. Huxley, _Quart. J. Geol. Soc._, vol. XV. p. 649, 1859. R.
Owen, _Catalogue of Fossil Reptiles of S. Africa in the British
Museum_, London, 1876. H.G. Seeley, various papers published in the
_P.R. Soc. London_, and _Phil. Trans._

[69] See pp. 281-283.

[70] An ent-epicondylar foramen is one piercing the humerus on its
inner side just above the condyle.

[71] According to Hulke they should be regarded as the
omosternum,--the clavicles and interclavicle being wanting.

[72] See p. 272.

[73] R. Lydekker, _Nat. Sci._ vol. I. p. 514, 1892. Further references
are there given.

[74] The exact position of the suture between the prefrontal and
postfrontal is not known.

[75] A. Günther, On the Anatomy of Hatteria, _Phil. Trans_, vol. 157,
1867, p. 595.

[76] =Zygosphenes= are extra articulating surfaces borne upon the
anterior face of the neural arch; they fit into corresponding
structures, the =zygantra=, which are borne on the posterior surface
of the neural arch of the preceding vertebra. Ordinary zygapophyses
always accompany them.

[77] See E.D. Cope, _P. Amer. Phil. Soc_. vol. xxx. p. 185.

[78] See W.K. Parker, _Phil. Trans._ vol. 170, 1879, p. 595.

[79] See p. 281.

[80] Often called the columella cranii.

[81] See C.K. Hoffmann, in Bronn's _Klassen und Ordnungen des
Thierreichs_, Bd. VI., 3 abth. 1885-90.

[82] Some anatomists consider that the closing in of the brain case in
front is entirely due to the frontals and parietals.

[83] E.D. Cope, _Rep. U.S. Geol. Surv._, 1875, vol. II., The
vertebrata of the Cretaceous formations of the west. E.D. Cope, _P.
Boston Soc._ 1862, XII. p. 250. O.C. Marsh, _Amer. J. Sci._, 1872,
vol. 3. R. Owen, _Quart. J. Geol. Soc._, 1877, and 1878.

[84] J.W. Hulke, _Presidential address to the Geol. Soc. of London_,
1883 and 1884. O.C. Marsh, many papers in the _Amer. J. Sci._ from
1878 onwards, also in the _Geol. Mag._ R. Owen, _History of British
fossil reptiles: Dinosauria_ (Palaeont. Soc.).

[85] The diagnostic characters of the different groups of Dinosaurs
are in the main those given by von Zittel.

[86] See O.C. Marsh, _Amer. J. Sci._ (3), vol. 48, 1894, p. 85.

[87] See C.B. Brühl, _Das Skelet der Krokodiliden_, Wien, 1862. C.K.
Hoffmann in Bronn's _Klassen und Ordnungen des Thierreichs_, Bd. VI.
Abth. III. 1881-85. T.H. Huxley, _Proc. Linn. Soc._ (Zoology) 1860
vol. IV. p. 1. R. Owen, _History of British fossil Reptiles_.
_Crocodilia_ (Palaeont. Soc.). A. Smith Woodward, _Geol. Mag._ 1885,
3rd dec. II. p. 496. A. Smith Woodward, _Proc. of Geologists' Assoc._
vol. IX. p. 288, 1886.

[88] See H.G. Seeley, On the Organisation of the Ornithosauria,
_Journ. Linn. Soc._ (Zoology) vol. XIII. p. 84. K.A. Zittel, Ueber
Flugsaurier aus dem lithographischen schiefer, _Palaeontograph._ XXIX.
p. 49.




CHAPTER XIV.

THE SKELETON OF THE GREEN TURTLE.

(_Chelone midas._)


The most striking feature as regards the skeleton of the Turtle is
that the trunk is enveloped in a bony box, the dorsal portion of which
is called the =carapace=, while the ventral portion is the =plastron=.


I. EXOSKELETON.

_a._ The =epidermal exoskeleton= in the Green Turtle as in all other
Chelonia except _Dermochelys_, _Trionyx_ and their allies is strongly
developed, its most important part consisting of a series of horny
=shields= which cover over the bony plates of the carapace and
plastron but do not at all correspond to them in size and arrangement.

The shields covering over the =carapace= consist of three rows of
larger central shields,--five (=vertebral=) shields being included in
the middle row and four (=costal=) in each lateral row,--and of a
number of smaller =marginal= shields.

Of the marginal shields, that lying immediately in front of the first
vertebral is termed the =nuchal=, while the two succeeding the last
vertebral are called sometimes =pygal=, sometimes =supracaudal=; the
remainder are the marginal shields proper.

The epidermal covering of the =plastron= consists principally of six
pairs of symmetrically arranged shields, called respectively the
=gular=, =humeral=, =pectoral=, =abdominal=, =femoral=, and =anal=,
the gular being the most anterior. In front of the gular shields is an
unpaired =intergular=, and the shields of the plastron are connected
laterally with those of the carapace, by five or six pairs of rather
irregular =infra-marginal= shields. Smaller horny plates occur on
other parts of the body, especially on the limbs and head.

Two other sets of structures belong also to the epidermal exoskeleton,
viz. (_a_) horny =beaks= with denticulated edges which ensheath both
upper and lower jaws, (_b_) =claws=, which as a rule are borne only by
the first digit of each limb. Sometimes in young individuals the
second digit is also clawed.

_b._ The =dermal exoskeleton= is strongly developed, and is combined
with endoskeletal structures derived from the ribs and vertebrae to
form the carapace.

The =Carapace= (fig. 36) consists of a number of plates firmly united
to one another by sutures. They have a very definite arrangement and
include:

(_a_) the =nuchal= plate (fig. 36, 1), a wide plate forming the whole
of the anterior margin of the carapace. It is succeeded by three
series of plates, eight in each series, which together make up the
main part of the carapace. Of these the small

(_b_) =neural plates=[1] (fig. 36, A, 2) form the middle series. They
are closely united with the neural arches of the underlying vertebrae;

(_c_) the =costal plates=[89] (fig. 36, A, 3) are broad arched plates
united to one another by long straight sutures. They are united at
their inner extremities with the neural plates, but the boundaries of
the two sets of plates do not regularly correspond. Each is united
ventrally with a rib which projects beyond it laterally for some
distance; (_d_) the =marginal plates= (fig. 36, 4) are twenty-three
in number, eleven lying on each side, while an unpaired one lies in
the middle line posteriorly. Many of them are marked by slight
depressions into which the ends of the ribs fit;

(e) the =pygal= plates (fig. 36, 5) are two unpaired plates lying
immediately posterior to the last neural.

[Illustration Fig. 36. A, DORSAL AND B, VENTRAL VIEW OF THE CARAPACE
OF A LOGGERHEAD TURTLE (_Thalassochelys caretta_), (after Owen).

    1. nuchal plate.
    2. first neural plate.
    3. second costal plate.
    4. marginal plate.
    5. pygal plate.
    6. rib.
    7. thoracic vertebra.
    8. first vertebral shield.
    9. costal shield.]

The sculpturing due to the epidermal shields is very obvious on the
carapace.

The =plastron= (fig. 37) consists of one unpaired ossification, the
=entoplastron=, and four pairs of ossifications called respectively
the =epiplastra=, =hyoplastra=, =hypoplastra=, and =xiphiplastra=.

The =epiplastra= (fig. 37, 1) are the most anterior, they are expanded
and united to one another in the middle line in front, while behind
each tapers to a point which lies external to a process projecting
forwards from the hyoplastron. They are homologous with the
_clavicles_ of other vertebrates.

The =entoplastron= or =episternum= (fig. 37, 2) which is homologous
with the _interclavicle_ of other reptiles, is expanded at its
anterior end and attached to the symphysis of the epiplastra, while
behind it tapers to a point and ends freely.

The =hyoplastra= are large irregular bones each closely united
posteriorly with the corresponding hypoplastron, and drawn out
anteriorly into a process which lies internal to that projecting
backwards from the epiplastron. Each gives off on its inner surface a
slender process which nearly meets its fellow, while the anterior half
of the outer surface is drawn out into several diverging processes.

The =hypoplastra= (fig. 37, 4) are flattened bones resembling the
hyoplastra, with which they are united by long sutures; the posterior
half of both outer and inner surfaces is drawn out into a number of
pointed processes.

The =xiphiplastra= are small flattened elongated bones meeting one
another in the middle line posteriorly. In front they are notched and
each interlocks with a process from the hypoplastron of its side. The
hyoplastra, hypoplastra and xiphiplastra are homologous with the
abdominal ribs of Crocodiles.

[Illustration FIG. 37. THE PLASTRON OF A GREEN TURTLE (_Chelone
midas_). × 1/7. (Camb. Mus.)

    1. epiplastron (clavicle).
    2. entoplastron (interclavicle).
    3. hyoplastron.
    4. hypoplastron.
    5. xiphiplastron.]


II. ENDOSKELETON.

1. THE AXIAL SKELETON.

The axial skeleton includes the vertebral column, the ribs, and the
skull.

A. THE VERTEBRAL COLUMN AND RIBS.

The number of vertebrae in the Green Turtle is thirty-eight, not a
great number as compared with that in many reptiles, and of these
eighteen are caudal.

The vertebral column is divisible into four regions only--=cervical=,
=thoracic=, =sacral=, and =caudal=.


THE CERVICAL VERTEBRAE.

These are eight in number, and are chiefly remarkable for the great
variety of articulating surfaces which their centra present, and for
their mobility upon one another.

The first or =atlas= vertebra differs much from all the others and
consists of the following parts:--

_a._ the =neural arch=, formed of two separate ossifications united in
the mid-dorsal line;

_b._ the =inferior arch=;

_c._ the =centrum=, which is detached from the rest and forms the
odontoid process of the second vertebra.

Each half of the =neural arch= consists of a ventral portion, the
=pedicel=, which lies more or less vertically and is united ventrally
to the inferior arch, and of a dorsal portion, the =lamina=, which
lies more or less horizontally and meets its fellow in the middle line
in front, partially roofing over the neural canal. Each pedicel bears
a facet on its anterior surface, which, with a corresponding one on
the inferior arch, articulates with the occipital condyle of the
skull. Three similar facets occur also on the posterior surface of the
pedicel and inferior arch, and articulate with the odontoid process.
The laminae meet one another in front, but do not fuse, while behind
they are separated by a wide triangular space. They bear a pair of
small downwardly-directed facets, the =postzygapophyses=, for
articulation with the prezygapophyses of the second vertebra.

The =inferior arch= is a short irregular bone bearing two converging
facets for articulation with the occipital condyle and odontoid
process respectively.

The =centrum= or =odontoid process= has a convex anterior surface for
articulation with the neural and inferior arches, and a concave
posterior surface by which it is united with the centrum of the second
or axis vertebra. It bears posteriorly a small epiphysis which is
really a detached portion of the inferior arch.

The second or =axis= and following five cervical vertebrae, though
showing distinct differences, resemble one another considerably, each
having a fairly elongated centrum with a keel-like =hypapophysis=,
each having also a neural arch with prominent articulating surfaces,
the anterior of which, or =prezygapophyses=, look upwards and inwards,
while the posterior ones, the =postzygapophyses=, look downwards and
outwards. They however, as was previously mentioned, differ very
remarkably in the character of the articulating surfaces of the
centra. Thus the second and third vertebrae are convex in front and
concave behind, the fourth is biconvex, the fifth is concave in front
and convex behind. The sixth is concave in front and attached to the
seventh by a flat surface behind, the seventh has a flat anterior face
and two slightly convex facets behind. The vertebrae all have short
blunt transverse processes and the second has a prominent =neural
spine=.

The =eighth cervical vertebra= is curiously modified, the centrum is
very short, has a rather prominent hypapophysis, and is convex behind,
while in front it articulates with the preceding centrum by two
concave surfaces. The neural arch is deeply notched in front and bears
two upwardly-directed prezygapophyses, while behind it is very massive
and is drawn out far beyond the centrum, bearing a pair of flat
postzygapophyses. The top of the neural arch almost or quite meets a
blunt outgrowth from the nuchal plate.


THE THORACIC VERTEBRAE.

These are ten in number and are all firmly united with the ribs and
elements forming the carapace.

The first thoracic vertebra differs from the others, the centrum is
short and has a concave anterior surface articulating with the centrum
of the last cervical vertebra, and a pair of prezygapophyses borne on
long outgrowths. The neural spine arises only from the anterior half
of the centrum, and is not fused to the carapace. Arising laterally
from the anterior part of the centrum are a small pair of ribs each of
which is connected with a process arising from the rib of the
succeeding vertebra.

The next seven thoracic vertebrae are all very similar, each has a
long cylindrical centrum, expanded at the ends, and firmly united to
the preceding and succeeding vertebrae. The neural arches are
flattened and expanded dorsally, and are united to one another and to
the overlying neural plates; each arises only from the anterior half
of its respective centrum, and overlaps the centrum of the vertebra in
front of it. Between the base of the neural arch and its successor is
a small foramen for the exit of the spinal nerve. There are no
transverse processes or zygapophyses.

To each thoracic vertebra from the second to ninth inclusive, there
corresponds a pair of =ribs= (fig. 36, 6) of a rather special
character. Each is suturally united with the anterior half of the edge
of its own vertebra, and overlaps on to the posterior half of the edge
of the next preceding vertebra. The ribs are much flattened, and each
is fused with the corresponding costal plate, beyond which it projects
to fit into a pit in one of the marginal plates.

The tenth thoracic vertebra is smaller than the others, and its neural
arch does not overlap the preceding vertebra, it bears a pair of small
ribs which are without costal plates, but meet those of the ninth
vertebra.

There are no =lumbar= vertebrae.


THE SACRAL VERTEBRAE.

The =sacral vertebrae= are two in number, they are short and wide,
their centra are ankylosed together, and their neural arches are not
united to the carapace.

The first has the anterior face of the centrum concave and the
posterior flat, while both faces of the second are flat. Each bears a
pair of short ribs which meet the ilia, but are not completely
ankylosed either with them or the centra.


THE CAUDAL VERTEBRAE.

The =caudal vertebrae= are eighteen in number. The centrum of the
first is flat in front and is ankylosed to the second sacral; behind
it is convex. The others are all very similar to one another, and
decrease gradually in size when followed back. Each has a moderately
long centrum, concave in front and convex behind, both terminations
being formed by epiphyses. The neural arch arises only from the
anterior half of the vertebra; it bears a blunt truncated neural spine
and prominent pre- and post-zygapophyses. The first seven caudal
vertebrae bear short ribs attached to their lateral margins, the
similar outgrowths on the succeeding vertebrae do not ossify from
distinct centres, and are transverse processes rather than ribs.


B. THE SKULL.

The skull of the Turtle is divisible into the following three parts:--

(1) the cranium;

(2) the lower jaw or mandible;

(3) the hyoid.

(1) THE CRANIUM.

The =cranium= is a very compact bony box, containing a cavity in which
the brain lies, and which is a direct continuation of the neural canal
of the vertebrae.

[Illustration FIG. 38. THE SKULL OF THE GREEN TURTLE (_Chelone
midas_). × 1/2. A, POSTERIOR HALF, B, ANTERIOR HALF. (Brit. Mus.)

    1. parietal.
    2. squamosal.
    3. quadrate.
    4. basisphenoid.
    5. basi-occipital.
    6. quadratojugal.
    7. opisthotic.
    8. exoccipital.
    9. foramen magnum.
    10. splenial.
    11. articular.
    12. dentary.
    13. angular.
    14. supra-angular.
    15. premaxilla.
    16. maxilla.
    17. jugal.
    18. postfrontal.
    19. vomer.
    20. prefrontal.
    21. frontal.
    22. external auditory meatus leading into tympanic cavity.]

Like those of the skull as a whole its component bones may be
subdivided into three sets:--

1. those forming the brain-case or =cranium proper=;

2. those developed in connection with the special sense organs;

3. those forming the upper jaw and suspensorial apparatus.

Both cartilage and membrane bones take part in the formation of the
skull, and a considerable amount of cartilage remains unossified,
especially in the ethmoidal and sphenoidal regions.


1. THE CRANIUM PROPER OR BRAIN-CASE.

The cartilage and membrane bones of the brain-case when taken together
can be seen to be more or less arranged in three rings or segments,
called respectively the occipital, parietal, and frontal segments.

The =occipital segment= is the most posterior of these, and consists
of four cartilage bones, the =basi-occipital=, the two =exoccipitals=
and the =supra-occipital=; these bound the =foramen magnum=.

The =basi-occipital= (figs. 38 and 39, 5) lies ventral to the foramen
magnum and only bounds a very small part of it; it forms one-third of
the =occipital condyle= by which the skull articulates with the atlas
vertebra. It unites dorsally with the exoccipitals and anteriorly with
the basisphenoid.

The =exoccipitals= are rather small bones, which form the sides and
the greater part of the floor of the foramen magnum, and two-thirds of
the occipital condyle. Laterally each is united with the pterygoid and
opisthotic of its side. At the sides of the occipital condyle each
exoccipital is pierced by a pair of foramina, the more dorsal and
posterior of which transmits the hypoglossal nerve.

The =supra-occipital= (fig. 39, 14) is a larger bone than the others
of the occipital segment. It forms the upper border of the foramen
magnum and is drawn out dorsally into a large crest which extends back
far beyond the occipital condyle. In the adult the supra-occipital is
completely ankylosed with the epi-otics.


The =Parietal segment=.

The ventral portion of the parietal segment is formed by the
=basisphenoid= (figs. 38 and 39, 4) which lies immediately in front of
the basi-occipital. A triangular portion of it is seen in a ventral
view of the skull, but it is quickly overlapped by the pterygoids. It
gives off dorsally a pair of short processes which meet the pro-otics.

The alisphenoidal region is unossified and the only other constituents
of the parietal segment are the _parietals_ (fig. 39, 1). These are
large bones which, after roofing over the cranial cavity, extend
upwards and become expanded into a pair of broad plates which unite
with the squamosal and bones of the frontal segment to form a wide,
solid, false roof to the skull. Each also sends ventralwards a plate
which meets an upgrowth from the pterygoid and acts as an alisphenoid.


The =Frontal segment=.

Of the frontal segment the basal or presphenoidal and lateral or
orbitosphenoidal portions do not become ossified, the dorsal portion
however includes three pairs of membrane bones, the _frontals_,
_prefrontals_ and _postfrontals_.

The _frontals_ are a pair of small bones lying immediately in front of
the parietals, and in front of them are the _prefrontals_ (figs. 38
and 39, 20), a pair of similar but still smaller bones, which are
produced ventrally to meet the vomer and palatines. They form also the
dorsal boundary of the anterior nares. The _postfrontals_ (figs. 38
and 39, 18) are larger bones, united dorsally to the frontals and
parietals, posteriorly to the squamosals, and ventrally to the jugals
and quadratojugals. All three pairs of frontal bones, especially the
postfrontals, take part in the bounding of the orbits.

[Illustration FIG. 39. LONGITUDINAL VERTICAL SECTION THROUGH THE
CRANIUM OF A GREEN TURTLE (_Chelone midas_). × 2/3. (Camb. Mus.)

    1. parietal.
    2. squamosal.
    3. quadrate.
    4. basisphenoid.
    5. basi-occipital.
    6. quadratojugal.
    7. pro-otic.
    8. opisthotic.
    9. pterygoid.
    10. palatine.
    11. rod passed into narial passage.
    12. exoccipital.
    13. epi-otic fused to supra-occipital.
    14. supra-occipital.
    15. premaxillae.
    16. maxillae.
    17. jugal.
    18. postfrontal.
    19. vomer.
    20. prefrontal.
    21. frontal.
    V, VII, VIII, IX, X, XI, XII,
    foramina for the exit of
    cranial nerves.]


2. THE SENSE CAPSULES.

Skeletal structures occur in connection with each of the three special
sense organs of hearing, sight, and smell.


The =Auditory capsules=.

The auditory or periotic capsule of the turtle is rather large and its
walls are well ossified, epi-otic, pro-otic and opisthotic bones being
present.

The =epi-otic= (fig. 39, 13) is the more dorsal of the three bones,
and in the adult is completely ankylosed with the supra-occipital.

The =opisthotic= (fig. 39, 8) is the ventral posterior element. On its
inner side it is united to the supra-occipital above, and to the
exoccipital below; it sometimes becomes completely fused with the
exoccipital. In front it meets the pro-otic, and on its outer side the
squamosal and quadrate. Its anterior portion is hollowed out by the
cavity in which the auditory organ lies, it gives off also a process
which is separated from the exoccipital by an oval foramen through
which the glossopharyngeal, pneumogastric, and spinal accessory nerves
leave the cranial cavity.

The =pro-otic= is the anterior element; it meets the supra-occipital
and opisthotic posteriorly, while anteriorly it is separated from the
alisphenoidal plate of the parietal and pterygoid by a large oval
foramen through which the maxillary and mandibular branches of the
trigeminal nerve pass out (fig. 39, V 1 & 2). It is hollowed out
posteriorly by the cavity in which the auditory organ lies, and its
inner wall as seen in longitudinal section is pierced by a foramen
through which the external carotid artery and facial nerve leave the
cranial cavity,--the nerve finally leaving the skull through a small
oval foramen on the anterior face of the pro-otic near its junction
with the quadrate.

Between the pro-otic and opisthotic as seen in a longitudinal section
of the skull is a large opening constricted in the middle. This is
the =internal auditory meatus= (fig. 39, VIII.). Through it the
auditory nerve leaves the cranial cavity and enters the ear. The ramus
vestibularis leaves through the dorsal part of the hole, the ramus
cochlearis through the ventral.

The cavity of the auditory or periotic capsule communicates with the
exterior by a fairly large hole, the =fenestra ovalis=, which lies
between the opisthotic and pro-otic, and opens into a deep depression,
the =tympanic cavity=, which is seen in a posterior view of the skull
lying just external to the exoccipital. The cavity communicates with
the exterior by a large opening, the =external auditory meatus= (fig.
38, 22).

Several other openings are seen in the tympanic cavity; through one at
the extreme posterior end the pneumogastric and spinal accessory
nerves leave the skull, and through another, a little further
forwards, the glossopharyngeal.

The auditory ossicles consist of a long bony =columella=, whose inner
end fits into the fenestra ovalis, while the outer end is attached to
a small cartilaginous plate, the =extra-columella=, which is united to
the tympanum.


The =Optic capsules=.

The skeletal structures developed in connection with the optic capsule
do not become united to the skull. They consist of:--

(_a_) the =sclerotic=, a cartilaginous sheath investing the eye and
bearing

(_b_) a ring of ten small bony scales.

There is no _lachrymal_ bone.


The =Olfactory= or =Nasal capsules=.

The basicranial axis in front of the basisphenoid remains
cartilaginous, neither presphenoid nor mesethmoid bones are developed,
and the orbits in a dry skull communicate by a wide space through
which the second, third, fourth, and sixth cranial nerves pass out.
Separate _nasal_ bones also do not occur, the large prefrontals
extending over the area usually occupied by both nasals and
lachrymals.

The only bone developed in connection with the nasal capsules is the
_vomer_ (fig. 39, 19), an unpaired bone lying ventral to the
mesethmoid cartilage, and in contact laterally with the maxillae,
premaxillae and palatines.


3. THE UPPER JAW AND SUSPENSORIAL APPARATUS.

A number of pairs of bones are developed in connection with the upper
jaw and suspensorial apparatus, one pair, the =quadrates=, being
cartilage bones, while the rest are all membrane bones.

The _squamosals_ (fig. 38, 2) are large bones which, lying external to
the auditory bones, extend dorsalwards to meet the parietals and
postfrontals, and form a large part of the false roof of the skull.
They are united ventrally with the quadrates and quadratojugals.

Each =quadrate= (fig. 38, 3) forms the outer boundary of the tympanic
cavity, and is firmly united on its inner side with the opisthotic,
exoccipital, and pterygoid. Dorsally it is fixed to the squamosal and
anteriorly to the quadratojugal. Its outer surface is marked by a deep
recess, and it ends below in a strong condyle with which the mandible
articulates. In front of the quadrates are a pair of thin plate-like
bones, the _quadratojugals_ which are united in front to the jugals or
malars.

The _jugals_ (fig. 38, 17) are also thin plate-like bones, and form
part of the posterior boundary of the orbit. They are attached
dorsally to the postfrontals, and anteriorly to the maxillae, while
each also sends inwards a horizontal process which meets the pterygoid
and palatine.

The _maxillae_ (figs. 38 and 39, 16) are a pair of large
vertically-placed bones, each drawn out ventrally into a straight,
sharp, cutting edge. They form the lateral boundaries of the anterior
nares, and each sends dorsalwards a process which meets the
postfrontal. Each also sends inwards a horizontal =palatine process=,
which meets the palatine and vomer, and also forms much of the floor
of the narial passage.

The _premaxillae_ (figs. 38 and 39, 15) are a pair of very small bones
forming the floor of the anterior narial opening, they are wedged in
between the two maxillae, and send back processes which meet the vomer
and palatines.

The _palatines_ (fig. 39, 10) are a pair of small bones firmly united
with the pterygoids behind, with the maxillae and jugals externally,
and with the vomer in the middle line. Each also gives off a palatine
plate which unites with the expanded lower edge of the vomer, and
forms the ventral boundary of the posterior nares. Anteriorly the
palatines form the posterior boundary of a large foramen through which
the ophthalmic branches of the fifth and seventh nerves pass to the
olfactory organs.

The _pterygoids_ (fig. 39, 9) are a pair of large bones which unite
with one another by a long median suture. They are united also with
the palatines in front, and with the quadrate, basisphenoid,
basi-occipital, and exoccipitals behind. Each also sends dorsalwards a
short =alisphenoid plate= which meets that from the parietal.

Piercing the posterior end of the _pterygoid_ is the prominent opening
of the carotid canal; a bristle passed into this hole emerges through
a foramen lying between the pro-otic and the alisphenoid process of
the pterygoid.


(2) THE LOWER JAW OR MANDIBLE.

The =mandible= consists of one unpaired bone, formed by the fusion of
the two _dentaries_, and five pairs of bones, called respectively the
=articular=, _angular_, _supra-angular_, _splenial_ and _coronoid_.

The fused _dentaries_ (fig. 38, 12) form by far the largest of the
bones; they constitute the flattened anterior part of the mandible,
and extend back below the other bones almost to the end of the jaw.

The _coronoid_ is the most anterior of the paired bones, it forms a
prominent process to which the muscles for closing the jaw are
attached.

The =articular= (fig. 38, 11) is expanded, and with the
_supra-angular_ forms the concave articulating surface for the
quadrate.

The _splenial_ (fig. 38, 10) is a thin plate applied to the inner
surface of the posterior part of the mandible.

The _angular_ (fig. 38, 13) is a slender plate of bone lying below the
supra-angular and splenial.


(3) THE HYOID.

The hyoid apparatus is well developed, parts of the first two
branchial arches being found, as well as of the hyoid proper. It
consists of a more or less oblong flattened =basilingual plate= or
=body of the hyoid= which represents the fused ventral ends of the
hyoid and branchial arches of the embryo, and is drawn out into a
point anteriorly. The greater part is formed of unossified cartilage,
but at the posterior end it is bilobed, and a pair of ossified tracts
occur. To its sides are attached three pairs of structures, which are
portions of the hyoid and first and second branchial arches
respectively.

The free part of the =hyoid= consists of a small piece of cartilage
attached to the anterior part of the basilingual plate at its widest
portion (fig. 53, 2).

The =anterior cornu= or free part of the =first branchial arch= is
much the largest of the three structures. Its proximal portion
adjoining the basilingual plate is cartilaginous, as is its distal
end; the main part is however ossified.

The =posterior cornu= or free part of the =second branchial arch=
(fig. 53, 4) consists of a short flattened cartilaginous bar arising
from the bilobed posterior end of the basilingual plate.

The hyoid apparatus has no skeletal connection with the rest of the
skull.


2. THE APPENDICULAR SKELETON.

This includes the skeleton of the two pairs of limbs and their
girdles.


THE PECTORAL GIRDLE.

The pectoral girdle has an anomalous position, being situated
internal or ventral to the ribs. It consists of three bones, a dorsal
bone, the =scapula=, an anterior ventral bone, the =precoracoid=, and
a posterior ventral bone, the =coracoid=.

The =scapula= is a small somewhat rod-shaped bone forming about
two-thirds of the glenoid cavity. At its proximal end it is closely
united with the precoracoid, the two bones ossifying continuously. It
tapers away distally, and is directed dorsalwards towards the
carapace.

The =precoracoid= forms an angle of about 130° with the scapula, with
which it is completely fused at its proximal end. Its distal end is
somewhat expanded and flattened, and is terminated by a
fibrocartilaginous =epiprecoracoid= which meets its fellow. It takes
no part in the formation of the glenoid cavity.

The =coracoid= is a large flattened blade-shaped bone forming about
one-third of the glenoid cavity. It does not meet its fellow in a
ventral symphysis, and is terminated by a cartilaginous =epicoracoid=.
The glenoid articulating surfaces of both scapula and coracoid are
lined by a thick pad of cartilage.


THE ANTERIOR LIMB.

This is divisible into three portions, the =upper arm=, =fore-arm= and
=manus=.

The =upper arm= contains a single bone, the =humerus=.

The =humerus= (fig. 40, A, 1) is a stout, nearly straight, somewhat
flattened bone widely expanded at both ends. At the proximal end is
the large hemispherical =head=, which articulates with the glenoid
cavity. Behind the head the bone is drawn out into another large
rounded process. Below the head the shaft bears a small outgrowth
which is continuous with a larger one on the flexor surface (see p.
29). The bone is terminated distally by the =trochlea=, consisting of
three partially distinct convex surfaces which articulate with the
bones of the fore-arm.

The =fore-arm= includes two bones, the =radius= and =ulna=; both these
are small bones, and are immovably fixed to one another proximally and
distally.

The =radius= or pre-axial bone is the larger of the two, and is a
rod-like bone terminated at either end by an epiphysis. It articulates
at its proximal end with the humerus, and at its distal end with the
radiale or scaphoid bone of the carpus.

The =ulna= (fig. 40, A, 3) or postaxial bone is shorter than the
radius, and more expanded at its proximal end, where it articulates
with the humerus. It articulates distally with the intermedium (lunar)
and the ulnare (cuneiform) bones of the carpus. All three bones of the
arm have their terminations formed by epiphyses which ossify from
centres distinct from those forming the shafts.

The =Manus= consists of the =carpus= or =wrist= and the =hand= which
includes the metacarpals and phalanges.

The =carpus= consists of a series of ten small bones, one of which,
the =pisiform= (fig. 40, A, 10), differs from the others in being
merely an ossification in the tendon of a muscle. The remaining nine
bones are arranged in a proximal row of three, the =ulnare= (fig. 40,
A, 6), =intermedium=, and =radiale=, and a distal row of five
(carpalia 1-5), each of which supports one of the metacarpals. A ninth
bone, the =centrale= (fig. 40, A, 7), is wedged in between the two
rows. The ulnare, intermedium and pisiform are comparatively large
flattened bones, the others are small and cubical.

The =hand=. This is composed of five digits, each of which consists of
a metacarpal and of a varying number of phalanges.

The =metacarpals=. The first metacarpal (fig. 40, A, 11) is a short
flattened bone, the others are all elongated and cylindrical, and are
terminated proximally by slightly concave surfaces, and distally by
slightly convex ones.

The =phalanges=. The first and fifth digits both have two phalanges,
the second, third, and fourth have each three. The distal phalanx of
the first digit is stout and curved, and bears a horny claw; those of
the other digits are flattened and more or less pointed.

[Illustration FIG. 40. A. ANTERIOR LIMB OF A YOUNG HAWKSBILL TURTLE
(_Chelone imbricata_) × 1/4 (Brit. Mus.). B. POSTERIOR LIMB OF A LARGE
GREEN TURTLE (_Chelone midas_) × 1/8 (Camb. Mus.).

    1. humerus.
    2. radius (almost hidden by the ulna).
    3. ulna.
    4. radiale.
    5. intermedium.
    6. ulnare.
    7. centrale.
    8. carpale I.
    9. carpale IV.
    10. pisiform.
    11. first metacarpal.
    12. femur.
    13. tibia.
    14. fibula.
    15. tibiale intermedium and centrale fused.
    16. fibulare.
    17. tarsale 1.
    18. tarsale 2.
    19. tarsalia 4 and 5 fused.
    20. first metatarsal.
    21. fifth metatarsal.
    I, II, III, IV, V, digits.]


THE PELVIC GIRDLE.

The pelvic girdle consists of three bones; a dorsal bone, the =ilium=,
an anterior ventral bone, the =pubis=, and a posterior ventral bone,
the =ischium=. All three bones contribute largely to the formation of
the =acetabulum=, with which the head of the femur articulates.

The =ilium= is a small slightly curved bone, which unites ventrally
with the pubis and ischium, and extends dorsalwards and backwards to
meet the distal ends of the sacral ribs.

The =pubis= is the largest bone of the three; its distal end forms a
wide bilobed plate, the inner lobe meeting its fellow in a median
symphysis, while the other lobe or lateral process extends outwards.
Attached to the symphysis in front is a cartilaginous =epipubis=,
while behind, the two pubes are terminated by a wide rounded
cartilaginous area.

The =ischium=, the smallest bone of the three, is flattened and like
the pubis meets its fellow in a median symphysis. A narrow band of
cartilage connects the symphysis pubis with the symphysis ischii, and
separates the two =obturator foramina= from one another.


THE POSTERIOR LIMB.

This is divisible into three portions, the =thigh=, the =crus= or
=shin=, and the =pes=.

The =thigh= includes a single bone, the =femur=.

The =femur= (fig. 40, B, 12) is a short thick bone, with a prominent
rounded =head= articulating with the acetabulum. Behind this head is a
deep pit, beyond which is a roughened area corresponding with the
great trochanter of mammals. The distal end is expanded and somewhat
convex.

The bones of the =crus= or =shin= are the =tibia= and =fibula=. These
are both straight rod-like bones with expanded terminations which
closely approach one another, while elsewhere the bones diverge
considerably.

The terminations of all three of the leg bones are formed by
epiphyses.

The =Pes= consists of the =tarsus= or =ankle=, and the =foot=, which
is made up of five digits.

The =tarsus=. The tarsal bones of the Turtle do not retain their
primitive arrangement to such an extent as do the carpals. They are
arranged in a proximal row of two and a distal row of four. Of the
bones in the proximal row the postaxial one is much the smaller and is
the =fibulare=; the larger pre-axial one (fig. 40, B, 15) represents
the =tibiale=, =intermedium=, and =centrale= fused, and articulates
with both tibia and fibula. The first three distal tarsalia are all
small bones and are very similar in size, and each articulates
regularly with the corresponding metatarsal. The fourth bone (fig. 40,
B, 19) is much larger, and represents tarsalia 4 and 5 fused. The
first two distal tarsalia articulate with the pre-axial tarsal of the
proximal row, the third only with its neighbours the second, and the
fused fourth and fifth. The latter articulates with both bones of the
proximal row.

Each =digit= consists of a metatarsal and of a varying number of
phalanges.

The =metatarsals=. The first metatarsal (fig. 40, B, 20) is broad and
flattened, the second, third and fourth, are all elongated bones with
nearly flat terminations formed by small epiphyses. The fifth is large
and flattened, and the articular surface for the phalanx is situated
somewhat laterally.

The =phalanges=. The first digit has two phalanges and is the stoutest
of them all; its distal phalanx is sheathed in a large horny claw. The
other digits, of which the third is the longest, have each three
phalanges. The distal phalanges of the second and third digits are
flattened and pointed and bear small horny claws.


FOOTNOTES:

[89] Another view commonly held is that the neural and costal plates
are respectively formed by the expanded neural arches and ribs.




CHAPTER XV.

THE SKELETON OF THE CROCODILE.


The species chosen for description is _C. palustris_, a form occurring
throughout the Oriental region, but the description would apply almost
equally well to any of the other species of the genus _Crocodilus_,
and with comparatively unimportant modifications to any of the living
Crocodilia.


I. EXOSKELETON.

The exoskeleton of the Crocodile is strongly developed and includes
elements of both epidermal and dermal origin.

_a._ The =epidermal exoskeleton= is formed of a number of horny
=scales= or plates of variable size covering the whole surface of the
body. Those covering the dorsal and ventral surfaces are oblong in
shape, and are arranged in regular rows running transversely across
the body. The scales covering the limbs and head are mostly smaller
and less regularly arranged, and are frequently raised into a more or
less obvious keel. Those covering the dorsal surface of the tail are
very prominently keeled.

The epidermal exoskeleton also includes the horny =claws= borne by the
first three digits of both manus and pes.

_b._ The =dermal exoskeleton=. This has the form of bony =scutes=
which underlie the epidermal scales along the dorsal surface of the
trunk and anterior part of the tail. Except in very young individuals
the epidermal scales are rubbed off from these scutes, which
consequently come to project freely on the surface of the body. Each
scute is a nearly square bony plate, deeply pitted or sculptured, and
marked by a strong ridge on its dorsal surface, while its ventral
surface is smooth. Contiguous scutes are united to one another by
interlocking sutures.

The scutes are arranged in two distinct areas, viz. (1) a small
anterior =nuchal shield= which lies just behind the head and is formed
of six large scutes more or less firmly united together, and (2) a
larger posterior =dorsal shield= covering the whole of the back and
anterior part of the tail, and formed of smaller scutes, which are
arranged in regular transverse rows, and progressively diminish in
size when followed back.

The =teeth= are exoskeletal structures, partly of dermal, partly of
epidermal origin. They lie along the margins of the jaws and are
confined to the premaxillae, maxillae and dentaries. They are simple
conical structures, without roots; each is in the adult placed in a
separate socket, and is replaced by another which as it grows comes to
occupy the pulp cavity of its predecessor. In the young animal the
teeth are not placed in separate sockets but in a continuous groove.
This feature is met with also in the Ichthyosauria. The groove
gradually becomes converted into a series of sockets by the ingrowth
of transverse bars of bone. The anterior teeth are sharply pointed and
slightly recurved, the posterior ones are more blunt.

The upper jaw bears about nineteen pairs of teeth, the lower jaw about
fifteen pairs. The largest tooth in the upper jaw is the tenth, and in
the lower jaw the fourth.

The three living families of Crocodilia, the Crocodiles, Alligators
and Garials, can be readily distinguished by the characters of the
first and fourth lower teeth. In Alligators both first and fourth
lower teeth bite into pits in the upper jaw; in Garials they both bite
into notches or grooves in the upper jaw. In Crocodiles the first
tooth bites into a pit, the fourth into a notch in the upper jaw.


II. ENDOSKELETON.

1. THE AXIAL SKELETON.

This includes the vertebral column, the skull, and the ribs and
sternum.

A. THE VERTEBRAL COLUMN.

The vertebral column is very long, consisting of some sixty vertebrae.
It can be divided into the usual five regions, the cervical, thoracic,
lumbar, sacral, and caudal regions.

[Illustration FIG. 41. FIRST FOUR CERVICAL VERTEBRAE OF A CROCODILE
(_C. vulgaris_). (Partly after VON ZITTEL.)

    1. pro-atlas.
    2. lateral portion of atlas.
    3. odontoid process.
    4. ventral portion of atlas.
    5. neural spine of axis.
    6. postzygapophysis of fourth vertebra.
    7. tubercular portion of fourth cervical rib.
    8. first cervical rib.
    9. second cervical rib.
    10. convex posterior surface of centrum of fourth vertebra.]

THE CERVICAL VERTEBRAE.

Counting as cervical all those vertebrae which are anterior to the
first one whose ribs meet the sternum, there are nine cervical
vertebrae, all of which bear ribs.

As a type of the cervical vertebrae the fifth may be taken. It has a
short cylindrical =centrum= deeply concave in front and convex behind.
From the anterior part of the ventral surface of the centrum arises a
short =hypapophysis=, and on each side is a facet with which the lower
limb (=capitulum=) of the cervical rib articulates. The =neural arch=
is strongly developed and drawn out dorsally into a long =neural
spine=, in front of which are a pair of upstanding processes bearing
the prominent upwardly and inwardly directed =prezygapophyses=. At the
sides and slightly behind the neural spine are a corresponding pair of
processes bearing the =postzygapophyses=, which look downwards and
outwards. At the point where it joins the centrum the neural arch is
drawn out into a short blunt =transverse process= with which the upper
limb (=tuberculum=) of the cervical rib articulates. The sides of the
neural arch are slightly notched behind for the exit of the spinal
nerves.

The first or =atlas= vertebra differs much from any of the others, and
consists of four quite detached portions, a ventral arch, with two
lateral portions and one dorsal. The =ventral arch= (fig. 41, 4) is
flat below and slightly concave in front, forming together with two
flattened surfaces on the lateral portions a large articulating
surface for the occipital condyle of the skull. Its posterior face is
bevelled off and forms with a second pair of facets on the lateral
portions a surface with which the odontoid process of the second
vertebra articulates. The postero-lateral surfaces of the ventral arch
also bear a pair of little facets with which the cervical ribs
articulate. The lateral portions are somewhat flattened and expanded,
and bear in addition to those previously mentioned a pair of small
downwardly directed facets, the postzygapophyses, which articulate
with the prezygapophyses of the second vertebra. The dorsal portion
(fig. 41, 1) is somewhat triangular in shape, and overhangs the
occipital condyle. It is often regarded as the neural arch of a
vertebra in front of the atlas and is called the _pro-atlas_; but as
it is a membrane bone it is not properly a vertebral element.

The second or =axis vertebra= also differs a good deal from the other
cervicals. The centrum is massive, and is terminated in front by a
very large slightly concave articulating surface formed by the
=odontoid process= (fig. 41, 3) which is united with the centrum by
suture only, and is really the detached centrum of the first vertebra.
The cervical rib (fig. 41, 9) articulates with two little
irregularities on the odontoid process. The posterior surface of the
centrum is convex. The neural arch is strongly developed and
terminated dorsally by a long neural spine (fig. 41, 5), its sides are
notched, slightly in front and more prominently behind for the exit of
the spinal nerves. It is drawn out in front into two little processes
bearing a pair of upwardly and outwardly directed prezygapophyses,
while the postzygapophyses are similar to those of the other cervical
vertebrae.

The last two cervical vertebrae resemble the succeeding thoracic
vertebrae, in the increased length of the transverse processes, and
the shifting dorsalwards of the facet with which the capitulum of the
rib articulates.

THE THORACIC VERTEBRAE.

The thoracic vertebrae commence with the first of those that bears
ribs reaching the sternum. They are ten in number, and the first eight
are directly connected with the sternum by ribs.

The =third= of them may be taken as a type. It has a thick cylindrical
centrum, concave in front and convex behind, there is a slight
hypapophysis, and the centrum is suturally united with a strong neural
arch enclosing a narrow neural canal. The neural arch is drawn out
dorsally into a wide truncated neural spine, and laterally into two
prominent transverse processes, with the ends of which the tubercula
of the ribs articulate, while the capitulum articulates in each case
with a step-like facet (fig. 42, A, 3) on the anterior face of the
transverse process. The prezygapophyses (fig. 42, A, 2) are borne on
outgrowths from the bases of the transverse processes, and the
postzygapophyses on outgrowths at the base of the neural spine.

The thoracic vertebrae behind the third have no hypapophyses, and the
capitular facets gradually come to be placed nearer and nearer the
ends of the transverse processes, at the same time becoming less
prominent; otherwise these vertebrae are just like the third.

[Illustration FIG. 42. ANTERIOR VIEW OF A, A LATE THORACIC AND B, THE
FIRST SACRAL VERTEBRA OF A YOUNG CROCODILE (_C. palustris_). × 1/3.

    1. neural spine.
    2. process bearing prezygapophysis.
    3. facet for articulation with the capitulum of the rib.
    4. sacral rib.
    5. surface which is united with the ilium.
    6. concave anterior face of centrum.]

In the first and second thoracic vertebrae the capitulum of the rib
articulates, not with a facet on the transverse process, but with a
little elevation borne at the line of junction of the centrum and
neural arch.

THE LUMBAR VERTEBRAE.

These are five in number, and are precisely like the posterior
thoracic vertebrae, except in the fact that the transverse processes
have no facets for the articulation of ribs.

THE SACRAL VERTEBRAE.

These are two in number, and while the centrum of the first is concave
in front (fig. 42, B, 6) and nearly flat behind, that of the second is
flat in front and concave behind. Each has a pair of strong =ribs=
(fig. 42, B, 4) firmly ankylosed in the adult with a wide surface
furnished partly by the centrum, partly by the neural arch. The distal
ends of these ribs are united with the ilia. The character of the
neural spines and zygapophyses is the same as in the thoracic
vertebrae.

THE CAUDAL VERTEBRAE.

These are very numerous, about thirty-four in number. The first
differs from all the other vertebrae of the body in having a biconvex
centrum. The succeeding ones are procoelous and are very much like the
posterior thoracic and lumbar vertebrae, having high neural spines and
prominent straight transverse processes. They differ however in having
the neural spines less strongly truncated above, and the transverse
processes arise from the centra and not from the neural arches. When
followed further back the centra and neural spines gradually lengthen
while the transverse processes become reduced, and after the twelfth
vertebra disappear. Further back still the neural spines and
zygapophyses gradually become reduced and disappear, as finally the
neural arch does also, so that the last few vertebrae consist simply
of cylindrical centra.

Each caudal vertebra, except the first and the last eleven or so, has
a =V=-shaped =chevron bone= attached to the postero-ventral edge of
its centrum. The anterior ones are the largest and they gradually
decrease in size till they disappear.

B. THE SKULL[90].

The skull of the Crocodile is a massive depressed structure
presenting a number of striking characteristics, some of the more
important of which are:--

1. All the bones except the mandible, hyoid, and columella are firmly
united by interlocking sutures. In spite of this, however, growth of
the whole skull and of the component bones goes on continuously
throughout life, this growth being especially marked in the case of
the facial as opposed to the cranial part of the skull.

2. All the bones appearing on the dorsal surface are remarkable for
their curious roughened and pitted character; this feature is
prominent also in many Labyrinthodonts.

3. The size of the jaws and teeth is very great.

4. The mandibular condyle is carried back to some distance behind the
occipital condyle.

5. The occipital plane (see p. 386) of the skull is vertical.

6. The length of the secondary palate is remarkably great, and the
vomer takes no part in its formation.

7. The posterior nares are placed very far back, the nasal passages
being as in mammals separated from the mouth by the long secondary
palate.

8. There is a complicated system of Eustachian passages communicating
at one end with the tympanic cavity and at the other end with the
mouth cavity.

9. The interorbital septum is mainly cartilaginous, the presphenoidal
and orbitosphenoidal regions remaining unossified.

The =skull= is divisible into three parts:--

(1) the cranium, (2) the lower jaw, (3) the hyoid.

The =cranium= may again for purposes of description be divided into:--

1. the cranium proper or brain case;

2. the bones connected with the several special sense organs;

3. the bones of the upper jaw, and suspensorial apparatus.


1. THE CRANIUM PROPER OR BRAIN CASE.

[Illustration FIG. 43. PALATAL ASPECT A, OF THE CRANIUM, B, OF THE
MANDIBLE OF AN ALLIGATOR (_Caiman latirostris_). × 1/3. (Brit. Mus.)

    1. premaxillae.
    2. maxillae.
    3. palatine.
    4. pterygoid.
    5. posterior nares.
    6. transpalatine.
    7. posterior palatine vacuity.
    8. anterior palatine vacuity.
    9. basi-occipital.
    10. opening of median Eustachian canal.
    11. jugal.
    12. quadratojugal.
    13. quadrate.
    14. dentary.
    15. splenial.
    16. coronoid.
    17. supra-angular.
    18. angular.
    19. articular.
    20. lateral temporal fossa.
    21. openings of vascular canals leading into alveolar sinus.]

The cartilage and membrane bones of the cranium proper when taken
together can in most vertebrates be seen to be more or less arranged
in three rings or segments called respectively the =occipital=,
=parietal= and =frontal= segments; in the Crocodile however only the
occipital and parietal segments are clearly seen.

The =occipital segment= consists of four cartilage bones, three of
which together surround the =foramen magnum=.

The most ventral of these, the =basi-occipital= (figs. 43 and 45, 9),
forms the single convex =occipital condyle= for articulation with the
atlas, bounds the base of the foramen magnum, and is continuous
laterally with two larger bones, the =exoccipitals= (fig. 45, 24),
which meet one another dorsally and form the remainder of the boundary
of the foramen magnum. Each is drawn out externally into a strong
process, which is united below with the quadrate, and above with the
squamosal by a surface seen in a disarticulated skull to be very rough
and splintered. In a longitudinal section the anterior face of the
exoccipital is seen to be closely united with the opisthotic.

The exoccipital is pierced by a number of foramina, four lying on the
posterior surface. Just external to the foramen magnum is a small
foramen for the exit of the hypoglossal nerve (figs. 44 and 45, XII).
External to this is the foramen for the pneumogastric (fig. 44, X),
while more ventrally still is the foramen (fig. 44, 15) through which
the internal carotid artery enters the skull. Some distance further to
the side, and more dorsally, is a larger foramen which gives passage
to the facial nerve and certain blood-vessels.

In a median longitudinal section of the skull the hypoglossal foramen
is seen, and just in front of it a small foramen for a vein. Further
forwards the long slit-like opening between the exoccipital and
opisthotic is the =internal auditory meatus= (fig. 45, VIII) through
which the auditory nerve leaves the cranial cavity and enters the
internal ear.

The =supra-occipital= (fig. 45, 5) is a small bone which takes no part
in the formation of the foramen magnum, and is closely united in front
with the epi-otic. It is characteristic of Crocodiles that all the
bones of the occipital segment have their longer axes placed
vertically, and that they scarcely if at all appear on the dorsal
surface.

In front of the occipital segment is the =parietal segment=. The
dorsal and ventral portions of the two segments are in contact with
one another, but the lateral portions are widely separated by the
interposition of the =auditory= and =suspensorial bones=.

The =basisphenoid= (fig. 45, 12) is an unpaired wedge-shaped bone,
united along a deep vertical suture with the basi-occipital. The two
bones are, however, partially separated in the mid-ventral line by a
foramen, the opening of the =median Eustachian canal=, which leads
into a complicated system of Eustachian passages ultimately
communicating with the tympanic cavity.

The dorsal surface of the basisphenoid is well seen in a section of
the skull, but owing to the way it tapers ventrally, it appears on the
ventral surface only as a very narrow strip of bone wedged in between
the basi-occipital and pterygoids. In a lateral view it is seen to be
drawn out in front into an abruptly truncated process, the =rostrum=,
which forms part of the =interorbital septum.= On the anterior part of
the dorsal surface is a deep pit, the =pituitary fossa= or =sella
turcica=, at the base of which are a pair of foramina, through which
the carotid arteries pass. Dorsolaterally the basisphenoid articulates
with the =alisphenoids=.

[Illustration FIG. 44. LATERAL VIEW OF THE SKULL OF AN ALLIGATOR
(_Caiman latirostris_). × 1/3. (Brit. Mus.)

    1. premaxillae.
    2. maxillae.
    3. lachrymal.
    4. prefrontal.
    5. jugal.
    6. postfrontal.
    7. squamosal.
    8. quadrate.
    9. palatine.
    10. pterygoid.
    11. transpalatine.
    12. quadratojugal.
    13. exoccipital.
    14. basi-occipital.
    15. foramen by which carotid artery enters skull.
    16. external auditory meatus.
    17. frontal.
    18. supra-angular.
    19. articular.
    20. dentary.
    21. coronoid.
    22. angular.
    III, VI, opening for exit of oculomotor and abducens nerves.
    V, foramen ovale.
    X, pneumogastric foramen.
    XII, hypoglossal foramen.]

The =alisphenoids= (fig. 45, 13) are a pair of irregular bones which
arise from the basisphenoid antero-laterally, and are united dorsally
with the parietal, frontal, and postfrontals. They bound most of the
anterior part of the brain case, and each presents on its inner face a
deep concavity which lodges the cerebral hemisphere of its side.
Viewed from the ventral side the two alisphenoids are seen to almost
or quite meet one another immediately below the frontal, and then to
diverge, forming an irregular opening--partially closed by cartilage
in the fresh specimen,--through which the optic nerves leave the
cranial cavity. Further back the alisphenoids meet one another for a
narrow area, and then diverge again, so that between each and the
rostrum of the basisphenoid there appears an opening (fig. 44, III,
VI) through which the oculomotor and abducens nerves leave the
cranium. Further back still each is united for a short space with the
basisphenoid, pterygoid and quadrate, and then becomes separated from
the quadrate by a large foramen, the =foramen ovale= (fig. 44, V),
through which the whole of the trigeminal nerve passes out.

The dorsal portion of the parietal segment is formed by the _parietal_
(fig. 45, 4), which though double in the embryo, early comes to form a
single bone. It extends over the posterior part of the cranial cavity,
and is continuous in front with the frontal, behind with the
supra-occipital, and laterally with the postfrontals, squamosals,
alisphenoids, pro-otics and epi-otics. It forms the inner boundary of
a large rounded vacuity on the roof of the skull, the =supratemporal
fossa=.

The =frontal segment= is very imperfectly ossified, there being no
certain representatives of either the ventral member, the presphenoid,
or the lateral members, the orbitosphenoids. On the dorsal side there
is, however, a large development of membrane bones. There is a large
_frontal_ (fig. 45, 3), unpaired, except in the embryo, united behind
with the parietal and postfrontal, and drawn out in front into a long
process which is overlapped by the prefrontals and posterior part of
the nasals. The frontal ends off freely below, owing to the
orbitosphenoidal region being unossified, it forms a considerable part
of the roof of the cranial cavity, but takes no part in the formation
of the wall.

Each _prefrontal_ (fig. 45, 14) forms part of the inner wall of the
orbit and sends ventralwards a process which meets the palatine.

The _postfrontals_ (fig. 44, 6) are small bones lying at the sides of
the posterior part of the frontal. Each is united with a number of
bones, on its inner side with the frontal and parietal, behind with
the squamosal, and ventrally with the alisphenoid. It also unites by
means of a strong descending process with an upgrowth from the jugal,
and thus forms a =postorbital bar= separating the orbit from the
lateral temporal fossa. The postfrontal forms also part of the outer
boundary of the supratemporal fossa.


2. THE SENSE CAPSULES.

Skeletal capsules occur in connection with each of the three special
sense organs of sight, of hearing and of smell.

The =Auditory capsules= and associated bones.

Three bones, the =epi-otic=, =opisthotic= and =pro-otic=, together
form the auditory or =periotic= capsule of each side. They are wedged
in between the lateral portions of the occipital and parietal segments
and complete the cranial wall in this region. Their relations to the
surrounding structures are very complicated, and many points can be
made out only in sections of the skull passing right through the
periotic capsule. The relative position of the three bones is,
however, well seen in a median longitudinal section. The =opisthotic=
early becomes united with the exoccipital, while the =epi-otic=
similarly becomes united with the supra-occipital, the =pro-otic=
(fig. 45, 7),--seen in longitudinal section to be pierced by the
prominent =trigeminal foramen=--alone remaining distinct throughout
life. The three bones together surround the essential organ of hearing
which communicates laterally with the deep tympanic cavity by the
=fenestra ovalis=.

The =tympanic cavity=, leading to the exterior by the =external
auditory meatus= (fig. 44, 16), is well seen in a side-view of the
skull; it is bounded on its inner side by the periotic bones,
posteriorly in part by the exoccipital, and elsewhere mainly by the
quadrate. A large number of canals and passages open into it. On its
inner side opening ventro-anteriorly is the =fenestra ovalis=, opening
ventro-posteriorly the =internal auditory meatus= (fig. 45, VIII),
while dorsally there is a wide opening which forms a communication
through the roof of the brain-case with the tympanic cavity of the
other side. On its posterior wall is the prominent foramen through
which the facial nerve passes on its way to its final exit from the
skull through the exoccipital, this foramen is bounded by the
quadrate, squamosal, and exoccipital.

The opening of the fenestra ovalis is in the fresh skull occupied by
the expanded end of the auditory ossicle, the =columella=, whose outer
end articulates by a concave facet with a trifid =extra-columellar=
cartilage which reaches the tympanic membrane. The lower process of
this extra-columella passes into a cartilaginous rod which lies in a
canal in the quadrate and is during life continuous with Meckel's
cartilage within the articular bone of the mandible.

The columella and extra-columella are together homologous with the
chain of mammalian auditory ossicles.

       *       *       *       *       *

The =Optic capsules= and associated bones.

Two pairs of bones are associated with the optic capsules, viz. the
_lachrymals_ and the _supra-orbitals_. The _lachrymal_ (fig. 44, 3) is
a fairly large flattened bone lying wedged in between the maxillae,
nasal, jugal, and prefrontal. It forms a considerable part of the
anterior boundary of the orbit, and is pierced by two foramina. On the
orbital edge is a large hole leading into a cavity within the bone
which lodges the naso-lachrymal sac, and communicates with the narial
passage by a wide second foramen near the anterior end of the bone.
The _supra-orbital_ is a very small loose bone lying in the eyelid
close to the junction of the frontal and prefrontal.

       *       *       *       *       *

The =Olfactory capsules= and associated bones.

Two pairs of membrane bones, the _vomers_ and _nasals_, are developed
in association with the olfactory organ, but the =mesethmoid= is not
ossified.

The _vomers_ form a pair of delicate bones, each consisting of a
vertical plate (fig. 45, 15), which with its fellow separates the two
narial passages, and of a horizontal plate which forms much of their
roof. The vomers articulate with one another and with the pterygoids,
palatines, and maxillae.

The _nasals_ (fig. 45, 2) are very long narrow bones extending along
the middle line from the frontal almost to the anterior nares. They
are continuous laterally with the premaxillae, maxillae, lachrymals and
prefrontals. They form the roof of the narial passages.

3. THE UPPER JAW AND SUSPENSORIAL APPARATUS.

These are enormously developed in the Crocodile and are firmly united
to the cranium. It will be most convenient to begin by describing the
bones at the anterior end of the jaw and to work back thence towards
the brain-case. The most anterior bones are the _premaxillae_. The
_premaxillae_ (figs. 44 and 45, 1) are small bones, each bearing five
pairs of teeth, set in separate sockets in their alveolar borders.
They constitute almost the whole of the boundary of the =anterior
nares=, which are confluent with one another and form a large
semicircular opening in the roof of the skull, leading into the wide
narial passage. They are also partially separated from one another in
the ventral middle line, by the small =anterior palatine vacuity=
(fig. 43, A, 8). They form the anterior part of the broad =palate=.
The alveolar border on each side between certain of the teeth is
marked by pits which receive the points of the teeth of the other jaw.
The first pair of these pits in the premaxillae are often so deep as to
be converted into perforations. Pits of the same character occur
between the maxillary and mandibular teeth.

[Illustration FIG. 45. LONGITUDINAL SECTION THROUGH THE SKULL OF AN
ALLIGATOR (_Caiman latirostris_). × 1/3. (Brit. Mus.)

    1. premaxilla.
    2. nasal.
    3. frontal.
    4. parietal.
    5. supra-occipital.
    6. epi-otic.
    7. pro-otic.
      immediately in front of the figure 7 is the prominent foramen
      for the trigeminal nerve.
    8. opisthotic.
    9. basi-occipital.
    10. quadrate.
    11. pterygoid.
    12. basisphenoid.
    13. alisphenoid.
    14. prefrontal.
    15. vomer.
    16. maxilla.
    17. palatine.
    18. dentary.
    19. splenial.
    20. angular.
    21. supra-angular.
    22. articular.
    23. coronoid.
    24. exoccipital.
    25. squamosal.
    26. jugal.
    27. external mandibular foramen.
    28. internal mandibular foramen.
    VIII. internal auditory meatus.
    XII. hypoglossal foramen.]

The _maxillae_ (figs. 43, A, 2 and 44, 2) are a pair of very large
bones and bear the remaining teeth of the upper jaw, set in sockets
along their alveolar borders. On the dorsal side each maxillae is
continuous with the premaxillae, nasal, lachrymal, and jugal, while
ventrally it meets its fellow in a long straight suture and forms the
greater part of the long bony palate. The maxillae are separated in the
middle line posteriorly by processes from the palatines, while further
back they meet the transpalatines. The internal or nasal surface, like
that of the premaxillae, is excavated by a deep longitudinal groove,
the =narial passage=. In a ventral view of the skull a number of small
openings (fig. 43, A, 21) are seen close to the alveolar border, these
are the openings of small vascular canals which lead into the
=alveolar sinus=, a passage traversing the maxillae, and transmitting
the superior maxillary branch of the trigeminal nerve and certain
blood-vessels. This alveolar sinus opens posteriorly by the more
external of the two large holes in the maxillae, which lie close to the
anterior edge of the posterior palatine vacuity, to be described
immediately. The more internal of these holes, on the other hand,
leads into a cavity lodging the nasal sac. Behind the maxillae the
completeness of the palate is broken up by the large oval =posterior
palatine vacuities= (fig. 43, A, 7); these are separated from one
another in the middle line by the palatines, and are bounded elsewhere
by the maxillae, transpalatines, and pterygoids.

The _palatines_ (fig. 43, A, 3) are long and rather narrow bones
interposed between the maxillae in front and pterygoids behind. They
meet one another in a long suture and form much of the posterior part
of the palate, while the whole length of their dorsal surface
contributes to the floor of the narial passage. The dorsal surface of
each bone is also drawn out on its outer side into a prominent ridge
which forms much of the side and roof of the narial passage, being in
contact with the vomer and pterygoid, and at one point by means of a
short ascending process with the descending process of the
prefrontal.

The _pterygoids_ (figs. 43, A, 4, and 45, 11) are a pair of large
bones, each consisting of a median more or less vertical part, which
becomes ankylosed to its fellow in the middle line early in life, and
of a wide horizontal part which meets the transpalatine. They
completely surround the posterior nares (fig. 43, A, 5) and their
median portions form the whole boundary of the posterior part of the
narial passage, and assist the palatines and vomers in bounding the
middle part. The horizontal parts form the posterior part of the
secondary palate, while the dorsal surface of each looks into the
=pterygoid fossa=, a large cavity lying below the quadrate and
quadratojugal at the side of the skull. The lateral margin adjoining
the transpalatine is in the fresh skull terminated by a plate of
cartilage against which the mandible plays. Dorsally the pterygoid
articulates with the basisphenoid, quadrate, and alisphenoid.

The _transpalatines_ (fig. 44, 11) connect the pterygoids with the
jugals and maxillae, articulating with each of the three bones by a
long pointed process. The jugal process meets also a down-growth from
the postfrontal.

The _jugals_ or _malars_ (fig. 44, 5) are long somewhat flattened
bones which are united to the lachrymals and maxillae in front, while
passing backwards each is united behind to the _quadratojugal_ (fig.
44, 12), the two forming the =infratemporal arcade= which constitutes
the external boundary of the orbit and lateral temporal fossa. The
jugal is united below to the transpalatine, and the two bones together
form an outgrowth, which meeting that from the postfrontal forms the
=postorbital bar=, and separates the orbit from the lateral temporal
fossa. The quadratojugals are small bones and are united behind with
the quadrates.

The =quadrate= (figs. 43, A, 13 and 44, 8) of each side is a large
somewhat flattened bone firmly fixed in among the other bones of the
skull. It is terminated posteriorly by an elongated slightly convex
surface, coated with cartilage in the fresh skull, by which the
mandible articulates with the cranium. The dorsal surface of the
quadrate is flat behind, further forwards it becomes much roughened
and articulates with the exoccipital and squamosal; further forwards
still it becomes marked by a deep groove which forms the floor of the
external auditory meatus and part of the tympanic cavity. The anterior
boundary of the quadrate is extremely irregular, it is united dorsally
with the postfrontal, pro-otic, and squamosal, and more ventrally with
the alisphenoid. The smooth ventral surface looks into the pterygoid
fossa. In front the quadrate forms the posterior boundary of the
supratemporal fossa and foramen ovale, and is continuous with the
alisphenoid, while it sends down a thin plate meeting the pterygoid
and basisphenoid. On the inner side of the dorsal surface of the
quadrate near the condyle, is a small foramen which leads into a tube
communicating with the tympanic cavity, by a foramen lying in front of
and ventral to that for the exit of the facial nerve. By this tube air
can pass from the tympanic cavity into the articular bone of the
mandible.

The _squamosal_ (fig. 44, 7) meets the quadrate and exoccipital below,
and forms part of the roof of the external auditory meatus, while
above it forms part of the roof of the skull and has a pitted
structure like that of the other bones of the roof. It is continuous
with the postfrontal in front, forming with it the =supratemporal
arcade= which constitutes the outer boundary of the supratemporal
fossa. It meets also the parietal on its inner side, forming the
=post-temporal bar=, the posterior boundary of the supratemporal
fossa.

It may be useful to recapitulate the large vacuities in the surface of
the Crocodile's cranium.

_Dorsal surface._

1. =The Supratemporal fossae=. Each is bounded internally by the
parietal, behind by the =post-temporal bar= formed by the parietal and
squamosal, and externally by the =supratemporal arcade= formed by the
squamosal and postfrontal. The postfrontal meets the parietal in front
and forms the anterior boundary of the supratemporal fossa.

2. The =Lateral temporal= or =infratemporal fossae=. These lie below
and to the outer side of the supratemporal fossae. Each is bounded
dorso-internally by the supratemporal arcade; and behind by a
continuation of the post-temporal bar formed by the quadrate and
quadratojugal. The external boundary is the =infratemporal arcade=
formed of the quadratojugal and jugal, while in front the fossa is
separated from the orbit by the =postorbital bar= formed by the
junction of outgrowths from the postfrontal and jugal.

3. The =Orbits=. Each is bounded behind by the postorbital bar,
externally by the jugal forming a continuation of the infratemporal
arcade, in front by the lachrymal, and internally by the frontal and
prefrontal.

4. The =Anterior nares=. These form an unpaired opening bounded by the
premaxillae.

_Posterior surface._

5. The =Foramen magnum=. The exoccipitals form the chief part of its
boundary, but part of the ventral boundary is formed by the
basi-occipital.

6. The =Pterygoid fossae=. These form a pair of large cavities at the
sides of the occipital region of the skull. The dorsal boundary is
formed by the quadrate and quadratojugal, the ventral by the
pterygoid, the internal chiefly by the quadrate, pterygoid,
alisphenoid, and basisphenoid. The transpalatine forms a small part of
the external boundary which is incomplete.

_Ventral surface._

7. The =Posterior nares=. These form a median unpaired opening (fig.
43, A, 5) bounded by the pterygoids.

8. The =Posterior palatine vacuities=. Each is bounded by the maxillae
in front, the maxillae and transpalatine externally, the transpalatine
and pterygoid behind, and the palatine on the inner side (fig. 43, A,
7).

9. The =Anterior palatine vacuity=. This is unpaired and is bounded by
the premaxillae (fig. 43, A, 8).


(_b_) THE LOWER JAW OR MANDIBLE.

The mandible is a strong compact bony structure formed of two halves
or =rami=, which are suturally united at the symphysis in the middle
line in front. Each ramus is formed of six separate bones.

The most anterior and largest of these is the _dentary_ (figs. 44, 20,
and 45, 18), which forms the symphysis, and greater part of the
anterior half of the jaw, and bears along the outer part of its dorsal
border a number of sockets or =alveoli= in which the teeth are placed.
Lying along the inner side of the dentary is a large splint-like bone,
the _splenial_ (fig. 45, 19), which does not extend so far forwards as
the symphysis, and is separated from the dentary posteriorly by a
large cavity. Forming the lower part of all the posterior half of the
jaw is the large _angular_ (figs. 44, 22, and 45, 20), which underlies
the posterior part of the dentary in front and sends a long process
below that bone to the splenial. On the inner side of the jaw there is
an oval vacuity, the =internal mandibular foramen= (fig. 45, 28),
between the angular and the splenial; through this pass blood-vessels
and branches of the inferior dental nerve. Lying dorsal to the angular
is another large bone, the _supra-angular_ (figs. 44, 18, and 45, 21).
It extends back as far as the posterior end of the jaw and forwards
for some distance dorsal to the dentary and splenial. It forms part of
the posterior margin of a large vacuity, the =external mandibular
foramen=, which is bordered above and in front by the dentary and
below by the angular; it gives passage to the cutaneous branch of the
inferior dental nerve. The concave surface for articulation with the
mandible and much of the posterior end of the jaw is formed by a short
but solid bone, the =articular= (fig. 45, 22), which in young skulls
rather readily becomes detached. The remaining mandibular bone is the
_coronoid_ (fig. 45, 23), a very small bone of irregular shape
attached to the angular below, and to the supra-angular and splenial
above.

(_c_) THE HYOID.

The hyoid of the Crocodile consists of a wide flattened plate of
cartilage, the =basilingual plate= or =body of the hyoid=, and a pair
of =cornua=.

The =basilingual plate= (fig. 53, 1) is rounded anteriorly and marked
by a deep notch posteriorly. The =cornua= (fig. 53, 3), which are
attached at a pair of notches near the middle of the outer border of
the basilingual plate, are partly ossified, but their expanded ends
are formed of cartilage. They pass at first backwards and then upwards
and inwards. They are homologous with part of the first branchial
arches of Selachians.

The columella and extra-columella have been already described (p.
251).

C. THE RIBS AND STERNUM.

=Thoracic ribs.=

The Crocodile has ten pairs of =thoracic ribs=, all except the last
one or two of which consist of three parts,--a vertebral rib, an
intermediate rib and a sternal rib.

Of the =vertebral ribs= the third may be taken as a type, it consists
of a curved bony rod which articulates proximally with the transverse
process of the vertebra by two facets. The terminal one of these, the
=capitulum= or =head=, articulates with a notch on the side of the
transverse process; the other, the =tuberculum=, which lies on the
dorsal surface a short distance behind the head, articulates with the
end of the transverse process. From near the distal end an imperfectly
ossified uncinate =process= (see p. 190) projects backwards.

The =intermediate ribs= are short and imperfectly ossified; they are
united with the =sternal ribs= (fig. 46, 3), which are large,
flattened, likewise imperfectly ossified structures, and articulate at
their distal ends with a pair of long divergent =xiphisternal horns=
(fig. 46, 5), which arise from the posterior end of the sternum
proper. The last pair of sternal ribs are attached to the preceding
pair, not to the xiphisternal horns.

The first and second vertebral ribs differ from the others in the fact
that the tuberculum forms a fairly long outstanding process.


=Cervical ribs.=

Movable ribs are attached to all the cervical as well as to the
thoracic vertebrae. Those borne by the atlas and axis are long, narrow
structures attached by a fairly broad base, and tapering gradually.
The ribs borne by the third to seventh cervical vertebrae are shaped
like a =T= with a double base, one limb of which, corresponding to the
tuberculum (fig. 41, 7), articulates with a short transverse process
arising from the neural arch, while the other, corresponding to the
capitulum, articulates with a surface on the centrum. The ribs
attached to the eighth and ninth cervical vertebrae are intermediate
in character between the =T=-shaped ribs and the ordinary thoracic ribs.
The anterior limb of the =T= is shortened, the posterior one is drawn
out, forming the shaft of the rib. The distal portion of the rib of
the ninth cervical vertebra is unossified.

The =Sacral ribs= have been described in connection with the sacral
vertebrae.


THE STERNUM.

The =sternum= of Crocodiles is a very simple structure, consisting of
a plate of cartilage (fig. 46, 2) lying immediately dorsal to the
interclavicle, and drawn out posteriorly into a pair of long
=xiphisternal horns= (fig. 46, 5).


THE ABDOMINAL SPLINT RIBS.

Lying superficially to the recti muscles of the ventral body-wall,
behind the sternal ribs, are seven or eight series of slender curved
bones, the _abdominal ribs_ (fig. 46, 4). Each series consists of four
or more bones, arranged in a =V=-like form with the angle of the =V=
directed forwards. They show a considerable amount of variability in
number and character. They are really membrane bones, and are in no
way homologous with true ribs, but correspond rather with the more
posterior of the bones constituting the plastron of Chelonia.

[Illustration FIG. 46. STERNUM AND ASSOCIATED MEMBRANE BONES OF A
CROCODILE (_C. palustris_) × 1/3. (Brit. Mus.)

The last pair of abdominal ribs which are united with the epipubes by
a plate of cartilage have been omitted.

    1. interclavicle.
    2. sternum.
    3. sternal rib.
    4. abdominal splint rib.
    5. xiphisternal horn.]


2. THE APPENDICULAR SKELETON.

This includes the skeleton of the two pairs of limbs and their
respective girdles.

[Illustration: FIG. 47. LEFT HALF OF THE PECTORAL GIRDLE OF AN
ALLIGATOR (_Caiman latirostris_) × 2/3. (Brit. Mus.)

    1. scapula.
    2. coracoid.
    3. interclavicle.
    4. glenoid cavity.]


THE PECTORAL GIRDLE.

The pectoral girdle of the Crocodile is less complete than is that of
most reptiles. It consists of a dorsal bone, the =scapula=, and a
ventral bone, the =coracoid=, with a median unpaired element, the
_interclavicle_; but there is no separate representative either of the
clavicle or precoracoid.

The =scapula= (fig. 47, 1) is a large bone, flattened and expanded
above where it is terminated by an unossified margin, the
=suprascapula=, and thickened below where it meets the coracoid. The
scapula forms about half the =glenoid cavity= (fig. 47, 4) for
articulation with the humerus, and has the lower part of its anterior
border drawn out into a roughened ridge.

The =coracoid= (fig. 47, 2) is a flattened bone, much expanded at
either end; it bears on its upper posterior border a flattened surface
which forms half the glenoid cavity, and is firmly united to the
scapula at its dorsal end. Its ventral end meets the sternum.

The _interclavicle_ (figs. 46, 1, and 47, 3) is a long narrow
blade-shaped bone lying along the ventral side of the sternum; about a
third of its length projects beyond the sternum in front.


THE ANTERIOR LIMB.

This is as usual divisible into three portions, the upper arm,
fore-arm and manus.

The =upper arm= or =brachium= contains one bone, the =humerus.=

The =humerus= (fig. 48, A, 1) is a fairly long stout bone,
considerably expanded at either end. The proximal end or head is
evenly rounded and is formed by an epiphysis ossifying from a centre
different from that forming the shaft. It articulates with the glenoid
cavity. The shaft bears on the flexor surface, at some little distance
behind the head, a prominent rounded protuberance, the =deltoid
ridge.= The distal end or trochlea is also formed by an epiphysis and
is partially divided by a groove into two convex surfaces; it
articulates with the two bones of the fore-arm, the radius and ulna.

[Illustration FIG. 48. A, RIGHT ANTERIOR, AND B, RIGHT POSTERIOR LIMB
OF A YOUNG ALLIGATOR (_Caiman latirostris_). (Brit. Mus.)

A × 1/2. B × about 1/3.]

    1. humerus.
    2. radius.
    3. ulna.
    4. radiale.
    5. ulnare.
    6. pisiform.
    7. patch of cartilage representing carpalia 1 and 2; between it
      and the radiale should be another flattened patch, the centrale.
    8. carpalia 3, 4, and 5 (fused).
    9. first metacarpal.
    10. proximal phalanx of second digit.
    11. second phalanx of fifth digit.
    12. femur.
    13. tibia.
    14. fibula.
    15. tibiale, intermedium and centrale (fused).
    16. fibulare.
    17. tarsalia 1, 2, and 3 (fused).
    18. tarsalia 4 and 5 (fused).
    19. first metatarsal.
    20. ungual phalanx of second digit.
    21. fifth metatarsal.

The =radius= and =ulna= are nearly equal in size and each consists of
a long shaft terminated at either end by an epiphysis.

The =radius= (fig. 48, A, 2) or pre-axial bone is slightly the smaller
of the two. It has a straight cylindrical shaft and is slightly and
nearly evenly expanded at either end. The proximal end which
articulates with the humerus is flat or slightly concave, the distal
end which articulates with the carpus is slightly convex.

The =ulna= (fig. 48, A, 3) or postaxial bone is a curved bone rather
larger than the radius. Its proximal end is large and convex, but is
not drawn out into an olecranon process.

The =Manus= consists of the =carpus= or =wrist=, and the =hand=.

The =Carpus=. This differs considerably from the more primitive type
met with in the Turtle. It consists of six elements arranged in a
proximal row of three and a distal row of two, with one intervening.
The bones of the proximal row are the radiale, the ulnare, and the
pisiform. The =radiale= (fig. 48, A, 4) is the largest bone of the
carpus: it is a somewhat hour-glass shaped bone, with its ends formed
by flattened epiphyses. It articulates by its proximal end with the
whole of the radius, and partly also with the ulna, and by its distal
end with the centrale.

The =ulnare= (fig. 48, A, 5) is a smaller bone, also somewhat
hour-glass shaped; it articulates proximally with the pisiform and
radiale, not quite reaching the ulna. The third bone of the proximal
row is the =pisiform= (fig. 48, A, 6), an irregular bone, articulating
with the ulna, radiale, and fifth metacarpal. The =centrale= is a
flattened cartilaginous element applied to the distal surface of the
radiale.

The distal row of carpals consists of two small structures. The first
of these forms a small cartilaginous patch, which is wedged in between
the first and second metacarpals, the centrale and the bone
representing carpalia 3, 4 and 5; this cartilaginous patch represents
=carpalia 1 and 2= (fig. 48, A, 7). The bone representing =carpalia 3,
4 and 5= is a good deal larger, rounded, and well-ossified; it
articulates with the ulnare, the pisiform, and the third, fourth, and
fifth metacarpals.

The =hand=. Each of the five digits consists of an elongated
metacarpal, terminated at each end by an epiphysis, and of a varying
number of phalanges. The terminal phalanx of each digit has an
epiphysis only at its proximal end, the others have them at both ends.

The first digit, or =pollex=, is the stoutest, and has two phalanges,
the second has three, the third four, the fourth three, and the fifth
two. The terminal phalanx of each of the first three digits is pointed
and sheathed in a horny claw; and is also marked by a pair of
prominent lateral grooves.

THE PELVIC GIRDLE.

The pelvic girdle of the Crocodile consists of four parts, a dorsal
element, the =ilium=, an anterior ventral element, the =pubis=, a
posterior ventral element, the =ischium=, and an accessory anterior
ventral element, the =epipubis=. All except the epipubis take part in
the formation of the =acetabulum=, which is perforated by a prominent
hole.

The =ilium= (fig. 49, 1) is a thick strong bone, firmly united on its
inner side with the two sacral ribs. Its dorsal border is rounded, its
ventral border bears posteriorly two irregular surfaces, completed by
epiphyses, which are united respectively with the ischium and pubis.

The =ischium= (fig. 49, 2)--the largest bone of the pelvis, is
somewhat contracted in the middle and expanded at either end. Its
proximal end, which is formed by an epiphysis, bears two surfaces, one
of which is united to the ilium, while the other forms part of the
acetabulum. The anterior border is also drawn out dorsally into a
strong process, which is terminated by a convex epiphysis, and is
united to the pubis. The ventral end of the ischium forms a flattened
blade, meeting its fellow in a median symphysis.

The =pubis= (fig. 49, 3) is much smaller than either the ilium or
ischium; it forms a small patch of unossified cartilage, interposed
between the anterior parts of the ilium and ischium.

[Illustration FIG. 49. PELVIS AND SACRUM OF AN ALLIGATOR (_Caiman
latirostris_) × 1/2. (Brit. Mus.)

    1. ilium.
    2. ischium.
    3. true pubis.
    4. epipubis (so-called pubis).
    5. acetabular foramen.
    6. neural spines of sacral vertebrae.
    7. symphysis ischii.
    8. process bearing prezygapophysis.]

The =epipubis= (fig. 49, 4) is a large bone with a thickened proximal
end, which is loosely articulated to the ischium, and a flattened
expanded distal end, which is united with its fellow, and with the
last pair of abdominal ribs by a large plate of cartilage. This bone
is generally described as the pubis.


THE POSTERIOR LIMB.

This is as usual divisible into three portions, the =thigh=, the
=crus= or =shin=, and the =pes=.

The =thigh= is formed by the =femur= (fig. 48, B, 12), a moderately
long stout bone, not unlike the humerus; it articulates with the
acetabulum by a fairly prominent rounded =head=. The distal end
articulating with the tibia and fibula is also expanded, and is
partially divided into equal parts by anterior and posterior grooves.
The flexor surface bears a fairly prominent trochanteric ridge. Each
end of the femur is formed by an epiphysis.

The =crus= or =shin= includes two bones, the =tibia= and =fibula=.
Both are well developed, but the tibia is considerably the larger of
the two.

The =tibia= (fig. 48, B, 13) is a strong bone with a flattened
expanded proximal end articulating with almost the whole of the end of
the femur, and a similarly expanded distal end articulating with a
bone representing the fused astragalus and centrale.

The =fibula= (fig. 48, B, 14) is flattened proximally, and articulates
with only quite a small part of the femur, while distally it is more
expanded, and articulates with the fibulare (calcaneum) and with a
facet on the side of the fused astragalus and centrale.

The =Pes= consists of the =tarsus= or =ankle=, and the =foot=.

The =Tarsus=. This, like the carpus, is much reduced and modified from
the primitive condition. It consists of only four bones, arranged in
two rows of two each. The two bones of the proximal row are much
larger than are those of the distal row. The pre-axial of them (fig.
48, B, 15) representing the fused =astragalus= (tibiale and
intermedium) and =centrale=, articulates proximally with the tibia and
fibula, and distally with the first metatarsal, and a small bone
representing the first three tarsalia. The postaxial bone, the
=calcaneum= (fibulare) (fig. 48, B, 16), is drawn out into a
prominent posterior process forming a heel such as is almost unknown
elsewhere except in mammals. It articulates with the fibula, the
tibiale-centrale, and distally with a bone representing the fourth and
fifth tarsalia, and with the fifth metatarsal.

The two bones forming the distal row of tarsals are both small and
rounded; one represents the first three tarsalia fused together, the
other tarsalia 4 and 5.

The =Foot=. The =foot= has five digits, but the fifth is much reduced,
consisting only of a short metatarsal. The first four =metatarsals=
are all long bones, slightly expanded at each end, and terminated by
small epiphyses. The first digit has two phalanges, the second three,
the third four, and the fourth five. The terminal or =ungual phalanx=
in each instance is grooved and pointed, and in the case of the first
three digits bears a horny claw. The ungual phalanx progressively
decreases in size from the first to the fourth. The fifth digit
consists only of a small, somewhat square metatarsal (fig. 48, B, 21),
attached to the bone representing the fused fourth and fifth
tarsalia.


FOOTNOTES:

[90] Free use has been made of L.C. Miall's _Studies in Comparative
Anatomy_, I., _The Skull in Crocodilia_, London, 1878. See also W.K.
Parker, _Tr. Z.S._, vol. XI. 1885, p. 263.




CHAPTER XVI.

GENERAL ACCOUNT OF THE SKELETON IN REPTILES.


EXOSKELETON.

The exoskeleton both epidermal and dermal is exceedingly well
developed in reptiles.


EPIDERMAL EXOSKELETON.

This generally has the form of overlapping horny =scales= which invest
outgrowths of the dermis, and are found covering the whole body in
most Rhynchocephalia, Ophidia, and Lacertilia, and many Crocodilia. In
the Ophidia the ventral surface of the tail is commonly covered by a
double row of broad scales, while the ventral surface of the precaudal
part of the body is covered by a single row. In the burrowing snakes
(Typhlopidae) and some sea snakes (Hydrophidae) these broad scales do
not occur, the scales of the ventral surface being similar to those of
the dorsal.

In the Chelonia with the exception of _Dermochelys_, _Trionyx_ and
their allies there is a well-developed system of horny shields having
a regular arrangement which has been described in the account of the
Turtle's skeleton[91].

The =rattle of the rattlesnake= is an epidermal structure formed of
several loosely articulated horny rings, produced by the modification
of the epidermal covering of the end of the tail, which instead of
being cast off when the rest of the outer skin is shed is retained
loosely interlocked with the adjoining ring or joint. New rings are
thus periodically added to the base of the rattle, and in old animals
the terminal ones wear away and are lost.

Horny claws occur on the ends of some or all of the digits in most
living reptiles.

Owen's Chameleon bears three epidermal horns, one arising from the
nasal and two from the frontal region.

In the Chelonia, some of the Theromorpha such as _Udenodon_ and
_Dicynodon_, probably also in the Pterosauria and _Polyonax_ among the
Dinosaurs, the jaws are more or less cased in horny beaks. The horny
beaks of Chelonia are variable; sometimes they have cutting edges,
sometimes they are denticulated, sometimes they are adapted for
crushing.


DERMAL EXOSKELETON.

Nearly all Crocodilia, many Dinosauria, some Rhynchocephalia and
Pythonomorpha, and some Lacertilia such as _Tiliqua_, _Scincus_ and
_Anguis_ have a dermal exoskeleton of bony scutes, developed below and
corresponding in shape to the epidermal scales. Sometimes as in
_Caiman sclerops_, _Jacare_ and _Teleosaurus_, the scutes completely
invest the body, being so arranged as to form a dorsal and a ventral
shield, and a continuous series of rings round the tail. In
_Crocodilus_ they are confined to the dorsal surface, and in
_Alligator_ to the dorsal and ventral surfaces. The scutes of some
extinct forms articulate with one another by a peg and socket
arrangement as in some Ganoid fish.

The =carapace= of most Chelonia is a compound structure, being partly
endoskeletal and formed from the ribs and vertebrae, partly from
plates derived from the dermal exoskeleton. The common arrangement is
seen in fig. 36. All the surface plates are probably exoskeletal in
origin, but united with the ventral surfaces of the costal and neural
plates respectively are the expanded ribs and neural arches of the
vertebrae.

The =plastron= in the common genus _Chelone_ (fig. 37) includes nine
plates of bone, one unpaired and four pairs; they will be referred to
in connection with the ribs and pectoral girdle.

In the Leathery Turtle (_Dermochelys_) the carapace and plastron
differ completely from those of any other living form. The carapace
consists of a number of polygonal ossifications fitting closely
together and altogether distinct from the vertebrae and ribs. The
plastron is imperfectly ossified, and not united with the pelvis, and
the whole surface of both carapace and plastron is covered with a
tough leathery skin, without horny shields.

Some of the extinct Dinosauria have an enormously developed dermal
exoskeleton. Thus in _Stegosaurus_ and _Omosaurus_ the dorsal surface
is provided with flattened plates or with spines reaching a length of
upwards of two feet. In _Polacanthus_ the posterior part of the body
is protected by a bony shield somewhat recalling that of the little
armadillo _Chlamydophorus_. No exoskeleton is known in Ichthyosauria,
Sauropterygia, Pterosauria, many Dinosauria and Theromorpha, and some
Lacertilia, such as _Chamaeleon_ and _Amphisbaena_.


TEETH.

The teeth of reptiles are generally well developed, and in the great
majority of forms are simple conical structures, uniform in character,
generally somewhat recurved, and often with serrated edges. Another
common type of tooth is that with a laterally compressed triangular
crown provided with a double cutting edge which may or may not be
serrated. The teeth are mainly formed of dentine, with usually an
external layer of enamel, and often a coating of cement on the root.
Vasodentine is found below the dentine in _Iguanodon_. The teeth of
reptiles never have the enamel deeply infolded, nor do they have
double roots.

Teeth may be present not only on the jaw-bones, but as in many
_Squamata_, also on the palatines, pterygoids or vomers. The method
by which they are attached to the bones varies much. Sometimes as in
_Iguana_ and some other lizards, they are pleurodont[1], sometimes
they are acrodont[92], as in the Rhynchocephalia, Pythonomorpha,
Ophidia and some Lacertilia such as _Agama_. Again they may be set in
a continuous groove as in the Ichthyosauria and young Crocodilia.
Finally the teeth may be _thecodont_ or placed in distinct sockets as
in the Theromorpha, Sauropterygia, adult Crocodilia, Sauropoda and
Theropoda. In _Iguanodon_ the teeth are set in shallow sockets in a
groove one side of which is higher than the other; the method of
attachment thus shows points of resemblance to the thecodont
condition, the pleurodont condition, and that met with in the
Ichthyosauria.

In _Ichthyosaurus_ the teeth are marked by a number of vertical
furrows, and it is from a furrow of this nature greatly enlarged and
converted into a tube that the channel down which flows the poison of
venomous snakes is derived.

In most reptiles the dentition is more or less homodont. The only
reptiles in which a definite heterodont dentition is known are the
extinct Theromorpha, and in them the teeth vary greatly. Thus
_Udenodon_ is toothless, the jaws having been probably cased in a
horny beak. In _Dicynodon_ the jaws are likewise toothless with the
exception of a pair of permanently growing tusks borne by the maxillae.
_Dicynodon_ is the only known reptile whose teeth have permanently
growing pulps. In _Pariasaurus_ the teeth are uniform and very
numerous, and though placed in distinct sockets are ankylosed to the
jaw. In _Galesaurus_ and _Cynognathus_ three kinds of teeth can be
distinguished, slender conical incisor-like teeth, large canine-like
teeth, and cheek teeth with two or three cusps. The teeth in
_Galesaurus_ are confined to the jaws, in _Placodus_ and its allies,
however, large flat crushing teeth are attached to the palatines as
well as to the jaw-bones, and in _Pariasaurus_ the vomer, palatine and
pterygoid all bear teeth as well as the jaw bones. The upper jaw of
_Sphenodon_ and other Rhynchocephalia is provided with two parallel
rows of teeth, one borne on the maxillae and one on the palatines, the
mandibular teeth bite in a groove between these two rows. The bone of
the jaws in _Sphenodon_ is so hard that when the teeth get worn away,
it can act as a substitute. In the young _Sphenodon_ the vomers bear
teeth, as they do also in _Proterosaurus_.

There is generally a continuous succession of teeth throughout life,
the new tooth coming up below, or partly at the side of the one in
use, and causing the absorption of part of its wall or base. In this
way the new tooth comes to lie in the pulp cavity of the old one. This
method of succession is well seen in the Crocodilia.

[Illustration FIG. 50. PREPARATION OF PART OF THE RIGHT MANDIBULAR
RAMUS OF _Crocodilus palustris_ × 1/2. (Brit. Mus.)

    1. tooth in use.
    2. fairly old germ of future
    tooth.
    3. symphysial surface of the
    mandible.]

Teeth have been detected in embryos of _Trionyx_, but otherwise no
teeth are known to occur in Chelonia, or in _Pteranodon_
(Pterosauria), while the anterior part of the jaw is edentulous in
_Iguanodon_, _Polyonax_ and some other Dinosaurs, and in
_Rhamphorhynchus_.


ENDOSKELETON.


VERTEBRAL COLUMN.

The vertebral column is commonly divisible into the usual five
regions, but in the Ophidia, Ichthyosauria, and Amphisbaenidae among
Lacertilia, only into caudal and precaudal regions. In the Chelonia
there are no lumbar vertebrae.

The form of the vertebral centra is very variable. A large proportion
of extinct reptiles,--several entire orders,--and the earlier and more
primitive forms in some of the other groups have amphicoelous
vertebrae. Vertebrae of this type occur in the Theromorpha,
Ichthyosauria, most Sauropterygia and Rhynchocephalia, and many
Dinosauria, also in some of the early Crocodilia such as _Belodon_,
_Teleosaurus_ and _Goniopholis_, and the Geckonidae among Lacertilia.

The majority of living reptiles have procoelous vertebrae. Thus they
occur in the Lacertilia (excluding the Geckos), the Ophidia, and the
Crocodilia, also among extinct forms in the Pterosauria and many
Dinosauria. On the other hand some Dinosauria such as _Iguanodon_ have
opisthocoelous cervical vertebrae, while others have opisthocoelous
thoracic vertebrae. The vertebrae of the Ceratopsidae and some
Sauropterygia, the thoracic vertebrae of _Iguanodon_, and the sacral
vertebrae of Crocodilia have flat centra. The first caudal vertebra of
modern Crocodilia is biconvex, and in the Chelonia all types of
vertebral centra are found. The cervical vertebrae of _Sphenodon_ are
noticeable for the occurrence of a small pro-atlas, which may
represent the neural arch of a vertebra in front of the atlas.

In most reptiles the vertebrae are fully ossified, but in some of the
more primitive forms the notochord persists in the centre of the
vertebra (i.e. intervertebrally), this is the case for instance in
many of the Theromorpha and Rhynchocephalia, and also in the Geckos.
In other reptiles it persists longest intravertebrally.

The centrum of each of the caudal vertebrae of most Lacertilia is
traversed by an unossified septum along which it readily breaks.

Chevron bones occur below the caudal vertebrae in Lacertilia,
Chelonia, Ichthyosauria, many Dinosauria, and _Sphenodon_,
articulating with quite the posterior part of the centrum which bears
them. In Lacertilia and Crocodilia (fig. 41, 3) the axis has a
well-marked odontoid process. The ventral portions of the
intervertebral discs are sometimes ossified, forming wedge-shaped
inter centra, as in Geckos, and the cervical vertebrae of _Sphenodon_.

In snakes, Theropod Dinosaurs, and the iguanas among lizards, the
neural arches are provided with _zygosphenes_, and _zygantra_.

The neural arches are usually firmly ankylosed to the centra, but in
the Crocodilia and some Chelonia, Sauropterygia, and Dinosauria, the
suture between the centrum and neural arch persists at any rate till
late in life. In the Ichthyosauria the neural arches were united to
the centra by cartilage only.

The thoracic vertebrae of some of the Theromorpha (_Dimetrodon_) are
remarkable for the extraordinary development of the neural spine, and
those of Chelonia for the absence of transverse processes.

In living reptiles the number of sacral vertebrae is nearly always
two, but in the Theromorpha, Dinosauria, and Pterosauria, as many as
five or six bones may be ankylosed together in the sacral region. In
Crocodiles the two halves of the pelvis sometimes articulate with
different vertebrae. The vertebrae of some of the great Sauropoda are
remarkably hollowed out, having a large vacuity on each side of the
centrum communicating with a series of internal cavities. The whole
structure of these vertebrae shows a combination of great strength
with lightness.


THE SKULL.

The reptilian skull is well ossified and the bones are noticeable for
their density. The true cranium is often largely concealed by a
secondary or false roof of membrane bones, which is best seen in the
Ichthyosauria and some of the Chelonia. In other reptiles the false
roof is more or less broken up by vacuities exposing the true cranial
walls. The ethmoidal region is the only one in which much of the
primordial cartilaginous cranium remains. The lateral parts of the
sphenoidal region are also as a rule not well ossified.

In some reptiles, such as most Lacertilia and Chelonia, the orbits are
separated only by the imperfect interorbital septum, while in others,
such as the Ophidia, Crocodilia and Amphisbaenidae, the cranial cavity
extends forwards between the orbits.

In the occipital region all four bones are ossified. The great
majority of reptiles have a single convex occipital condyle, but some
of the Theromorpha such as _Cynognathus_ have two distinct condyles as
in mammals. Sometimes, as in Chelonia, Ophidia and Lacertilia, the
exoccipitals, as well as the basi-occipital, take part in the
formation of the single condyle; sometimes, as in Crocodiles, it is
formed by the basi-occipital alone, as in birds. The relations of the
bones to the foramen magnum vary considerably, in Chelonia the
basi-occipital generally takes no part in bounding it, and in the
Theromorpha, Crocodilia, and Ophidia, the supra-occipital is excluded.
The parietals are paired in Geckos and Chelonia alone among living
forms, and in the extinct Ichthyosauria and some Theromorpha; in all
other reptiles they are united.

The frontals are paired in Ichthyosauria (fig. 32, 5), Chelonia,
Ophidia, _Sphenodon_ (fig. 52, B, 4) and some extinct crocodiles, such
as _Belodon_. They are completely fused in living Crocodilia and some
Lacertilia and Dinosauria. In the gigantic _Polyonax_ they are drawn
out into a pair of enormous horns, and the parietals and squamosals
are greatly expanded behind.

An interparietal foramen occurs in the Theromorpha, the Ichthyosauria
(fig. 32, 10), _Sphenodon_, the Sauropterygia and most Lacertilia.
The posterior part of the skull is curiously modified in some
Chamaeleons, the parietals and supra-occipitals being drawn out into a
backwardly-projecting sagittal crest which unites with the two
prolongations from the squamosals. In other Chamaeleons (_C. bifidus_)
prolongations of the prefrontals and maxillae form large
forwardly-projecting bony processes.

The roof of the skull is characterised by the development of
prefrontals and postfrontals, which lie respectively near the anterior
and posterior extremity of the orbit. In Theromorpha, Squamata,
Crocodilia, and some Dinosauria lachrymals are developed. There is a
ring of bones in the sclerotic in the Ichthyosauria (fig. 32, 15), the
Metriorhynchidae among Crocodiles and some Rhynchocephalia,
Dinosauria, and Pterosauria.

The pro-otic lies in front of the exoccipital and together with the
opisthotic forms the hind border of the fenestra ovalis. In Chelonia
the opisthotic remains separate, in all other living reptiles it fuses
with the exoccipital. The epi-otic fuses with the supra-occipital.

The parasphenoid, so important in Ichthyopsids, has very often
disappeared completely; it is present, however, in the Ichthyosauria,
the Plesiosauridae, and a number of Squamata, in many Ophidia its
anterior part forming the base of the interorbital septum.

In the Plesiosauridae and most Lacertilia, but not in the
Amphisbaenidae, a slender bone, the epipterygoid, occurs uniting the
parietal or the anterior end of the pro-otic with the pterygoid. A
homologous arrangement occurs in the Ichthyosauria and some Chelonia.

In most reptiles a transpalatine occurs, connecting the maxillae with
the pterygoid, but this is absent in the Chelonia, and some
Dinosauria, and in the Typhlopidae among snakes.

The quadrate is always well developed, and except in the Squamata is
firmly fixed to the surrounding bones. The Chamaeleons also, among the
Squamata, have a fixed quadrate, and in them too the quadratojugal is
absent. Separate nasal bones do not occur in any living Chelonia.

The vomers are generally paired as in Squamata, sometimes unpaired as
in Chelonia.

[Illustration FIG. 51. DORSAL (TO THE LEFT) AND VENTRAL (TO THE RIGHT)
VIEWS OF THE SKULL OF THE COMMON SNAKE (_Tropidinotus natrix_). (After
PARKER.)

     1. premaxillae (fused).
     2. anterior nares.
     3. nasal.
     4. prefrontal.
     5. frontal.
     6. parietal.
     7. maxillae.
     8. transpalatine.
     9. palatine.
    10. pterygoid.
    11. pro-otic.
    12. exoccipital.
    13. supra-occipital.
    14. opisthotic.
    15. epi-otic.
    16. quadrate.
    17. parasphenoid.
    18. basisphenoid.
    19. basi-occipital.
    20. occipital condyle.
    21. splenial.
    22. dentary.
    23. angular.
    24. articular.
    25. supra-angular.
    26. coronoid.
    27. vomer.
    28. squamosal.

    IX, X foramina for the ninth
    and tenth cranial nerves.]

The disposition of the bones of the jaws is subject to much
modification in the Ophidia in order to adapt them for swallowing very
large prey. The arrangements again differ greatly in the venomous and
non-venomous snakes. In the non-venomous snakes, such as _Python_ and
_Tropidonotus_, the palatine is large and is fixed to the pterygoid
which extends outwards (fig. 51, 10) so as to be united to the
quadrate, and is at the same time firmly connected by the
transpalatine with the maxillae. The quadrate is united to the
squamosal, which is loosely attached to the cranium. The premaxillae is
moderately developed and bears teeth, and the maxillae forms a long bar
loosely connected with the rest of the skull. The rami of the mandible
are united only by an extremely elastic ligament. It is as regards the
maxillae and premaxillae that the skulls of venomous and non-venomous
snakes differ most. In the rattlesnake (_Crotalus_) and other venomous
snakes the premaxillae is extremely small and toothless. The maxillae is
small and subcylindrical, and is movably articulated to the lachrymal,
which also is capable of a certain amount of motion on the frontal.
The maxillae is connected by means of the transpalatine with the
pterygoid, which in its turn is united to the quadrate. When the mouth
is shut the quadrate is directed backwards, and carrying back the
pterygoid and transpalatine pulls at the maxillae and causes its
palatal face, to which the poison teeth are attached, to lie back
along the roof of the mouth. When the mouth opens the distal end of
the quadrate is thrust forward, and this necessitates the pushing
forward of the pterygoid and transpalatine, causing the tooth-bearing
surface of the maxillae to look downwards and the tooth to come into
the position for striking.

The Ophidian skull is also noticeable for the absence of the jugals
and quadratojugals. In poisonous snakes the place of the jugal is
taken by the zygomatic ligament which connects the quadrate and
maxillae.

The extent to which the palate is closed in reptiles varies much. In
many reptiles, such as the Squamata and Ichthyosauria, the palate is
not complete, both palatines and pterygoids being widely separated in
the middle line. In others, such as the Crocodilia, Sauropterygia, and
Chelonia, there is a more or less complete bony palate. In many
Chelonia this is chiefly formed of the vomer, palatines, and
pterygoids, the posterior nares being mainly bounded by the palatines.
In living Crocodilia, however, outgrowths are formed from the
pterygoids and palatines which arch round and meet one another
ventrally, forming a secondary palate (fig. 43, A), which completely
shuts off the true sphenoidal floor of the skull, and causes the
posterior nares which are bounded by the pterygoids to open very far
back. Though this feature is common to all postsecondary crocodiles,
it is interesting to notice that it is not found in the earlier forms,
but that its gradual evolution can be traced. In the Triassic
_Belodon_, for instance, the posterior nares open far forwards, and
are not surrounded by either the palatines or pterygoids. In the
Jurassic crocodile, _Teleosaurus_, the posterior nares lie further
back, being surrounded by the palatines, but the pterygoids do not
meet them. Finally, in the Tertiary forms the arrangements are as in
living crocodiles.

A short secondary hard palate is found also in the Theriodontia. The
palatines of _Ichthyosaurus_ are noticeable for their transverse
position, which recalls that in the Frog.

The various =fossae= or =vacuities= in the false roof of the skull are
important, and their relations may best be understood by a description
of their mode of occurrence in _Sphenodon_, a form in which they are
very completely developed.

[Illustration FIG. 52. SKULL OF HATTERIA. (_Sphenodon punctatus_). A,
lateral; B, dorsal; C, ventral; D, posterior. (After VON ZITTEL.)

     1. premaxillae.
     2. nasal.
     3. prefrontal.
     4. frontal.
     5. postfrontal.
     6. parietal.
     7. squamosal.
     8. quadratojugal.
     9. quadrate.
    10. postorbital.
    11. jugal.
    12. maxillae.
    13. vomer.
    14. palatine.
    15. pterygoid.
    16. transpalatine.
    17. exoccipital.
    18. epipterygoid.
    19. basisphenoid.
    20. supratemporal fossa.
    21. infratemporal or lateral temporal
    fossa.
    22. orbit.
    23. post-temporal fossa.
    24. foramen magnum.
    25. anterior nares.
    26. interparietal foramen.
    27. dentary.
    28. supra-angular.
    29. articular.]

In _Sphenodon_, then, on the dorsal surface of the skull, are the
large =supratemporal fossae= (fig. 52, 20). Their inner margins are
separated from one another by the parietal walls of the cranium, while
externally each is bounded by a bony arch, the =supratemporal arcade=,
formed of the postfrontal, postorbital, and squamosal. Posteriorly the
boundary is formed by a =post-temporal bar=, formed by the parietal
and squamosal. Below the supratemporal arcade is another large
vacuity, the =infratemporal= or =lateral temporal fossa= (fig. 52,
21). This is bounded above by the supratemporal arcade, and is
separated from the orbit in front by the =postorbital bar=, formed by
the union of outgrowths from the jugals and postorbitals. Behind it is
bounded by a continuation of the post-temporal bar formed of the
squamosal and quadratojugal, and below by an =infratemporal arcade=,
which is chiefly composed of the quadratojugal and jugal.

Below the post-temporal bar is a third vacuity, the =post-temporal
fossa= (fig. 52, D, 23), bounded above by the post-temporal bar and
below by the exoccipital and opisthotic.

_Sphenodon_ and the Crocodilia are the only living reptiles with
complete supratemporal and infratemporal arcades, but they are both
present in the extinct Pterosauria and some Dinosauria.

Supratemporal fossae, bounded below by supratemporal arcades, occur in
all reptiles except some Chelonia, the Ophidia, the Geckonidae among
Lacertilia, and the Pariasauria and others among Theromorpha; they are
specially large in _Nothosaurus_ among the Sauropterygia, _Dicynodon_
among the Theromorpha, and many Crocodilia and Pterosauria. In some
Dinosaurs, such as _Ceratosaurus_, they are very small, while the
infratemporal fossae are correspondingly large.

In _Elginia_[93] (Theromorpha) and some Chelonia, such as _Chelone_,
there are no fossae on the surface of the skull, a complete false roof
being developed; in other Chelonia, such as _Trionyx_, the true
cranium is freely visible, the only part of the false roof developed
being the infratemporal arcade.

In many reptiles large =pre-orbital vacuities= occur; they are
specially large in the Pterosauria and in some of the Crocodilia and
Dinosauria (fig. 35, 3). In some Pterosauria they are confluent with
the orbits.

The premaxillae are usually separate, but sometimes, as in some Ophidia
(fig. 51, 1), Chelonia, Lacertilia (Agamidae), and Dinosaurs
(Ceratopsia) they are united. In the Dinosaur _Hadrosaurus_ they are
exceedingly large and spatulate. In the Rhynchocephalian
_Hyperodapedon_ they are drawn out into a strongly curved beak.

As regards the mandible, sometimes, as in most Rhynchocephalia,
Ophidia and Pythonomorpha, the rami have only a ligamental union;
sometimes, as in Crocodilia, the Rhynchosauridae and the majority of
Lacertilia, they are suturally united. In Chelonia (fig. 28, B, 12),
and apparently in Pterosauria, the two dentaries are completely fused
together. The sutures between the various bones of the lower jaw
usually persist, but in Ophidia those between the angular,
supra-angular, articular and coronoid are obliterated. There are
sometimes large vacuities in the mandible, as in Theromorpha,
Crocodilia, and some Dinosauria. In _Iguanodon_, _Polyonax_,
_Hypsilophodon_ and _Hadrosaurus_ among Dinosaurs the mandible has a
predentary or mento-meckelian bone which, in some cases at any rate,
was probably sheathed in a horny beak.

The principal part of the auditory ossicular chain is formed by a
rod-like columella. The development of the hyoid apparatus varies, and
it often happens that the first branchial arch is better developed
than is the hyoid arch. In the Crocodilia and Chelonia there is a
large basilingual plate or body of the hyoid (fig. 53, 1); but while
in the Crocodilia the first branchial forms the only well-developed
arch, in the Chelonia the first and second branchials are both
strongly developed, and the hyoid is often fairly large.


THE RIBS.

[Illustration FIG. 53. HYOIDS OF AN ALLIGATOR (_Caiman latirostris_)
(TO THE LEFT) AND OF A GREEN TURTLE (_Chelone midas_) (TO THE RIGHT) ×
5/8. (Brit. Mus.)

The cartilaginous portions are dotted.

    1. basilingual plate or body of
    the hyoid.
    2. hyoid arch.
    3. first branchial arch (anterior
    cornu).
    4. second branchial arch (posterior
    cornu).]

Ribs are always present, and may be attached to any of the precaudal
vertebrae. In most reptiles the posterior cervical vertebrae bear
ribs, while the atlas and axis are ribless; in Crocodiles and Geckos,
however, ribs are borne even by the atlas and axis. On the other hand,
in the Chelonia none of the cervical vertebrae bear obvious ribs. In
the following groups the thoracic ribs have both capitula and
tubercula--Theromorpha, Ichthyosauria, Crocodilia, Dinosauria,
Pterosauria. In the other groups each rib articulates by a single
head, and the position of the facet is subject to a considerable
amount of variation, thus in the Squamata it lies on the centrum, and
in the Sauropterygia on the neural arch, while in the Chelonia the rib
articulates with the contiguous parts of two centra instead of
directly with one.

In most reptiles a greater or smaller number of ribs are united
ventrally with a sternum; but in snakes a continuous series of similar
ribs, all articulating freely with the vertebral column, extends from
the third cervical vertebra to the end of the trunk. The number of
ribs connected with the sternum varies from three or four in Lizards
to eight or nine in Crocodiles. Those which reach the sternum are
nearly always divided into vertebral, sternal, and intermediate
portions, and as a rule only the vertebral portion is completely
ossified. In Crocodiles a number of sternal ribs are connected with a
cartilaginous arch, which is attached to the hind end of the sternum,
and represents the xiphisternum. The sacral ribs connecting the
vertebral column with the ilia are very distinct in Crocodiles; in
these animals and _Sphenodon_ the vertebral ribs have backwardly
projecting uncinate processes as in birds.

In the curious arboreal lizard, _Draco volans_, the posterior ribs are
long and straight, and support a parachute-like expansion of the
integument used in its long flight-like leaps. In Chelonia the ribs
are generally combined with the carapace.

In Ichthyosauria, Sauropterygia, Crocodilia and _Sphenodon_, abdominal
splint ribs occur; and probably all except the first of the paired
ossifications forming the plastron of Chelonia are of similar
character. Abdominal ribs have quite a different origin from true
ribs, for while true ribs are cartilage bones, abdominal ribs have no
cartilaginous precursors, but are simply the ossified tendons of the
rectus abdominalis muscle.


THE STERNUM.

A sternum occurs in the following groups of reptiles: Rhynchocephalia,
nearly all Lacertilia, Pythonomorpha, Crocodilia, and Pterosauria, and
is generally more or less rhomboidal or shield-shaped. In Pterosauria
it is keeled and bears some resemblance to that of birds. It may have
been replaced by membrane bone.

[Illustration FIG. 54. VENTRAL VIEW OF THE SHOULDER-GIRDLE OF STERNUM
OF A LIZARD (_Loemanctus longipes_) × 2. (After PARKER.)

    1. interclavicle.
    2. clavicle.
    3. scapula.
    4. coracoid.
    5. precoracoidal process.
    6. glenoid cavity.
    7. sternum.
    8. xiphisternum.
    9. sternal rib.]

The sternum is absent in Sauropterygia, Ichthyosauria, Chelonia,
Ophidia, and most of the snake-like Amphisbaenidae among Lacertilia;
while it is not well known in Theromorpha and Dinosauria. In the
Sauropod _Brontosaurus_, however, two rounded bones occur near the
base of the coracoids, and these probably represent ossified patches
in a sternum, which was mainly cartilaginous; similar structures occur
in _Iguanodon_.

The sternum frequently remains wholly cartilaginous, especially in
Lacertilia; sometimes it becomes calcified, but true ossification does
not as a rule take place.


APPENDICULAR SKELETON.


THE PECTORAL GIRDLE.

The pectoral girdle is well developed in all groups of reptiles except
the Ophidia, occurring even in the limbless Amphisbaenidae. It is very
solid in the Theromorpha. As a rule all three cartilage bones,
scapula, coracoid, and precoracoid are represented, and frequently
also the membrane bones,--clavicles, and interclavicle.

The coracoids are generally flat expanded bones, which sometimes, as
in Sauropterygia and Ichthyosauria, meet in a ventral symphysis;
sometimes, as in Lacertilia, are united with the sides of the sternum.
In Chelonia neither the coracoids nor precoracoids meet one another,
but their free ends are connected by fibrocartilaginous bands. In
Lacertilia the coracoids are pierced by fenestrae.

The precoracoid is generally represented, but the Theromorpha are the
only reptiles in which it is separately ossified; it forms a
well-marked process on the coracoid in Lacertilia (fig. 54, 5). It is
absent in Ichthyosauria, and Dinosauria, and probably in
Sauropterygia. In some Lacertilia and Chelonia the sternal ends of the
coracoids are unossified and form epicoracoids; in some Chelonia there
are also epiprecoracoids; but neither these nor the epicoracoids
overlap their fellows of the opposite side as they do in arciferous
Anura (see p. 185). In some Lacertilia with degenerate limbs the
pectoral girdle is also much reduced, in _Ophisaurus apus_ the ventral
borders of the coracoids are widely separated.

A scapula is always present, and is generally expanded distally, but
in the Chelonia the distal end is cylindrical. In the Theromorpha it
has an acromial process with which the precoracoid articulates, and it
is very large in Dinosauria. In the Chelonia the scapula and
precoracoid are ossified continuously. Among the Pterosauria,
_Pteranodon_ has an unique pectoral girdle; the scapula and coracoid
are ankylosed and the scapula articulates with the neural spines of
several ankylosed vertebrae.

Clavicles occur in some Theromorpha such as _Pariasaurus_, and also in
the Ichthyosauria, Sauropterygia, Rhynchocephalia, and most
Lacertilia. They are absent in the Pterosauria, the Chamaeleons among
Lacertilia, the Ophidia and the Crocodilia. They are wanting too in
the Chelonia, unless the first pair of ossifications in the plastron
are to be regarded as clavicles. In the Sauropterygia bones regarded
as the clavicles and interclavicle are generally well developed. The
unpaired ossification in the plastron of Chelonia is an interclavicle,
and a representative of the same bone occurs arising from the sternum
in Pterosauria. A well developed =T=-shaped interclavicle is found in
Ichthyosauria, Rhynchocephalia, Lacertilia, and some Theromorpha, such
as _Pariasaurus_.


THE LIMBS.

In most reptiles there are two pairs of pentedactylate limbs provided
with claws, but in nearly all Ophidia and some Lacertilia
(_Amphisbaena_, _Lialis_, _Anguis_) the limbs have entirely
disappeared. In a few Ophidia such as _Python_ traces of the posterior
limbs occur, and in _Chirotes_ among the Amphisbaenidae there are
minute anterior limbs. The Lacertilians, _Chalcides_ (_Seps_) and
_Ophisaurus_ (_Bipes_, _Pseudopus_) have very small posterior limbs.

The limbs are as a rule adapted for walking, but in Ichthyosauria,
Sauropterygia, Pythonomorpha and some Chelonia, they have the form of
swimming paddles, the relative size of the manus and pes being
increased, while that of the proximal and middle portions of the limbs
is reduced. This reduction is carried to its furthest extent in the
Ichthyosauria in which radius and ulna, tibia and fibula, have the
form of short polygonal bones similar to those constituting the manus
and pes. In the Pythonomorpha the reduction of the limb bones is not
quite so marked, in the Sauropterygia it is less, and still less in
the Chelonia. In the earlier Ichthyosauria too, the limb bones are not
so short as they are in the later forms. The Ichthyosaurian limb is
also remarkable, firstly for the fact that both humerus and femur are
terminated by concave articulating surfaces instead of by convex
condyles, and secondly for the great multiplication of the phalangeal
bones, each digit being sometimes composed of a series of over twenty.
Sometimes too the number of series is increased, either by the
bifurcation of some of the digits or by the development of marginal
bones. In the Sauropterygia the phalanges are likewise increased above
the normal but not so much as in Ichthyosauria. The humerus and femur
of Sauropterygia are noticeable for the enormous size of the terminal
epiphyses which form in each case by far the greater part of the bone.


THE ANTERIOR LIMB.

The anterior limb is usually approximately equal in length to the
posterior, but in many Dinosauria it is considerably the shorter of
the two. The humerus is generally without distinct condyles, but they
are well developed in the Theromorpha, the Lacertilia and _Sphenodon_.

In the Theromorpha, some Rhynchocephalia, and some Sauropterygia, such
as _Mesosaurus_, the humerus has an ent-epicondylar foramen; in
Lacertilia, Chelonia and some Dinosauria there is an ect-epicondylar
foramen or groove; _Sphenodon_ possesses both ent- and ect-epicondylar
foramina. The radius and ulna are always separate. In some Chelonia,
such as _Chelydra_, the carpus has a very simple arrangement, namely,
a proximal row of three bones, the radiale, intermedium and ulnare,
and a distal row of five carpalia, with one bone, the centrale,
between the two rows. Many reptiles have a carpus only slightly
different from this. Thus the carpus in _Sphenodon_ differs mainly in
having two centralia, that of most Lacertilia, in having the centrale
and intermedium fused.

Crocodiles have a much reduced carpus with the radiale and ulnare
considerably elongated. The manus in Chamaeleons is curiously
modified, having the first three digits arranged in one group and
turned inwards, and the fourth and fifth in another group turned
outwards; carpalia 3 and 4 are united.

In the Pterosauria the anterior limbs form wings, the phalanges of the
fifth digit being very greatly elongated to support the wing membrane.
The first digit is vestigial and the second, third, and fourth are
clawed.


THE PELVIC GIRDLE.

The pelvic girdle is well developed in all reptiles which have
posterior limbs, but is absent or quite vestigial in Ophidia and those
Lacertilia which have no posterior limbs. The ilium and ischium agree
in their general characters throughout all the various groups of
reptiles, but that is not the case with the pubis.

In many reptiles such as Chelonia, Ichthyosauria and Lacertilia the
ilia are small, more or less cylindrical bones either directed
backwards, or vertically placed as in the Chamaeleons. In the
Crocodilia they are larger and more expanded, while in Dinosauria and
Pterosauria they are greatly elongated both in front of, and behind,
the acetabulum. The ischia are generally strongly developed somewhat
square bones meeting in a ventral symphysis. In Dinosauria the ischium
(fig. 35, 9) is a much elongated and backwardly-directed bone,
bearing a forwardly projecting obturator process. In Pterosauria the
ischium is fused with the ilium, and in both pterosaurs and crocodiles
the ilium and ischium are the only bones taking part in the formation
of the acetabulum. In most Lacertilia there is an unpaired structure,
the _hypo-ischium_ or _os cloacae_ projecting back from the symphysis
ischii, which is usually separated from the symphysis pubis by a large
space, the _foramen cordiforme_. In some Lacertilia and Chelonia there
is a cartilaginous bar dividing the foramen cordiforme into two
obturator foramina; in many Chelonia this bar is ossified. Among
_Ophidia_, _Python_, _Tortrix_, _Typhlops_ and their allies have a
structure representing a vestigial ischio-pubis: but in most Ophidia
there is no trace of the pelvis. In some Theromorpha all the bones of
the pelvis are completely fused, forming an os innominatum as in
mammals; the pubes and ischia are so completely fused that sometimes
as in _Pariasaurus_ even the obturator foramina are closed.

Concerning the reptilian pubis there are considerable difficulties.
Sometimes there is only a single pubic structure present, sometimes
there are two. The reptilian pubis is best understood by comparing the
arrangements met with in the various other groups with that in the
Orthopod Dinosaurs such as _Iguanodon_. In _Iguanodon_ the pubis
consists of two portions, viz. of a moderately broad pre-pubis
directed downwards and forwards, and of a narrow greatly elongated
post-pubis directed backwards parallel to the ischium. The pubis is
united to both ilium and ischium, the acetabulum has a large
unossified space, and neither pre-pubes nor post-pubes meet in ventral
symphyses. The arrangement bears a great resemblance to that of Ratite
birds. In Lacertilia, Chelonia, Rhynchocephalia and Ichthyosauria
together with Theropod and Sauropod Dinosaurs the pubis corresponds to
the pre-pubis of _Iguanodon_ and is a more or less cylindrical bone
expanded at both ends, meeting its fellow in a ventral symphysis. In
Chelonia and Lacertilia the pubis bears a lateral process which is
homologous with the post-pubis of Iguanodon. In Lacertilia and
sometimes in Chelonia there is a cartilaginous epipubis attached to
the anterior border of the pubic symphysis; this is well developed in
the Chamaeleons and Geckos. In Crocodilia there is, forming the
anterior and ventral portion of the acetabulum, a patch of cartilage
(fig. 49, 3) which is probably the true pubis homologous with that of
lizards and with the pre-pubis of _Iguanodon_. The large bone
generally called the pubis in Crocodiles is probably an epipubis.


THE POSTERIOR LIMB.

The posterior limb is entirely absent in some Lacertilia and in most
Ophidia, though traces occur in _Python_, _Tortrix_ and _Typhlops_. In
the Ichthyosauria, Sauropterygia and Pythonomorpha the posterior limbs
form swimming paddles and have been already referred to.

The arrangement of the proximal and middle segments of the limb is
fairly constant in all reptiles with limbs adapted for walking, and
the tibia and fibula are always separate. The pes is however subject
to a considerable amount of variation, especially as regards the
tarsus. In some Chelonia the tarsus like the carpus has an extremely
simple arrangement, consisting of a proximal row of three bones, the
tibiale, intermedium and fibulare, a centrale, and a distal row of
five tarsalia. In most living reptiles, however, the tibiale and
intermedium are as in mammals united, forming the astragalus. In
Crocodiles (fig. 48, B, 15) the centrale is also united with the
tibiale while the distal tarsalia are very slightly developed. The
calcaneum in Crocodiles is drawn out into a long process forming a
heel in a manner almost unique among Sauropsida. In _Sphenodon_ and
Lacertilia the tibia and fibula articulate with a single large bone
representing the whole proximal row of tarsalia.

The pes is generally pentedactylate, but in some Crocodiles the fifth
digit is vestigial (fig. 48, B), and in some Dinosauria (fig. 35)
there are only three digits. The North American Dinosaurs present a
continuous series ranging from a pentedactylate plantigrade form like
_Morosaurus_, to such a form as _Hallopus_ with a highly digitigrade
and specialised pes reduced to three functional digits, and a
vestigial fifth metatarsal. The second, third and fourth metatarsals
in this form are nearly two-thirds as long as the femur, and the
calcaneum is drawn out into a heel much as it is in most mammals.

In Lacertilia, Orthopoda and many Chelonia, the ankle joint comes to
lie between the proximal and distal row of tarsals as in birds.


FOOTNOTES:

[91] See pp. 214 and 215.

[92] These terms are defined on p. 199.

[93] E.T. Newton, _Phil. Trans._ vol. CLXXXIV, B, p. 431 (1893).




CHAPTER XVII.

CLASS. AVES[94].


Birds form a large and extremely homogeneous class of the vertebrata,
and are readily distinguished from all other animals by the possession
of an epidermal exoskeleton having the form of feathers. Feathers
differ from hairs in the fact that they grow from papillae formed of
both the horny and the Malpighian layer of the epidermis, which
papillae at first project from the surface, and only subsequently
become imbedded in pits of the dermis. A dermal exoskeleton does not
occur in birds.

The endoskeleton is characterised by its lightness, the large bones
being generally hollow; but the pneumaticity does not vary in
proportion to the power of flight. The cervical part of the vertebral
column is very long and flexible, while the post-cervical portion is
generally very rigid, owing to the fusion of many of the vertebrae,
especially in the lumbar and sacral regions. The vertebrae are
generally without epiphyses to their centra. The cervical vertebrae in
living forms have saddle-shaped articulating surfaces, and many of
them bear ribs. The thoracic ribs in almost all birds have large
uncinate processes. The sternum is very large, and the ribs are always
attached to its sides, not as in many reptiles to anybackwardly
projecting process or processes. The sternum ossifies from two or more
centres.

The skull is extremely light, and its component bones show a great
tendency to fuse together completely. The facial part of the skull is
prolonged into a beak, chiefly formed of the premaxillae; this beak is
in all modern birds devoid of teeth, and is coated externally with a
horny epidermal sheath. The quadrate is large and freely movable. The
supratemporal arcade[1] is imperfect, while the infratemporal
arcade[95] is complete. There are no postorbital or postfrontal bones.
Neither parotic processes nor an interparietal foramen occur. There
are commonly large pre-orbital vacuities. The palatines and pterygoids
never form a secondary bony palate as in Crocodiles. Part of the floor
of the skull is formed by a wide _basitemporal_ (paired in the embryo)
which is continued in front as a long slender _rostrum_; these
structures have replaced the parasphenoid of Ichthyopsids. Cartilage
or bone is always developed in the sclerotic. The first branchial arch
is well developed, the hyoid arch but slightly. The coracoids are
large, and the clavicles are nearly always united forming the
_furcula_. There is no separate interclavicle and hardly any trace of
a precoracoid.

The anterior limbs form wings, and the manus is in the adult always
much modified, never having more than three digits. The three bones of
the pelvis are, except in Archaeornithes, always ankylosed together in
the adult, and the ilium is greatly prolonged in front of the
acetabulum, which is perforated. The ilia are not connected with the
sacrum by ossified sacral ribs. The pubes and ischia are directed
backwards parallel to one another, and except in a very few forms
never meet their fellows in ventral symphyses. The fibula is generally
much reduced. The proximal tarsal bones are always ankylosed to the
tibia, and the distal tarsals to the metatarsals, so that the ankle
joint is _intertarsal_. The first metatarsal is nearly always free.
The pes never has more than four digits in the adult.

The class Aves is most conveniently divided into two subclasses: 1.
Archaeornithes. 2. Neornithes.


Subclass I. ARCHAEORNITHES.

The only form referred to this subclass of extinct birds is
_Archaeopteryx_[96], the earliest known bird. In this animal the
skeleton does not seem to be pneumatic. The cervical and trunk
vertebrae are generally thought to be flat, certainly their
articulating surfaces are not saddle-shaped. There is no long compound
sacrum as in modern birds. The tail is longer than the whole body, the
caudal vertebrae are twenty in number, they gradually taper as traced
away from the trunk, and each bears a pair of feathers. The posterior
caudal vertebrae are not united together to form a _pygostyle_. The
upper jaw bears thirteen pairs of conical teeth, planted in distinct
sockets in the maxillae and premaxillae, but the mandible has only three
pairs. The presence of these teeth forms the most essential difference
between the skull of _Archaeopteryx_ and that of modern birds, and the
fact that they occur on the premaxillae renders it improbable that a
horny beak was present. There is a ring of ossifications in the
sclerotic. The ribs do not show uncinate processes, and articulate
with the vertebrae by single heads not divided into capitula and
tubercula. Abdominal ribs appear to have been present. The furcula is
large, and the scapula has a well developed acromion. The sternum is
unknown. The radius and ulna are approximately equal in size. In the
manus the first, second and third digits[97] are present, each
terminated by a claw. The second digit is considerably the longest,
while the third includes four phalanges. The three bones of the pelvis
probably remained distinct throughout life. The tarsals are ankylosed
respectively to the tibia and metatarsals as in other birds. The
metatarsals are ankylosed together, and the pes has four digits.


_Subclass II._ NEORNITHES.

To this subclass may be referred all known birds except
_Archaeopteryx_. They all agree in having a short tail whose component
vertebrae are commonly ankylosed together forming a pygostyle. The
three metacarpals do not all remain distinct. The bones of the pelvis
are ankylosed together, and to a large though variable number of
vertebrae. There are three orders, the Ratitae, Odontolcae, and
Carinatae.


_Order_ 1. RATITAE.

The Ratitae differ from _Archaeopteryx_ and the great majority of
Carinatae in being flightless. The bones are generally not pneumatic,
containing marrow instead of air, in the Ostrich however they are very
pneumatic. The tail is short and the posterior caudal vertebrae are
generally ankylosed together forming a pygostyle. The pectoral girdle
has comparatively a much smaller size than in Carinatae, clavicles are
small or absent, and the scapula and coracoid lie nearly in the same
straight line. The ilium and ischium do not as in Carinatae unite
posteriorly, and enclose a foramen except in very old Rheas and Emeus.
The quadrate articulates with the cranium by a single head. The vomers
unite and form a broad plate, separating the palatines, pterygoids and
basisphenoidal rostrum.

The anterior limbs are greatly reduced in size or even absent, while
the posterior limbs are greatly developed and adapted for running. The
tibia and fibula are quite distinct.

Many ornithologists agree that the various forms grouped together as
Ratitae are not all very closely allied to one another, that they
resemble one another mainly in having lost the power of flight, and do
not form a natural group.

The Ratitae include the following groups:--

_Æpyornithes_[98], huge extinct birds from Madagascar.

_Apteryges_, including the Apteryx of New Zealand.

_Dinornithes_[99], the Moas, huge extinct birds from New Zealand, and
some of the neighbouring islands.

_Megistanes_, including the Cassowaries (_Casuarius_) of Australia,
New Guinea, and some of the neighbouring islands; and the Emeus
(_Dromaeus_) of Australia.

_Rheornithes_, including the Rheas of S. America.

_Struthiornithes_, including the Ostriches (_Struthio_) now living in
Africa, and found fossil in N. India and Samos.


_Order_ 2. ODONTOLCAE.

This order includes only an extinct N. American bird
_Hesperornis_[100]. The jaws are provided with a series of sharp teeth
placed in continuous grooves, but the premaxillae are toothless, and
were probably sheathed in a horny beak. The rami of the mandible are
not ankylosed together in front. The skeleton is not pneumatic. The
cervical vertebrae have saddle-shaped articulating surfaces as in
ordinary birds, and the thoracic vertebrae are not ankylosed together.
The tail is comparatively long, and formed of twelve vertebrae with
only slight indications of a pygostyle. The ribs have uncinate
processes. The anterior limb is quite vestigial, being reduced to a
slender humerus. The posterior limb is very powerful and adapted for
swimming.


_Order_ 3. CARINATAE.

This order includes the vast majority of living birds. The cervical
vertebrae have saddle-shaped articulating surfaces (except in the
Ichthyornithiformes). The posterior caudal vertebrae are ankylosed
forming a pygostyle. The quadrate articulates with the cranium by a
double head. In all except the Tinamidae the vomers are narrow behind
and not interposed between the palatines, pterygoids and
basisphenoidal rostrum. The sternum has a median keel, and the
anterior limbs are in the great majority of cases adapted for flight.
Clavicles are well developed, and the scapula and coracoid are nearly
at right angles to one another. The various groups into which the
Carinatae are divisible are shown in the table on pp. 40-42. Their
special characters will not be dealt with.

[Illustration FIG. 55. _Gallus bankiva_ var. _domesticus_. THE LEFT
HALF OF THE SKELETON. The skull, vertebral column, and sternum are
bisected in the median plane. (After Marshall and Hurst.)

A, acetabulum. B, cerebral fossa. CB, cerebellar fossa. CL, clavicle.
CO, coracoid. CR, cervical rib. C 1 = one, first cervical vertebra.
FE, femur. HC, ventral end of clavicle. HU, humerus. HY, hyoid. IF,
ilio-sciatic foramen. IL, ilium. IS, ischium. L, lachrymal. MC 3,
postaxial metacarpal. MN, mandible. MS, xiphoid processes. MT,
tarso-metatarsus. MT 1, first metatarsal. N, nasal. OP, optic foramen.
P, premaxillae. PB, pubis. PL, palatine. PY, pygostyle. R, radius. RC,
radial carpal. S, keel of sternum. SC, scapula. T, tibio-tarsus. TH 4,
fourth thoracic vertebra. U, ulna. UC, ulnar carpal. UP, uncinate
process. Z, infra-orbital bar. 1, 2, 3, 4, first, second, third and
fourth digits of pes. 3, pre-axial, 4, middle, and 5, postaxial digit
of manus.]


FOOTNOTES:

[94] M. Fürbringer, _Untersuchungen zur Morphologie und Systematik der
Vögel_, I. and II. Amsterdam, 1888. Cf. H. Gadow, _Nature_, XXXIX.
1888, pp. 150 and 177.

T.H. Huxley, "On the classification of birds." _P.Z.S._, London, 1867.

E. Selenka and H. Gadow, _Vögel_ in Bronn's _Classen und Ordnungen des
Thierreichs_ 1869-1890.

[95] See p. 283.

[96] R. Owen, _Phil. Trans._, vol. CLIII., p. 33; 1863. T.H. Huxley,
_P. R.S._, vol. XVI., p. 243; 1868. C. Vogt, _Rev. Scient._, ser. 2,
tom. 9, p. 241; 1879. C.H. Hurst, _Nat. Sci._, vol. III., p. 275;
1893; vol. VI., pp. 112, 180, 244; 1895. W.P. Pycraft, _Nat. Sci._,
vol. V., pp. 350 and 437; 1894; and vol. VIII., p. 261; 1896.

[97] According to Hurst the fourth and fifth digits are also present.

[98] See C.W. Andrews, _P.Z.S._, 1894, p. 108.

[99] See T.J. Parker, _Tr. Zool. Soc. London_, vol. XIII., pt. 2,
1895, and F.W. Hutton, several papers in _Tr. N. Zealand Inst._, 1893
and 1895.

[100] See O.C. Marsh. _Odontornithes. A monograph of the extinct
toothed birds of N. America._ New Haven, 1880.




CHAPTER XVIII.

THE SKELETON OF THE WILD DUCK (_Anas boschas_).


I. EXOSKELETON.

The exoskeleton of the Duck and indeed of all birds is entirely
epidermal in origin. Its most important part consists of =feathers=,
but it includes also the following horny structures:--

(_a_) =scales=, which cover the toes and tarso-metatarsus;

(_b_) =claws=, which are attached to the distal phalanges of the toes
and of the pollex;

(_c_) the wide =beak=, which sheaths both upper and lower jaws, and
whose edges are raised into lamellae, which act as strainers.


FEATHERS.

A well developed feather, such as one of the large quill feathers of
the wing or tail, consists of the following parts: A main stem, the
=scapus=, which forms the axis running along the whole length of the
feather, and is divided into (1) a proximal hollow cylindrical
portion, the =calamus= or =quill=, and (2) a distal solid portion, the
=rachis= or =shaft=, which is square in section, flexible and grooved
along its ventral surface, and bears a number of lateral processes,
the =barbs=. The =calamus= which is partly imbedded in a pit in the
dermis, bears two holes: one, the =inferior umbilicus=, is at its
proximal end, and into it enters a vascular outgrowth from the dermis;
the other, the =superior umbilicus=, lies on the ventral surface at
the junction of the calamus and scapus.

The =barbs= are a series of narrow elastic plates, attached by their
bases to the rachis, and with their edges looking upwards and
downwards. The barbs are connected together by a number of smaller
processes, the =barbules=, which interlock with one another by means
of hooklets, and bear the same relation to the barbs that the barbs do
to the rachis. The barbs and barbules, together with the rachis,
constitute the =vexillum= or =vane= of the feather. Any feather having
the above type of structure is called a =penna= or a =contour
feather=, from the fact that it helps to produce the contour of the
body.


VARIETIES OF FEATHERS.

1. =Pennae.= There are two kinds of pennae or contour feathers.

(_a_) The =quills=. These form the large feathers of the wing and
tail. They are divided into two groups, the =remiges=, or wing quills,
and the =rectrices=, or tail quills.

The =remiges=[101] include three sets of feathers, the =primaries= or
=metacarpo-digitals=, which are attached to the bones of the manus,
the =secondaries= or =cubitals=, which are attached to the ulna, and
the =humerals=, which are attached to the humerus.

The =primaries= differ from all the other quill feathers in having the
posterior half of the vane much wider than the anterior half. They are
ten in number, and of these six, the =metacarpal= quills (fig. 57,
14), are attached to the second and third metacarpals, one, the
=ad-digital= (fig. 57, 15), to the phalanx of the third digit, two,
the =mid-digitals= (fig. 57, 16), to the first phalanx of the second
digit, and two, the =pre-digitals= (fig. 51, 17), to the second
phalanx of the second digit. One of the pre-digitals is very small,
and is called the =remicle= (fig. 57, 11).

[Illustration FIG. 56. THE WING OF A WILD DUCK (_Anas boschas_).

The upper figure shows the dorsal side of a right wing, the lower
figure the ventral side of a left wing. × 1/3. (Brit. Mus.)

    1. scapulars.
    2. tectrices marginales.
    3. tectrices minores.
    4. bastard wing.
    5. tectrices majores.
    6. metacarpo-digitals or primaries.
    7. tectrices mediae.
    8. cubitals or secondaries.
    9. pennae humerales.
    10. pennae axillares.]

In addition, a group of three quill feathers is attached to the first
digit, constituting the =bastard wing= or =ala spuria= (fig. 56, 4).

The =secondaries= or =cubitals= (fig. 56, 8) form a group of seventeen
feathers, attached to the ulna; they are shorter than the primaries,
and do not have the posterior half of the vane much wider than the
anterior half.

The =humerals= (figs. 56, 9 and 57, 12) form a group of eight small
feathers, of varying length, attached to the anterior half of the
humerus.

[Illustration FIG. 57. WINGS OF A WILD DUCK WITH THE COVERTS REMOVED
(_Anas boschas_). × 1/3.

A. Right wing seen from the dorsal side. B. Left wing disarticulated
and seen from the ventral side. (Brit. Mus.)

    1. humerus.
    2. radius.
    3. ulna.
    4. radial carpal.
    5. ulnar carpal.
    6. first phalanx of first digit.
    7. second metacarpal.
    8. third metacarpal.
    9. first phalanx of second digit.
    10. second phalanx of second digit.
    11. remicle.
    12. pennae humerales.
    13. cubitals or secondaries.
    14. metacarpal quills.
    15. ad-digital.
    16. mid-digitals.
    17. pre-digital.]

(b) The =tectrices= or =coverts= are short feathers, which cover over
the quills of the rectrices and remiges, and clothe the body
generally. Their barbules are less developed than is the case with the
quill feathers, so that the barbs separate readily from one another,
especially at the base of the vane. The nomenclature of the various
patches of coverts on the wings is seen in fig. 56. A small patch of
backwardly-directed feathers surrounding the external auditory opening
are known as the =auriculars=.

2. The =filoplumes= are rudimentary feathers, consisting of a minute
stem and slightly developed vane. They are left in the skin after the
other feathers have been removed.

3. The =plumulae=, or down feathers, have the stem very slightly
developed, while the barbs are soft and free from one another. They
are distributed all over the body, not only among the contour
feathers, but also over the spaces (_apteria_) which bear no contour
feathers.

In the young bird the rudiments of the new feathers are formed at the
bases of the embryonic down feathers, and as they grow they push them
out from the skin. The embryonic down feathers however remain attached
to the apices of the new feathers till these have reached a length of
about an inch; they are then shed.


II. ENDOSKELETON.

As compared with that of the Turtle or Crocodile, the endoskeleton of
the Duck is characterised by:

1. The great lightness of the bones, many of which contain air
cavities.

2. The tendency to become ankylosed together shown by many of the
bones.

3. The modification of the anterior limbs and girdle for the purpose
of flight.


1. THE AXIAL SKELETON.

This, as in other vertebrates, is divisible into--

    A. The vertebral column.

    B. The skull.

    C. The ribs and sternum.


A. THE VERTEBRAL COLUMN.

The vertebral column of the duck, like that of the great majority of
birds, presents a number of well-marked characteristics, contrasting
strongly with those of the generality of higher vertebrates. The centra
are always without epiphyses. The neck is exceedingly long, about as
long as all the rest of the vertebral column put together, and is
remarkable for its flexibility. The trunk portion of the vertebral
column on the other hand is characterised by extreme rigidity, and
the marked tendency shown by the component vertebrae to fuse together
into one almost continuous mass. The most rigid part of the vertebral
column is that to which the pelvis is united, as no less than seventeen
vertebrae take part in the union. The tail of the duck, like that of
all living birds, is very short, and the posterior caudal vertebrae are
united together, forming the =pygostyle=. The vertebral column may be
divided into cervical, thoracic, lumbar, sacral, and caudal regions,
but the boundaries between the several regions are ill-defined.


THE CERVICAL VERTEBRAE.

All the vertebrae anterior to the first one that bears a rib meeting
the sternum are regarded as cervical vertebrae. There are therefore
sixteen cervical vertebrae, the last two of which bear well developed
ribs. All are freely movable on one another.

As a typical cervical vertebrae, any one from the fifth to the ninth
may be taken. The vertebra is rather elongated, and is very lightly
and strongly made, its most characteristic feature being the shape of
the articulating surfaces of the centra, which are generally described
as saddle-shaped. The anterior surface is convex from above downward,
and concave from side to side, while the posterior and more prominent
surface is concave from above downwards and convex from side to side.
The neural arch is low, and is drawn out into a slight blade-like
=neural spine=. Its base is deeply notched on both sides posteriorly
for the exit of the spinal nerves. Above these notches it is drawn out
into two rather prominent diverging processes, which bear the
=postzygapophyses=,--two flattened surfaces which look downwards and
outwards. The =transverse processes= form irregular outgrowths from
the anterior two-thirds of the sides of the vertebra; each projects
for a short distance downwards and outwards, and is terminated
posteriorly by a short backwardly-projecting spine. The transverse
processes are shown by development to ossify from separate centres,
and are therefore to be regarded as cervical ribs, and each is
perforated at its base by a canal for the passage of the vertebral
artery. Above the anterior end of the vertebrarterial canal are a pair
of thickened outgrowths, which bear upwardly and inwardly directed
=prezygapophyses=. Each transverse process is perforated near its
middle by a prominent foramen through which passes a vein which is
connected with the jugular vein.

The third and fourth cervical vertebrae resemble the succeeding ones
in most respects, but have small =hypapophyses=, and the neural spines
are less blade-like. The posterior cervical vertebrae (tenth to
sixteenth) differ somewhat from the middle ones. They are shorter and
more massive, the neural arch is much shorter, being deeply notched in
the middle line in front and behind. The transverse processes arise
from the anterior half of the vertebra only, and in the eleventh
vertebra each is drawn out below into a pair of rather prominent
downwardly and inwardly directed processes. In the twelfth vertebra
these processes have almost coalesced, and in the thirteenth vertebra
they have coalesced completely, forming a prominent =hypapophysis=. In
the succeeding vertebrae this hypapophysis rapidly decreases in size.

The fifteenth and sixteenth cervical vertebrae resemble the succeeding
thoracic vertebrae, having short thick centra and prominent squarely
truncated neural spines; the sides of the neural arches are very
deeply notched. The fifteenth vertebra has a short transverse process,
perforated by a wide vertebrarterial foramen, but this foramen is
absent in the sixteenth. The transverse processes of the fifteenth
vertebra bear two facets for the articulation of the capitulum and
tuberculum of the rib. The sixteenth vertebra has its tubercular facet
on the transverse process, but the capitular facet is borne on the
centrum.

The second or =axis= vertebra is small, and has the centrum drawn out
into a comparatively very large hypapophysis. The posterior
articulating surface of the centrum is saddle-shaped, the anterior
nearly flat: above it the centrum is prolonged into the prominent
=odontoid process=, which is shown by development to be the detached
centrum of the atlas. The neural arch is deeply notched in the middle
line in front, and at the sides behind. It is drawn out posteriorly
into a wide massive outgrowth, which overhangs the third vertebra and
bears the downwardly-directed postzygapophyses. The prezygapophyses
are situated at the sides of the anterior end of the neural arch, and
look directly outwards. The transverse processes are very slightly
developed, and are pierced by the vertebrarterial canals.

The =atlas= vertebra is a very slight ring-like structure, thickened
ventrally and bearing in front a prominent concave cavity for
articulation with the occipital condyle of the skull. Posteriorly it
bears a more or less flattened surface for articulation with the
centrum of the axis. It surrounds a large cavity partially divided
into a larger dorsal portion, which is the neural canal, and a smaller
ventral portion which lodges the odontoid process. The sides of the
atlas are pierced by the vertebrarterial canals, above which there are
two slight backwardly-projecting outgrowths bearing the
postzygapophyses on their inner faces.


THE THORACIC VERTEBRAE.

The thoracic region includes all the vertebrae bearing free ribs,
except the first two, viz. those whose ribs do not reach the sternum.
There are seven thoracic vertebrae. The first four have centra with
saddle-shaped articulating surfaces, but are more or less firmly
united together by their neural spines; the last two are completely
ankylosed by their centra to the lumbar vertebrae.

Each of the first five vertebrae has a prominent, vertical, abruptly
terminated neural spine, and straight transverse processes. The
zygapophyses and articulating surfaces at the ends of the centra are
well developed. The third, fourth, fifth, and sixth vertebrae have
very prominent hypapophyses. The articular facets for the ribs are
well marked, those for the tubercula lying at the free ends of the
transverse processes, and those for the capitula at the sides of the
anterior ends of the centra. The sixth and seventh thoracic vertebrae
are firmly fused by their centra and neural arches to one another and
to the lumbar vertebrae behind, and by their transverse processes to
the ilia. The sixth has its centrum terminated in front by a
saddle-shaped articulating surface, and bears a pair of prominent
prezygapophyses. Its transverse processes and centrum bear facets for
the tubercula and capitula of the ribs respectively. In the seventh
vertebra the tubercular facet is wanting.


THE SACRUM.

[Illustration FIG. 58. A, DORSAL AND B, VENTRAL VIEW OF THE PELVIS AND
SACRUM OF A DUCK (_Anas boschas_).

    1. ilium.
    2. ischium.
    3. pubis.
    4. pectineal process.
    5. lumbar vertebrae.
    6. true sacral vertebrae.]

The =sacrum= generally consists of seventeen vertebrae fused with one
another and with the ilia. Their number may be reckoned from the
number of foramina for the exit of spinal nerves. The two most
anterior of these vertebrae bear ribs and have been already described
with the other thoracic vertebrae. Their neural spines and those of
the four succeeding vertebrae are fused together, forming a continuous
crest of bone completely united laterally with the ilia. The
transverse processes of all these six vertebrae are well developed,
but those of the posterior two (fig. 58, B, 5) are much the stoutest.
The next three vertebrae have broad centra, but their transverse
processes are very slightly developed and have no ventral elements.
These seven vertebrae belong to the =lumbar= series. The remaining
eight vertebrae have well-developed transverse processes, which in the
case of the first three or four are divisible into dorsal and ventral
elements. All the dorsal elements are united to form a pair of
flattened plates, partially separated by a series of foramina from a
median plate formed by the united neural arches. Laterally they are
continuous with the ischia. The first two of this series of vertebrae
are shown by their relation to the nerves to be the true =sacrals=
(fig. 58, B, 6), the remaining six belonging to the =caudal= series.

Behind them come the six free caudal vertebrae, succeeded by a
terminal piece, the =pygostyle=, formed of a number of vertebrae fused
together; this bears the rectrices or tail quills.

[Illustration FIG. 59. SKULL OF A DUCK (_Anas boschas_). × 1.

A. Dorsal view of the cranium. B. Palatal view of the mandible. C. The
Hyoid.

For numbers see Fig. 60.]


B. THE SKULL.

[Illustration FIG. 60. A. Ventral view of the cranium of a Duck (_Anas
boschas_). B. Cranium and mandible seen from the left side. × 1.

    1. maxillae.
    2. premaxillae.
    3. anterior nares.
    4. nasal process of premaxillae (fig. 59).
    5. nasal.
    6. frontal (fig. 59).
    7. lachrymal.
    8. postfrontal process.
    9. parietal (fig. 59).
    10. jugal.
    11. quadratojugal.
    12. quadrate.
    13. condyle of mandible.
    14. posterior articular process.
    15. dentary at symphysis. }
    16. basi-hyal.            } (fig.
    17. uro-hyal.             } 59).
    18. basibranchial.        }
    19. vomer.
    20. palatine.
    21. pterygoid.
    22. anterior palatine foramen.
    23. basitemporal.
    24. foramen leading into tympanic cavity.
    25. bristle inserted into posterior opening of carotid canal.
    26. bristle inserted into posterior opening of Eustachian canal.
    27. bristle emerging through anterior opening of carotid canal.
         Close by is seen the bristle emerging through the anterior
          opening of the Eustachian canal.
    28. fenestral recess.
    29. maxillo-palatine.
    30. lambdoidal crest.
    31. rostrum.

I. II. IV. V. IX. X. nerve foramina.]

The skull of the duck, like that of birds in general, is characterised
(1) by its lightness, (2) by the contrast between the bones of the
cranium proper and those forming the rest of the skull, for the bones
forming the cranium proper are closely fused together, the sutures
between them being nearly all completely obliterated in the adult,
while the bones forming the face are loosely connected with the
cranium proper; (3) by the prolongation of the face into a long
toothless beak; (4) by the size of the orbits, and their position
entirely in front of the cranium, so that they are separated from one
another only by a thin interorbital septum.

For purposes of description the skull may be divided into

    (1) The cranial portion.
    (2) The facial portion.
    (3) The mandible.
    (4) The hyoid.


(1) THE CRANIAL PORTION.

This is a rounded box expanded dorsally and posteriorly, but tapering
antero-ventrally. In the young skull the divisional lines between the
several bones can be easily seen, but in the adult they are quite
obliterated.

(_a_) The _dorsal surface_ is rounded, expanded in front and behind,
but encroached upon in the middle by the cavities of the orbits. There
is a prominent divisional line in front, separating it from the facial
part of the skull. It is formed mainly by the _frontal_ (fig. 59, A,
6) and _parietal_ bones, but the frontals diverge a little anteriorly
and enclose between them the ends of the _nasal processes_ (fig. 59,
A, 4) of the _premaxillae_. Just in front of the orbit the outer
margins of the frontals are either notched or pierced by a pair of
foramina.

(_b_) At the _posterior end_ of the cranium the most prominent feature
is the large, almost circular =foramen magnum=, through which the
spinal cord and brain communicate; this in young birds is seen to be
bounded by four distinct bones, dorsally by the =supra-occipital=,
ventrally by the =basi-occipital=, and laterally by the
=exoccipitals=.

The =basi-occipital= forms the main part of a prominent convex knob,
the =occipital condyle=, with which the atlas articulates. The
occipital condyle is slightly notched above, and the ventral surface
of the cranium is deeply pitted just in front of it; the exoccipitals
also contribute slightly to its formation. Slightly in front of and
ventral to the foramen magnum is a small foramen through which the
hypoglossal nerve leaves the cranial cavity.

The =supra-occipital= is separated from the parietal by a suture line
along which run a pair of prominent ridges, the =lambdoidal crests=
(fig. 60, B, 30). There are often a pair of prominent vacuities in the
supra-occipital dorsal to the foramen magnum. The =epi-otics= and
=opisthotics= become completely fused with the bones of the occipital
segment at a very early stage.

(_c_) The _ventral surface_ of the cranium is wide behind, where it is
formed by a broad transverse membrane bone, the _basitemporal_ (fig.
60, A, 23), the sides of which are fused with the auditory capsules.
Slightly in front of and an eighth of an inch external to the
hypoglossal foramen the cranial wall is pierced by a pair of foramina
through which the tenth or pneumogastric nerves leave (fig. 60, A, X).
At the sides of the basitemporal are a pair of depressions, the
=tympanic recesses=, in each of which are three holes. Straight lines
joining these holes would form an isosceles triangle with its apex
directed forwards. Of the two holes at the base of the triangle, the
one nearer the middle line and leading into the cranial cavity, is for
the exit of the ninth or glossopharyngeal nerve (fig. 60, A, IX), it
lies just in front of the pneumogastric foramen. The more external
leads into the tympanic cavity, while the more anterior at the apex of
the triangle is the =posterior opening of the carotid canal= (fig. 60,
A, 25), which traverses the base of the cranium, and during life
lodges the carotid artery.

The anterior end of the basitemporal is pierced near the middle line
by a pair of holes, the =anterior openings of the Eustachian canals=;
while just in front of these and a little further removed from the
middle line are the anterior openings of the =carotid canals=.
Bristles passed in through the posterior openings of the carotid
canals will emerge here (fig. 60, A, 27). In front of the basitemporal
the base of the cranium is formed by the =rostrum= (fig. 60, A, 31),
or thickened basal portion of the interorbital septum; this bears two
prominent surfaces with which the pterygoids articulate. In some kinds
of duck these surfaces are borne by well-marked basi-pterygoid
processes.

(_d_) _The side of the cranium._ At the base of the posterior end is
seen the deep =tympanic cavity=. The dorsal part of this is divided by
a vertical partition into two halves; of these the more anterior is
the larger, and forms a deep funnel-shaped cavity, the =posterior
opening of the Eustachian canal= (fig. 60, B, 26). A bristle passed
into this opening emerges through the anterior opening of the
Eustachian canal. The more posterior of the two is the =fenestral
recess= (fig. 60, B, 28), and is in its turn divided by a slender
horizontal bar into a dorsal hole, the =fenestra ovalis=, and a
ventral hole, the =fenestra rotunda=. During life the fenestra ovalis
lodges the proximal end of the =columellar= chain. Lying at the outer
side and slightly dorsal to the tympanic cavity is a deep depression,
the =lateral tympanic recess=, and immediately in front of this is the
articular surface for the quadrate. The tympanic cavity is bounded
below by the basitemporal, posteriorly by the exoccipital, and above
by the _squamosal_, a membrane bone, which roofs over a good deal of
the side of the cranium, and bears ventrally a prominent surface with
which the quadrate articulates. Just in front of this is a large round
hole, the =trigeminal foramen= (fig. 60, B, V), behind which the
squamosal is drawn out into a short process.

In front of the squamosal there is a prominent forwardly-projecting
=postfrontal process= (fig. 60, 8), which ossifies from a different
centre from that forming the squamosal, but in the adult is completely
fused with it.

The =orbit= forms a large more or less hemispherical cavity which
lodges the eyeball. It is separated from its fellow of the opposite
side by an imperfect partition, the =interorbital septum=. In the
young skull it is seen to be bounded above by the frontal, with which
the _lachrymal_ (fig. 60, 7) is fused anteriorly, forming a large
backwardly-projecting process; while behind it is bounded by
the =alisphenoid=. The interorbital septum is formed by the
ossification and coalescence of the =mesethmoid= in front, with the
=orbitosphenoid= behind, and the =rostrum= below. The boundary of the
orbit below is very imperfect, the zygomatic arch being incomplete.

The interorbital septum is pierced by the very prominent =optic
foramen= (fig. 60, B, 2), just behind which are the two much smaller
foramina for the exit of the oculomotor and pathetic (fig. 60, B, IV)
nerves, the more anterior being that for the oculomotor.

Above and slightly in front of the optic foramen is a median opening,
the =olfactory foramen.= This leads into the cranial cavity behind,
and in front is continued forwards as a groove between the
interorbital septum and the frontal.


(2) THE FACIAL PART OF THE SKULL.

This includes the olfactory capsule and associated bones, and the
upper jaw.

The bones associated with the olfactory capsules are the _nasals_ and
_vomer_. The _nasals_ (figs. 59 and 60, 5) lie on the dorsal surface
immediately in front of the cranium, and are separated from one
another by the nasal processes of the premaxillae. Each is completely
fused in the adult with the corresponding maxillae and premaxillae, the
three bones together forming the boundary of the =anterior nares.= The
_vomer_ (fig. 60, 19) is unpaired and forms a small median vertical
plate lying ventral to the anterior continuation of the interorbital
septum.

The bones of the upper jaw consist on each side of two slender arcades
which in front converge and are attached to the large beak, while
behind they diverge but are united by the =quadrate=.

The =inner arcade= is formed by the pterygoid and palatine. The
_pterygoid_ (fig. 60, 21) is a short flattened bone, which articulates
behind with the quadrate, and on its inner side with a large flattened
surface borne by the rostrum, in front it meets the palatine, or
sometimes ends freely with a long antero-dorsally directed point.

The _palatine_ (fig. 60, 20) is a slender irregular bone flattened
dorso-ventrally at its anterior end where it articulates with the
beak, and laterally behind. It gives off at its posterior end a
process, which is sometimes united with the vomer, sometimes projects
forwards, and meets its fellow dorsal to the vomer. In the large space
between it and the vomer is the opening of the =posterior nares=.

The _premaxillae_ (figs. 59 and 60, 2) are very large, and form nearly
a third of the big shovel-shaped beak. They constitute the inner, and
part of the front boundary of the anterior nares, and send back a pair
of _nasal processes_ which partially separate the nasals from one
another.

The =outer arcade= forms the slender =suborbital bar=, and consists
mainly of two rod-like bones, which in the adult are completely fused
together. The posterior of these is the _quadratojugal_ (figs. 59 and
60, 11) which articulates with the quadrate, the anterior is the small
and slender _jugal_ or _malar_ (figs. 59 and 60, 10). The extreme
anterior part of the bar is formed by the _maxillae_. The main part of
the maxillae however lies anterior to the suborbital bar, and extends
forwards along the side of the premaxillae forming all the lateral part
of the beak (figs. 59 and 60, 1); it also sends inwards a plate, the
=maxillo-palatine= (fig. 60, A, 29), which completely fuses with its
fellow in the middle line, and forms the posterior boundary of the
anterior palatine foramen. The term =desmognathous= describes the
condition of the skull in which the maxillo-palatines fuse with one
another in the middle line in this way.

The =quadrate= (fig. 60, 12), which unites the two arcades behind, is
a stout irregular four-cornered bone forming the =suspensorium=. It
articulates by its dorso-posterior corner with the squamosal, and by
its antero-internal corner with the pterygoid. The middle of its
ventral surface forms a hemispherical knob with which the mandible
articulates, while its dorso-anterior border is drawn out into a long
point which extends towards the interorbital septum.


(3) THE MANDIBLE.

The =mandible= or lower jaw consists of two =rami= which are flattened
and fused together in the middle line in front, while behind they
diverge from one another and articulate with the quadrates.

Each ramus is composed of five bones fused together, one being a
cartilage bone, and the other four membrane bones. The =articular= is
the only cartilage bone of the mandible, it bears the double condyle
(figs. 59 and 60, 13) or concave articular surface for the quadrate,
and is drawn out behind into a large hooked =posterior articular
process=. The articular is also drawn out into a prominent process on
each side of the articular surface for the quadrate, and is marked by
a deep pit opening posteriorly. The articular is continuous in front
with =Meckel's cartilage= which forms the original cartilaginous bar
of the lower jaw, and is ensheathed by the membrane bones. Of these
the _supra-angular_ forms the upper part of the mandible in front of
the articular, its dorsal surface is drawn out into a small =coronoid
process=, its outer surface also bearing a prominent process. The
_angular_ is a small bone which underlies the articular and
supra-angular on the inner side of the jaw. The _dentary_ (fig. 59,
15) forms the anterior half of each ramus, and is the largest bone of
the mandible; it is fused with its fellow at the symphysis in front,
and extends back below the supra-angular. The _splenial_ is a small
bone lying along the middle half of the inner side of each ramus of
the mandible.


(4) THE HYOID.

With the hyoid apparatus is included the =columella=. This forms a
minute rod of bone, one end of which is expanded and fits into the
fenestra ovalis, while the other end, terminated by a triradiate piece
of cartilage, is attached to the tympanic membrane. The structure is
as a whole homologous with the auditory ossicles of mammals and the
hyomandibular of fish.

The =hyoid= consists of a median unpaired portion, formed of two
pieces of bone, the =basi-hyal= (fig. 59, C, 16) in front, and the
=uro-hyal= (fig. 59, C, 17) behind, the two being placed end to end
and terminated anteriorly by an unpaired cartilaginous plate, the =os
entoglossum.= At the posterior end there come off a pair of long
=posterior cornua=, each of which consists of two pieces, a longer
=basibranchial= (fig. 59, C, 18), and a shorter =cerato-branchial=.
For the homology of these parts see p. 336.


THE RIBS AND STERNUM.

The last two cervical vertebrae bear long movable ribs which
articulate by distinct capitular and tubercular processes, but do not
meet the sternum. The thoracic ribs are eight in number, and each is
divisible into a =vertebral= and a =sternal= portion. The first five
thoracic ribs are flattened curved bars of bone, which articulate by a
prominent =capitulum= with the centrum of the corresponding vertebra,
and by a =tuberculum= with the transverse process. Projecting
backwards from each is a large hooked =uncinate process.= The last
three ribs which are without uncinate processes, become progressively
more slender, and in the eighth the tubercular processes are lost.

The sternal portions of the ribs are imperfectly ossified pieces,
short and comparatively thick in the case of the anterior ribs, longer
and more slender in the case of the posterior ribs.


THE STERNUM[1].

The =sternum= or breast bone is exceedingly large in the Duck, as in
all birds, and projects back far beyond the thorax over much of the
anterior part of the abdomen. It is an irregularly oblong plate of
bone, abruptly truncated behind, somewhat concave dorsally, and drawn
out ventrally into a prominent keel, the =carina=, which projects for
some distance forwards beyond the body of the sternum, and tapers off
gradually behind. The point where the carina joins the body of the
sternum is at the anterior end drawn out into a small process, the
=rostrum=[102]. Just dorsolateral to this are a pair of deep grooves,
the =coracoid grooves=, with which the coracoids articulate.

The sides of the sternum are drawn out in front into a pair of short
blunt =costal processes;= and just behind these are a series of seven
surfaces with which the ends of the sternal ribs articulate.
Immediately behind these surfaces the sides are produced into a pair
of long backwardly-projecting =xiphoid processes= which nearly meet
processes from the posterior end of the sternum.


2. THE APPENDICULAR SKELETON.

This consists of the skeleton of the anterior and posterior limbs and
of their respective girdles.


A. THE PECTORAL GIRDLE[103].

The pectoral girdle in almost all birds is strongly constructed and
firmly united to the sternum. It consists of three bones, a dorsal
element, the =scapula=, a posterior ventral element, the =coracoid=,
and an anterior ventral element, the _clavicle_.

The =scapula= forms a long curved flattened bone expanded at its
anterior end, where it meets the coracoid, and lying across the ribs
at its tapering posterior end. It helps to form the imperfect =glenoid
cavity=, with which the humerus articulates. The =coracoid=, a shorter
but stouter bone than the scapula, has its upper end or =head=
thickened and bears on its posterior border an irregular surface, with
part of which the scapula articulates, while the rest forms part of
the glenoid cavity. The inner border of the coracoid adjoining the
articular facet for the scapula is produced into a strong process
which helps to complete the =foramen triosseum=, a space lying between
the adjoining ends of the scapula and coracoid, through which the
tendon of the second pectoral muscle passes. The lower part of the
coracoid, which is much flattened and expanded, and abruptly truncated
posteriorly, articulates with the coracoid groove of the sternum. The
_clavicle_ is a thickened curved membrane bone, which is fused with
its fellow in the middle line below, the two forming the _furcula_ or
merrythought. Its dorsal end is drawn out into a process which
articulates with the coracoid.


THE ANTERIOR LIMB OR WING.

This consists of three parts, a proximal part, the upper arm or
=brachium=, a middle part, the fore-arm or =antibrachium=, and a
distal part, the =manus=. When extended for flight the parts lie
almost in the same straight line, but when at rest they are folded on
one another in the form of a Z, the brachium and manus pointing
backwards, and the antibrachium forwards. When extended for flight the
surfaces and borders of the wing correspond in position with those of
the primitive vertebrate limb[104], the pre-axial border being
directed forwards and the postaxial backwards, while the dorsal and
ventral surfaces look respectively upwards and downwards. But when the
wing is at rest, the humerus as it extends backwards becomes slightly
rotated, so that its dorsal surface looks more inwards than upwards,
while the dorsal surface of the antibrachium looks partially outwards
and upwards, and that of the manus mainly outwards.

The =brachium= or =upper arm= contains only a single bone, the
=humerus= (fig. 57, 1). This is a large nearly straight bone expanded
at both ends. The proximal end is specially expanded, forming two
=tuberosities=, and a large convex =head= articulating with the
glenoid cavity. The =pre-axial tuberosity= is the smaller of the two,
but is continued by a prominent =deltoid ridge=, which extends for a
very short distance down the shaft. The =postaxial tuberosity= is the
larger, and below it there is a very deep pit, the =pneumatic
foramen=, which leads into an air cavity in the shaft of the bone. The
shaft is long and straight, and at the distal end of the bone is the
=trochlea= with two convex surfaces, one pre-axial with which the
radius articulates, the other postaxial for the ulna.

The =fore-arm= or =antibrachium= consists of two bones, the =radius=
and =ulna=. These are of nearly equal length, and are separated from
one another by a considerable space except at their terminations.

The =radius= (fig. 57, 2), the pre-axial and smaller bone, is straight
and fairly stout; its proximal end articulates with the humerus by a
slightly cupped surface, while its distal end, which articulates with
the carpus, is convex and somewhat expanded.

The =ulna= (fig. 57, 3) is longer, stouter, and slightly curved. Its
proximal end is expanded, forming two surfaces which articulate with
the trochlea of the humerus; behind them it is drawn out into a short
blunt =olecranon process=. Its distal end is less expanded, and
articulates with the carpus and also with the radius.

The =Manus=. This includes the carpus or wrist, and the hand.

The =Carpus=. While in the embryo the carpus consists of five
distinct elements arranged in a proximal row of two and a distal row
of three, in the adult only the proximal bones can be clearly
distinguished, the distal ones having become completely ankylosed with
the metacarpals to form the =carpo-metacarpus=.

The two distinct carpal bones are the radial carpal and the ulnar
carpal. The radial carpal (fig. 57, 4) is a small somewhat cubical
bone, wedged in between the manus and the radius and ulna. The ulnar
carpal (fig. 57, 5) is a somewhat larger, more irregular bone, lying
adjacent to the end of the ulna. It is deeply notched to receive the
carpo-metacarpus.

=The hand.= In the adult bird the hand is in a much modified
condition; only the first three digits are represented, and the
metacarpals are all fused with one another and with the distal
carpalia to form the =carpo-metacarpus=.

The most prominent part of the carpo-metacarpus is formed by the
=second metacarpal= (fig. 57, 7), a stout, straight bone expanded at
both ends. The =third metacarpal= (fig. 57, 8) is a more slender
curved bone fused at both ends with the second metacarpal. The =first
metacarpal= forms simply a small projection on the radial side of the
proximal end of the second metacarpal.

The =phalanges=. The first digit or =pollex= includes two phalanges,
the distal one being very small and bearing a claw.

The second digit includes three phalanges, the proximal one being
somewhat flattened. The third digit has a single small phalanx.


THE PELVIC GIRDLE.

The bones constituting the pelvic girdle are not only as in other
higher vertebrates ankylosed together forming the innominate bones,
but are also ankylosed with a series of some seventeen sacral and
pseudosacral vertebrae. The =acetabulum= (fig. 61, 5) with which the
head of the femur articulates is incompletely ossified.

The =ilium= (figs. 58 and 61, 1) is the largest bone of the pelvis. It
forms a long flattened plate extending for a considerable distance
both in front of and behind the acetabulum, and is fused along its
whole length with the transverse processes and neural spines of the
sacral and pseudosacral vertebrae. It forms more than half the
acetabulum, above and behind which it is produced to form a process,
the =antitrochanter= (fig 61, 8), with which the great trochanter of
the femur articulates.

[Illustration FIG. 61. LATERAL VIEW OF THE PELVIS AND SACRUM OF A DUCK
(_Anas boschas_) × 2/3.

    1. ilium.
    2. ischium.
    3. pubis.
    4. pectineal process.
    5. acetabulum.
    6. ilio-sciatic foramen.
    7. fused vertebrae.
    8. antitrochanter.]

The =ischium= (figs. 58 and 61, 2) is a flattened bone which forms
about one-third of the acetabulum, and lies ventral to the posterior
part of the ilium. Its anterior portion is separated from the ilium by
the large oval =ilio-sciatic foramen= (fig. 61, 6), while behind this
the two bones are completely fused.

The =pubis= (figs. 58 and 61, 3) is a very long slender bar of bone
which forms only a very small part of the acetabulum and runs back
parallel to the ventral surface of the ischium with which it is
loosely connected at its posterior end. For the greater part of their
length the two bones are separated by the long narrow =obturator
foramen=. Behind the ischium the pubis is produced into a long curved
downwardly-projecting process, and in front of the acetabulum it bears
a short blunt =pectineal= or =pre-pubic process= (fig. 61, 4) probably
homologous with the pre-pubis of Orthopod Dinosaurs. The remainder of
the pubis is homologous with the post-pubis of Orthopod Dinosaurs.


THE POSTERIOR LIMB.

The leg of the bird is somewhat differently constructed from that of
other vertebrates owing to the fact that there is no free tarsus, the
proximal tarsals having fused with the tibia, and the distal with the
metatarsals.

The =thigh= consists of a single bone, the femur. The =femur= is a
comparatively short bone with a straight shaft and expanded ends. The
proximal end bears on its inner side a rounded =head=, which
articulates with the acetabulum. On its outer side is an irregular
outgrowth, the =great trochanter=, while between the two is the
surface which meets the antitrochanter of the ilium. The posterior end
also is expanded and marked by a wide groove which lodges the
=patella=. On each side of the groove is a strong =condylar ridge= for
articulation with the tibia. The external condyle is deeply grooved
behind for articulation with the fibula.

The =crus= or =shin= consists of two separate bones, (1) the
=tibio-tarsus=, formed by the fusion of the tibia with the proximal
row of tarsals, and (2) the =fibula=.

The =tibio-tarsus= is a thick straight bone nearly twice as long as
the femur. Both ends of the bone are considerably expanded. The
proximal end bears two slight depressions which articulate with the
condyles of the femur, and a third depression which partly lodges the
patella. The proximal end of the anterior or extensor surface is drawn
out into a very prominent =cnemial crest= which bends over towards the
postaxial side of the bone; a slight ridge is continued from it all
the way down the shaft. The proximal part of the shaft of the
tibio-tarsus bears a roughened ridge with which the fibula is closely
connected. The distal end is expanded and rotated outwardly and forms
a prominent pulley-like surface which articulates with the
tarso-metatarsus.

The =fibula= is reduced to the proximal portion only, which is
expanded and articulates with a depression behind the external condyle
of the femur. The fibula further extends about a third of the way down
the shaft of the tibio-tarsus. The =patella= or =knee-cap= is a
sesamoid bone due to an ossification in the tendon of the extensor
muscles of the leg.

The =ankle joint= lies between the proximal and distal tarsals which
as previously mentioned fuse respectively with the tibia and
metatarsus.

The =Pes=. The pes includes four digits, and consists of the
tarso-metatarsus and the phalanges. The proximal tarsals which are
fused with the tibia also really belong to the pes.

The =tarso-metatarsus= is a strong straight bone nearly as long as the
femur, and is formed by the fusion of the distal tarsals with the
second, third and fourth metatarsals. The proximal end of the bone is
expanded and bears two facets for articulation with the tibio-tarsus,
and near them on the posterior surface is a large roughened
projection. The lines of junction between the several metatarsals are
marked along the shaft by slight ridges. At the distal end of the bone
the three metatarsals diverge from one another and each bears a
prominent convex pulley-like surface. The =first metatarsal= is
reduced to the distal end, which tapers to a point proximally, and is
attached by ligaments near the distal end of the tarso-metatarsus.

The =digits=. Four digits are present, each consisting of a metatarsal
(already described) and a certain number of phalanges, the terminal
one being in each case clawed. The first digit or =hallux= has two
phalanges, the second three, the third four, and the fourth five.


FOOTNOTES:

[101] See R.S. Wray, _P.Z.S._, 1887, p. 343.

[102] Often called the manubrium, but not homologous with the
manubrium of the mammalian sternum.

[103] Cp. fig. 63.

[104] See p. 28.




CHAPTER XIX.

GENERAL ACCOUNT OF THE SKELETON IN BIRDS.


EXOSKELETON.

The epidermal exoskeleton of birds is very greatly developed, feathers
constituting its most important part.

Three kinds of feathers are found, viz. (_a_) _pennae_ including
quills and coverts, (_b_) down feathers or _plumulae_, and (_c_)
_filoplumes_ which are rudimentary feathers. The structure of the
different kinds of feathers is described on pp. 303-306.

Sometimes a fourth class of feathers, the _semiplumae_, is recognised.
They have the stems of pennae, and the downy barbs and barbules of
plumulae.

In most birds the pennae are not uniformly distributed over the whole
surface of the body, but are confined to certain tracts, the
=pterylae=; while the intervening spaces or =apteria= are either bare
or covered only with down feathers. In some birds, however, such as
the Ratitae and the Penguins, pennae are evenly distributed over the
whole body.

In many birds the calamus or quill bears two vexilla or vanes, the
second of which, called the =aftershaft= or =hyporachis=, is generally
much the smaller, and is attached to the under surface of the main
vexillum. In the Moas, Emeu and Cassowary the two vexilla in the adult
bird are nearly equal in size; though in the nestling Emeu one is much
longer than the other. The aftershaft is very small in most Passeres
and gallinaceous birds, but is comparatively large in Parrots, Gulls,
Herons and most birds of prey. It is absent or extremely small in the
Ostrich, _Apteryx_, _Rhea_, Pigeons, Owls, Anseres, and others.

The quill feathers include two groups, the =remiges= or wing quills,
and the =rectrices= or tail quills. In most birds the primary remiges,
or those which are attached to the bones of the manus, are ten or
eleven in number, and are set in grooves in the bones, being firmly
attached to them. In the Ostrich however the primaries are little
specialised in character and are as many as sixteen in number. They
are also less definitely attached to the bones; as their ends do not
lie in grooves in the bones, but project beyond them.

The secondary quills or those attached to the ulna vary much in number
according to the length of the bone. The large dark quills in the
wings of Cassowaries are the secondaries.

The wing of Penguins is very little differentiated. It is covered at
the margin by overlapping scales which gradually merge into scale-like
feathers at the proximal end. The wing of the Penguin has nothing
comparable to the remiges of other birds.

In some birds, such as Herons (_Ardea_), there occur in places
plumulae of a peculiar kind, which grow persistently and whose summits
break off into fine powder as fast as they are formed. These feathers
are known as _powder-down_ feathers. They occur also in some Parrots
and are then scattered indiscriminately all over the body.

Other exoskeletal structures besides feathers are commonly well
developed. Thus the extremities of the jaws are sheathed in horny
=beaks= whose form varies enormously according to the special mode of
life.

In ducks and geese the beak with the exception of the anterior end is
soft, and its edges are raised into lamellae, while in the Mergansers
these lamellae become pointed processes supported by bony outgrowths.
These lamellae act as strainers. In Parrots and Hawks, on the other
hand, nearly the whole of the beak is hard.

The toes and tarso-metatarsus are usually featherless and are covered
either with granular structures or with well-formed scales. The toes
are nearly always provided with =claws=, and these vary in correlation
with the character of the beak. Claws[105] also sometimes occur on the
manus. Thus _Archaeopteryx_ and some Ostriches and Rheas have claws on
all three digits. Most Ostriches and Rheas, and many Anseres and birds
of prey, have them on the first two digits, while the Secretary bird
(_Gypogeranus_) and many fowls, ducks, and birds of prey, especially
kestrels, have a claw only on the pollex. In the Cassowary, Emeu,
Apteryx and some Ostriches and Rheas only the second digit is clawed.

Claws should not be confounded with =spurs=, which are conical horny
structures developed on bony outgrowths of the radial side of the
carpus, metacarpus, or metatarsus. They occur in a number of birds,
but are most commonly developed in gallinaceous birds, by which they
are used for fighting. A single spur occurs on the metacarpus in
_Megapodius_, in _Palamedea_, in _Parra jacana_ and in _Hoplopterus
spinosus_, the Spur-winged plover. The Derbian Screamer, _Chauna
derbiana_, has two metacarpal spurs, borne on the first and second
metacarpals. The Spur-winged goose, _Plectropterus gambensis_, has a
carpal spur borne on the radial carpal. Metatarsal spurs are quite
common.

The male Solitaire (_Pezophaps_) has large bony excrescences on the
wrist which may, like spurs, have been sheathed in horn and used for
fighting.

=Teeth= do not occur in any living birds, but conical teeth imbedded
in separate sockets are present in _Archaeopteryx_ and _Ichthyornis_,
while in _Hesperornis_ similar teeth occur implanted in continuous
grooves in the mandibles and maxillae, the premaxillae being toothless.

Except that teeth are partly dermal in origin, a dermal exoskeleton is
quite unrepresented in birds.


ENDOSKELETON.

Perhaps the most striking feature of the endoskeleton of birds is its
pneumaticity. In the embryo all the bones contain marrow, but as
growth proceeds this becomes replaced by air to a variable extent in
different forms. In all birds some part of the skeleton is pneumatic.
Many small birds and _Apteryx_ and Penguins among larger ones have air
only in the skull; in Pigeons air is present in all the bones except
the caudal vertebrae, the leg bones, and those of the antibrachium and
manus; in Hornbills every bone contains air.

[Illustration FIG. 62. THIRD CERVICAL VERTEBRA OF AN OSTRICH
(_Struthio camelus_). × 1. A anterior, B posterior, C dorsal view (A
and B after MIVART).

    1. neural spine.
    2. neural canal.
    3. prezygapophysis.
    4. postzygapophysis.
    5. posterior articular surface of centrum.
    6. anterior articular surface of centrum.
    7. vertebrarterial canal.
    8. hypapophysis.]


VERTEBRAL COLUMN.

The vertebral column of birds is readily divisible into a very mobile
cervical region, and an extremely rigid post-cervical region. In most
birds the vertebral centra are without terminal epiphyses, but these
structures are found in Parrots. The cervical vertebrae are generally
large and vary in number from eight or nine to twenty-three in Swans.
Except in some extinct forms, such as _Ichthyornis_ and _Apatornis_,
in which they are biconcave, the centra are characterised by having
saddle-shaped articulating surfaces, which in front are concave from
side to side and slightly convex from above downwards, while
posteriorly they are convex from side to side and concave from above
downwards. The atlas is small and ring-like, and its centrum is fused
with the axis forming the odontoid process. Cervical ribs are often
well developed, and in some of the Ratitae they remain for a long time
distinct from the vertebrae.

The thoracic vertebrae are distinguished from the cervical by the fact
that their true ribs are united to the sternum by means of sternal
ribs. This distinction, however, though convenient, is somewhat
arbitrary, as it has been shown that in the fowl and gannet, two pairs
of ribs which in the adult are free from the sternum, are connected
with it in the embryo. When, as in the Swans, the thoracic vertebrae
are not all fused together, they generally have saddle-shaped
articulating surfaces, but sometimes, as in the Penguins, Auks and
Plovers, the centra are convex in front and concave behind. The trunk
vertebrae generally have well-marked neural spines, while in the
Divers the anterior ones have peculiar bifurcating hypapophyses.

The trunk vertebrae are not readily divisible into thoracic and
lumbar. There are two true sacral vertebrae, but as development
proceeds a number of other vertebrae become fused with the true
sacrals, the whole forming a large compound sacrum. These pseudosacral
vertebrae generally include the lumbar, and some of the thoracic and
caudal vertebrae. Sixteen to twenty vertebrae or even more may be
included in the compound sacrum, and sometimes the whole of the trunk
vertebrae are fused together. In _Archaeopteryx_ however but five
vertebrae take part in the formation of the sacrum.

In _Archaeopteryx_ there are twenty long caudal vertebrae, of which
the last sixteen carry a pair of feathers apiece, but in all other
birds the tail is short and in the great majority of cases the
posterior vertebrae are fused together, forming the pygostyle. In the
Ratitae and Tinamidae a pygostyle is rarely or imperfectly developed.
In _Hesperornis_ there are twelve caudal vertebrae, six or seven of
which are united by their centra only, forming an imperfect pygostyle.

The free caudal vertebrae are generally amphicoelous.


THE SKULL.

The skull of all birds from _Archaeopteryx_ onwards is essentially
similar, differing from the skull of reptiles mainly in the extent to
which the cranium is arched, and its greater size in proportion to the
jaws.

Most of the bones of the cranium are pneumatic, and all show a marked
tendency to fuse together, and have their outlines obliterated by the
disappearance of the sutures. The several bones remain longest
distinguishable in the Ratitae and to a less extent in the Penguins.
The orbits are very large and lie almost entirely in front of the
cranium; they are separated by an interorbital septum which is
sometimes, as in _Chauna_ and _Scythrops_, very complete, sometimes,
as in Hornbills and the Common Heron, very slightly developed. As a
general rule the sclerotic is cartilaginous.

The anterior nares are almost always situated far back at the base of
the beak near the orbits, but in _Apteryx_ they are placed right at
its extremity. In _Phororhacos_ they are placed very high up on the
enormous beak and are not separated by any bony partition.

The skull of Parrots has some peculiarities. In some Parrots the
lachrymal sends back a process which meets the postorbital process of
the frontal and completes the orbit. In most birds the upper beak is
immovably fixed, but in some it is attached to the cranium, only by
the nasals and by flexible processes of the premaxillae, so that by
this means a kind of elastic joint is established and the beak is able
to be moved on the cranium. In the Parrots and _Opisthocomus_ there is
a regular highly movable joint.

In Cassowaries the fronto-nasal region of the skull is produced into
an enormous bony crest, and in Hornbills a somewhat similar structure
occurs. Although true teeth do not occur in any known bird except
_Archaeopteryx_, _Hesperornis_, and _Ichthyornis_, another extinct
bird, _Odontopteryx_, has the margins of both jaws provided with
forwardly-directed tooth-like serrations, formed of part of the actual
jawbone: a living hawk, _Harpagus_, too, has a deeply notched bill, to
which correspond serrations in the premaxillae.

A basi-pterygoid process of the basisphenoid abuts against the
pterygoid in Ratitae and in Tinamous, plovers, fowls, pigeons, ducks
and geese among Carinatae, recalling the arrangement met with in many
reptiles. The squamosal is sometimes, as in the fowl, united with the
postorbital process of the frontal. In the Carinatae the quadrate
articulates with the cranium by a double convex surface, in the
Ratitae by a single one. The premaxillae are always comparatively large
bones, the maxillae on the contrary are small, but give rise to
important inwardly-projecting maxillo-palatine processes.

The relations of the palatines, pterygoids, maxillae, and vomers vary
considerably, and on them Huxley has based a classification of
birds[106]. In the Ratitae and the Tinamous (Tinamidae), among
Carinatae the vomers unite and form a large broad bone, separating the
palatines and the pterygoids from the rostrum. Huxley uses the term
=Dromaeognathous= to describe this condition. In all other Carinatae
the vomers are narrow behind, and the palatines and pterygoids
converge posteriorly and articulate largely with the rostrum. Three
modifications of this condition are distinguished by Huxley, and
termed =Schizognathous=, =Ægithognathous=, and =Desmognathous=.

In the =Schizognathae= the vomers coalesce and form a narrow elongated
bone, pointed in front, separating the maxillo-palatine processes of
the premaxillae. Waders, fowls, penguins, gulls, some falcons and
eagles, American vultures, some herons and many owls have the
Schizognathous arrangement. In pigeons and sandgrouse there is no
vomer, but the other bones have the Schizognathous arrangement.

In the =Ægithognathae= the arrangement is the same as in the
Schizognathae, except that the vomers are truncated in front.
Passeres, swifts, woodpeckers, humming birds, rollers, hoopoes have
this arrangement.

In the =Desmognathae= (fig. 60, A) the maxillo-palatine processes
approach one another in the middle line, and either unite with the
vomers, or unite with one another, hiding the vomers. Thus a more or
less complete bony roof is formed across the palate. The vomers in
Desmognathae are small or sometimes absent. Ducks, storks, most
herons, most birds of prey and owls, pelicans, cormorants, parrots,
and flamingoes are Desmognathous.

The mandible, as in other Sauropsids, consists of a cartilage bone,
the articular, and a series of membrane bones, the dentary, splenial,
coronoid, angular, and supra-angular, developed round the unossified
Meckel's cartilage. The dentaries of the two rami are nearly always
fused together, but in _Ichthyornis_ and _Archaeopteryx_ the two rami
are but loosely united. There is often a fontanelle between the
dentary and the posterior bones, while the angle is sometimes, as in
the fowl, drawn out into a long curved process.

The hyoid apparatus (fig. 59, C) consists of a median portion, and a
pair of cornua. The median portion is composed of three pieces placed
end to end, and called respectively the os entoglossum, the basi-hyal,
and the uro-hyal. The os entoglossum is shown by development to be
formed by the union of paired structures and is probably homologous
with the hyoid arch of fishes. The basi-hyal and the long cornua, each
of which is composed of two or three pieces placed end to end, are
homologous with the first branchial arch of fishes, while the uro-hyal
is probably homologous with the second branchial arch of fishes. In
Woodpeckers the cornua are enormously long, and curve over the skull,
extending as far forwards as the anterior nares.


RIBS AND STERNUM.

Well-developed ribs are attached to the posterior cervical vertebrae
as well as to the thoracic vertebrae. The ribs generally have uncinate
processes and separate capitula and tubercula, but uncinate processes
are absent in _Chauna Palamedea_ and apparently in _Archaeopteryx_.

The sternum (fig. 63) is greatly developed in all birds. In the
embryo[107] it is seen to be derived from the union of right and left
plates of cartilage, formed by the fusion of the ventral ends of the
ribs. In the Ratitae and a few Carinatae, such as _Stringops_, it is
flat, but in the great majority of birds it is keeled, though the
development of the keel varies greatly. It is large in the flightless
Penguins, which use their wings for swimming. Traces of an
interclavicle may occur in the embryo.


PECTORAL GIRDLE.

[Illustration FIG. 63. SHOULDER-GIRDLE AND STERNUM OF

    A. BLACK VULTURE (_Vultur cinereus_) × 1/3.
    B. PEACOCK (_Pavo cristatus_) × 3/8.
    C. PELICAN (_Pelicanus conspicillatus_) × 1/3. (All Camb. Mus.)

    1. carina of the sternum.
    2. coracoid.
    3. scapula.
    4. clavicle.
    5. costal process.
    6. surfaces for articulation with the sternal ribs.
    7. xiphoid processes.
    8. fontanelle.]

The pectoral girdle is also strongly developed in all Carinatae, but
is much reduced in Ratitae. In some Moas the sternum has no facet for
the articulation of the coracoid, and the pectoral girdle appears to
have been entirely absent; it is extremely small also in _Apteryx_.
Clavicles are generally well developed in the Carinatae, and small
ones are found also in _Hesperornis_, and in Emeus and Cassowaries. In
the other living Ratitae and in _Stringops_ they are absent. In some
Parrots, Owls and Toucans they do not meet one another ventrally.
Clavicles are especially stout in some of the birds of prey. They do
not generally touch the sternum, but sometimes, as in the Pelican
(fig. 63, C), Adjutant and Frigate bird, they are fused with it.

In all Ratitae the scapula and coracoid lie almost in the same
straight line with one another, in the Carinatae they are nearly at
right angles to one another.


ANTERIOR LIMB.

In the wing of nearly all birds the ulna is thicker than the radius,
but in _Archaeopteryx_ the two bones are equal in size. In the wing of
_Archaeopteryx_ there are three long digits with distinct metacarpals.
In all other birds the digits are modified, the metacarpals being
commonly fused and the phalanges reduced in number. In _Palamedea_ and
some other birds the metacarpus bears a bony outgrowth, which when
sheathed in horn forms a spur.

In most of the Ratitae and in the extinct Dodo (_Didus_) and Solitaire
(_Pezophaps_) the wing is very small, but the usual parts are
recognisable. In _Hesperornis_ apparently only the humerus is present;
in some Moas, in which the wing is imperfectly known, the presence of
the humerus is indicated by traces of a glenoid cavity. In most Moas
the wing is apparently completely absent. As compared with those in
other Ratitae, the wings of the Ostrich and Rhea are well developed.
In the Ostrich (fig. 64, B) and Rhea, as in nearly all Carinatae, the
manus has three digits, but in _Apteryx_ there is only a single digit,
the second. The Penguins (fig. 64, A) too among Carinatae have only
two digits, but in their case it is the pollex which is missing. In
the Ostrich the third digit has two phalanges, in all other living
birds it has only one phalanx.


PELVIC GIRDLE.

[Illustration FIG. 64. BONES OF THE RIGHT WING OF

    A. A PENGUIN × 1/3. (Camb. Mus.)
    B. OSTRICH (_Struthio camelus_) × 1/7. (Partly after PARKER.)
    C. GANNET (_Sula alba_) × 1/3. (Camb. Mus.)

In C the distal phalanges of the pollex and second digit have been
omitted.

    1. humerus.
    2. radius.
    3. ulna.
    4. second metacarpal.
    5. third metacarpal.
    6. pollex.
    7. second digit.
    8. cuneiform.
    9. sesamoid bone.]

Birds have a very large pelvis and its characters are constant
throughout almost the whole group. The ilium is very large, and is
united along its whole length with the sacral and pseudosacral
vertebrae. The ischium is broad and extends back parallel to the ilium
with which in most birds it fuses posteriorly, further forward the
ilio-sciatic foramen separates the two bones. In _Tinamus_,
_Hesperornis_, _Apteryx_ (fig. 65, B, 2), and _Struthio_, the ischia
are separate from the ilia along their whole length except at the
acetabulum; in _Phororhacos_, on the other hand, the two bones are
fused along almost their whole length. The bone usually called the
pubis in birds corresponds to the post-pubis of Dinosaurs and forms a
long slender rod (fig. 65, 3) lying parallel to the ischium. In many
birds the ischia and pubes are united at their distal ends. This is
the case in the Ostrich (fig. 65, D), in which the ilia and ischia are
widely separated. In many birds the pubis is drawn out in front into
the pectineal process, this is specially large in _Apteryx_ (fig. 65,
B, 5), and in the embryos of many birds. It is probably homologous
with the pre-pubis of Dinosaurs but in some birds is formed in part by
the ilium. The acetabulum in birds is always perforate.

In _Rhea_ (fig. 65, C, 2) and probably in _Archaeopteryx_ a symphysis
ischii occurs, and in the ostrich alone among birds there is a
symphysis pubis. In _Archaeopteryx_ all three bones of the pelvis are
distinct, but they are imperfectly known. In Ichthyornis they are also
distinct, in all other known birds they are fused together to a
greater or less extent.

[Illustration FIG. 65. PELVIC GIRDLE AND SACRUM OF

    A. CASSOWARY (_Casuarius galeatus_) × 1/8.
    B. OWEN'S APTERYX (_A. oweni_) × 1/2.
    C. BROAD BILLED RHEA (_R. macrorhyncha_) × 1/6.
    D. OSTRICH (_Struthio camelus_) × 1/10. (All Camb. Mus.)

    1. ilium.
    2. ischium.
    3. pubis.
    4. acetabulum.
    5. pectineal process.]


POSTERIOR LIMB.

The tibia is always well developed and has a very strong cnemial
crest. The proximal tarsals are fused with its distal end, the whole
forming a compound bone, the tibio-tarsus. There is frequently an
oblique bar of bone crossing the anterior face of the tibio-tarsus at
the distal end, just above the articular surface of the
tarso-metatarsus, this is absent in Ostriches and _Æpyornis_. The
fibula though in the embryo and in _Archaeopteryx_ equal in length to
the tibia, is in the adult of other birds always imperfect, its
proximal end is often fused with the tibia, and its distal end is
commonly atrophied. In the Penguins however the distal end is
complete. The distal tarsals fuse with the second, third and fourth
metatarsals, forming a compound bone, the tarso-metatarsus. The first
metatarsal is nearly always free but occasionally as in _Phaëthon_ it
is fused with the others. No adult bird has more than four digits in
the pes. In the Penguins the metatarsals are separate, and in many
birds larger or smaller gaps exist between the fused metatarsals. In
most birds the third metatarsal is curved so as not to lie in the same
plane as the others, but in the Penguins they all three lie in the
same plane. The metatarsals are clearly separated in _Archaeopteryx_.
In Gallinaceous birds the tarso-metatarsus bears a bony outgrowth
which is sheathed in horn and forms a spur.

In most birds the first four toes are present while the fifth is
always absent. The first toe commonly has two phalanges, the second
three, the third four, and the fourth five. In Swifts the third and
fourth toes have only three phalanges. Many birds, such as all Ratitae
except _Apteryx_, have only three toes, the hallux being absent; in
the Ostrich the second toe is also gone with the exception of a small
metatarsal, so that the foot retains only the third and fourth digits,
the third being much the larger of the two and bearing a claw, while
the fourth is clawless.

In the Swifts, Cormorants, and Penguins, all four toes are directed
forwards. In most birds the hallux is directed backwards, and the
other toes forwards. In the Owls the fourth toe can be directed
backwards as well as the hallux, while in Parrots, Cuckoos,
Woodpeckers, and Toucans the fourth toe is permanently reversed. In
Trogons the second toe is reversed in addition to the hallux, but not
the fourth.


FOOTNOTES:

[105] W.K. Parker, _Phil. Trans._ vol. 179, p. 385, 1888; and _Ibis_,
1888, p. 124.

[106] See T.H. Huxley, "On the Classification of Birds," _P.Z.S._
1867.

[107] B. Lindsay, _P.Z.S._ 1885, p. 684.




CHAPTER XX.

CLASS MAMMALIA.


The skeleton of the members of this class, the highest of the
vertebrata, has the following characteristics:--

Some part of the integument at some period of life is always provided
with hairs; these are epidermal structures arising from short papillae
of the Malpighian layer of the epidermis, which at once grow inwards
and become imbedded in pits of the dermis. Sometimes scales or spines
occur, and epidermal exoskeletal structures in the form of hoofs,
nails, claws and horns are also characteristic. As regards the
endoskeleton, the vertebral centra have terminal epiphyses except in
the Ornithodelphia and some Sirenia. In the skull the cranial region
is greatly developed as compared with that in lower vertebrates, and
whereas in many reptiles the true cranium is largely concealed by a
false roof, in mammals the only relic of this secondary roof is found
in the zygomatic arch, and postorbital bar. In the adult all the bones
except the mandible, hyoid, and auditory ossicles are firmly united
together. The basisphenoid is well ossified, and there is no
parasphenoid. The pro-otic ossifies, and unites with the epi-otic and
opisthotic before they coalesce with any other bones.

The skull articulates with the vertebral column by means of two convex
occipital condyles formed mainly by the exoccipitals, and the mandible
articulates with the squamosal without the intervention of the
quadrate. The latter is much reduced, and is converted into the
tympanic ring, while the hyomandibular of fish is represented by the
auditory ossicles[108].

The teeth are always attached to the maxillae, premaxillae and
mandibles, never to any of the other bones. They are nearly always
implanted in distinct sockets, and are hardly ever ankylosed to the
bone. The teeth of mammals are generally markedly heterodont, four
forms, incisors, canines, premolars, and molars, being commonly
distinguishable. Some mammals are _monophyodont_, having only a single
set of teeth, but the great majority are _diphyodont_, having two
sets, a deciduous or milk dentition, and a permanent dentition.

The _incisors_, the front teeth, are simple, one-rooted, adapted for
cutting, and are nearly always borne by the premaxillae. Next come the
_canines_, one on each side in each jaw. They are generally large
teeth adapted for tearing or holding, and get their name from the fact
that they are largely developed in the dog. The remaining teeth form
the grinding series, the more posterior of them being the _molars_,
which are not preceded by milk teeth[109]. Between the molars and the
canines are the _premolars_, which do as a rule have milk or deciduous
predecessors, though very frequently the first of them is without a
milk predecessor.

In describing the dentition of any mammal, for the sake of brevity a
formula is generally made use of. Thus, the typical mammalian
dentition is expressed by the formula

    _i_ 3/3 _c_ 1/1 _pm_ 4/4 _m_ 3/3 = 11/11,

giving twenty-two teeth on each side, or forty-four altogether[110].
The incisors are represented by _i_, the canines by _c_, the premolars
by _p_ or _pm_, and the molars by _m_. The numbers above the lines
represent the teeth in the upper jaw, those below the lines the teeth
in the lower jaw. The milk dentition is expressed by a similar formula
with _d_ (deciduous) prefixed to the letter expressing the nature of
the tooth.

The following terms are of frequent use as characterising certain
forms of the grinding surfaces of teeth, and it will be well to define
them at once.

_Bunodont_ is a term applied to teeth with broad crowns raised into
rounded tubercles, e.g. the grinding teeth of Pigs and Hippopotami;

_Bilophodont_ to teeth marked by a simple pair of transverse ridges,
with or without a third ridge running along the outer border of the
tooth at right angles to the other two, e.g. the grinding teeth of
_Lophiodon_, Kangaroo, Manatee, Tapir, _Dinotherium_;

_Selenodont_ to teeth marked by crescentic ridges running from the
anterior towards the posterior end of the tooth, e.g. the grinding
teeth of the Ox and Sheep.

Teeth whose crowns are low so that their whole structure is visible
from the grinding surface are called _brachydont_, while those with
higher crowns, in which the bases of the infoldings of enamel are
invisible from the grinding surface are said to be _hypsodont_.
Bunodont teeth are brachydont, the teeth of the Horse and Ox are
hypsodont.

Passing now to the appendicular skeleton--the shoulder girdle differs
markedly from that of Sauropsids in the fact that the coracoid, except
in the Ornithodelphia, is greatly reduced, generally forming only a
small process on the scapula. In the pelvis the pubes meet in a
ventral symphysis, except in some Insectivora and Chiroptera. In many
mammals a fourth pelvic element, the _acetabular bone_, is
distinguishable. The ankle joint is _cruro-tarsal_, or situated
between the proximal tarsal bones and the tibia and fibula. Carpalia 4
and 5 are united forming the _unciform_; and the ulnar sesamoid bone
or _pisiform_ is generally well developed. In the proximal row of
tarsal elements there are only two bones, the calcaneum and
astragalus. Of these the calcaneum is the fibulare, and the
astragalus is generally regarded as the tibiale and intermedium
fused[111].


_Subclass I._ ORNITHODELPHIA OR PROTOTHERIA.

This subclass contains only a single order, the Monotremata, and the
following characteristics are equally applicable to the subclass and
to the order. The vertebral centra have no epiphyses, and the odontoid
process remains for a long time free from the centrum of the second
vertebra. With the exception of the atlas of _Echidna_ the cervical
vertebrae are without zygapophyses. The cranial walls are smooth and
rounded, and the sutures between the several bones early become
completely obliterated as in birds. The mandible is a very slight
structure, with no ascending ramus, and with the coronoid process (see
p. 398) and angle rudimentary. The auditory ossicles show a low state
of development. The tubercula of the ribs articulate with the sides of
the centra of the thoracic vertebrae, not with the transverse
processes. Some of the cervical ribs remain for a long time separate
from the vertebrae. Well ossified sternal ribs occur. No true teeth
are present in the adult. The young _Ornithorhynchus_ has functional
molar teeth, but in the adult their place is taken by horny plates. In
the Echidnidae neither teeth nor horny plates occur.

The coracoid (fig. 66, 3) is complete and well developed, and
articulates with the sternum. A precoracoid (epicoracoid) occurs in
front of the coracoid, and there is a large interclavicle (fig. 66,
6). The ridge on the scapula, corresponding to the spine of other
mammals, is situated on the anterior border instead of in the middle
of the outer surface. Epipubic bones are present. In the Echidnidae,
but not in _Ornithorhynchus_[112], the central portion of the
acetabulum is unossified as in birds. The humerus has a prominent
deltoid crest; its ends are much expanded, and the distal end is
pierced by an ent-epicondylar foramen. The fibula has a broad proximal
process resembling an olecranon. The limbs and their girdles bear a
striking resemblance to those of some Theromorphous reptiles.

[Illustration FIG. 66. VENTRAL VIEW OF THE SHOULDER-GIRDLE AND STERNUM
OF A DUCKBILL (_Ornithorhynchus paradoxus_) × 3/4 (after PARKER).

    1 and 2. scapula.
    3. coracoid.
    4. precoracoid (epicoracoid).
    5. glenoid cavity.
    6. interclavicle.
    7. clavicle.
    8. presternum.
    9. third segment of mesosternum.
    10. sternal rib.
    11. intermediate rib.
    12. vertebral rib.]

The order Monotremata includes only two living families, the
Echidnidae and Ornithorhynchidae.


MESOZOIC MAMMALIA[113].

It will be well here to briefly refer to certain mammals of small
size, the remains of which have been found in deposits of Mesozoic
age. In the great majority of cases they are known only by the lower
jaw, or sometimes only by isolated teeth. A large number of them are
commonly grouped together as the Multituberculata, and are sometimes,
partly owing to the resemblance of their teeth to those of
_Ornithorhynchus_, placed with the Prototheria, sometimes between the
Prototheria and the Metatheria. They are characterised by having a
single pair of large incisors in the lower jaw, and one large with one
or two smaller incisors in each premaxillae. The lower canines are very
small or altogether wanting. The incisors are separated by a diastema
from the grinding teeth, which are sometimes (_Tritylodon_)
characterised by the possession of longitudinal rows of little
tubercles separated by grooves, sometimes by having the premolars
provided with high cutting edges, whose surfaces are obliquely
grooved. Some of the Mesozoic mammals found associated with the
Multituberculata, have however a dentition of an altogether different
type, with at least three lower incisors, well developed canines and
premolars, and numerous molars with peculiar three-cusped or
tritubercular grinding surfaces. These mammals, one of the best known
of which is _Phascolotherium_, are commonly separated from the
Multituberculata, and are divided by Osborn into two groups, one
allied to the Marsupials, and one to the Insectivores. The group
showing Marsupial affinities is further subdivided into carnivorous,
omnivorous, and herbivorous subgroups. The members of both groups
commonly have four premolars, and six to eight molars in each
mandibular ramus.


_Subclass II._ DIDELPHIA OR METATHERIA.

This subclass, like the previous one, contains only a single order,
viz. the Marsupialia[114]; but the forms referable to it are far more
numerous than in the case of the Monotremata.

The integument is always furry, and the teeth are always
differentiated into incisors, canines, premolars and molars. Except in
_Phascolomys_, the number of incisors in the upper and lower jaws is
never equal, and the number in the upper usually exceeds that in the
lower jaw. There is no such regular succession and displacement of
teeth as in most mammals. Sometimes the anterior teeth are diphyodont,
and as a general rule the tooth commonly regarded as the last premolar
is preceded by a milk tooth. The majority of the permanent teeth of
most Marsupials are regarded as belonging to the milk series for two
reasons, (1) they are developed from the more superficial tissues of
the jaws, (2) a second set, the permanent teeth, begin to develop as
outgrowths from them, but afterwards become aborted[115].

The odontoid process at an early stage becomes fused with the centrum
of the second cervical vertebra, and the number of thoraco-lumbar
vertebrae is always nineteen. The skull has several characteristic
features. The tympanic bone remains permanently distinct, and the
anterior boundary of the tympanic cavity is formed by the alisphenoid.
The carotid canal perforates the basisphenoid, and the lachrymal canal
opens either outside the orbit or at its margin. There are generally
large vacuities in the palate. The angle of the mandible is (except in
_Tarsipes_) more or less inflected; and as a rule the jugal furnishes
part of the articular surface for the mandible. There is no
precoracoid (epicoracoid) or interclavicle, and the coracoid is
reduced to form a mere process of the scapula, not coming near the
sternum.

Epipubic, or so-called marsupial bones[116], nearly always occur, and
a fourth pelvic element, the acetabular bone, is frequently developed.
The fibula is always complete at its distal end, sometimes it is fused
with the tibia, but often it is not only free but is capable of a
rotatory movement on the tibia. This is the case in the families
Phascolomyidae, Didelphyidae, and Phalangeridae.

The Marsupialia can be subdivided into two main groups, according to
the character of the teeth:--


1. POLYPROTODONTIA.

In this group the incisors are small, subequal and numerous, not less
than 4/3. The canines are larger than the incisors, and the molars
have sharp cusps. The members of this group are all more or less
carnivorous or insectivorous. The group includes the families
Didelphyidae, Dasyuridae, Peramelidae, and Notoryctidae[117].


2. DIPROTODONTIA.

In this group the incisors do not exceed 3/3, and are usually 3/1,
occasionally 1/1. The first upper and lower incisors are large and
cutting. The lower canines are always small or absent, and so in most
cases are the upper canines. The molars have bluntly tuberculated, or
transversely ridged crowns. The group includes the families
Phascolomyidae, Phalangeridae, Macropodidae, and Epanorthidae.


_Subclass III._ MONODELPHIA OR EUTHERIA.

This great group includes all the Mammalia except the orders
Monotremata and Marsupialia. Coming to their general characteristics--
as in the Didelphia the odontoid process and cervical ribs early
become fused with the centra which bear them, while the coracoid is
reduced so as to form a mere process on the scapula, and there is no
precoracoid (epicoracoid), such as is found in Ornithodelphia.
Clavicles may be present or absent; when fully developed they
articulate with the sternum, usually directly, but occasionally, as in
some Rodents and Insectivores, through the remains of the sternal end
of the precoracoid. There is never any interclavicle in the adult,
though sometimes traces of it occur during development. In the pelvis
the acetabula are imperforate; and well-developed epipubic bones are
never found in the adult, though traces of them occur in some
Carnivores and foetal Ungulates.


_Order 1._ EDENTATA[118].

Teeth are not, as the name of the order seems to imply, always
wanting; and sometimes they are very numerous. They are, however,
always imperfect, and, with very few exceptions, are homodont and
monophyodont. They have persistent pulps, and so grow indefinitely and
are never rooted. In all living forms they are without enamel,
consisting merely of dentine and cement, and are never found in the
front part of the mouth in the situation occupied by the incisors of
other mammals. These characters derived from the teeth are the only
ones common to the various members of the order, which includes the
living sloths, ant-eaters, armadillos, pangolins and aard-varks,
together with various extinct forms, chiefly found in beds of late
tertiary age in both North and South America, the best known being the
Megatheridae and Glyptodonts.


_Order 2._ SIRENIA[119].

The skeleton of these animals has a general fish-like form, in
correlation with their purely aquatic habits. The fore limbs have the
form of paddles, but the number of phalanges is not increased beyond
the normal. There are no external traces of hind limbs.

The whole skeleton and especially the skull and ribs is remarkably
massive and heavy. The dentition varies; in the two living genera
_Manatus_ and _Halicore_, incisor and molar teeth are present, in one
extinct genus, _Rhytina_, teeth are entirely absent, while in another,
_Halitherium_, the dentition is more decidedly heterodont than in
living forms. In the two living genera the dentition is monophyodont,
but in _Halitherium_ the anterior grinding teeth are preceded by milk
teeth. The tongue and anterior part of the palate and lower jaw are
covered with roughened horny plates. The skull is noticeable for the
size and backward position of the anterior nares, also for the absence
or small size of the nasal bones. There is no union of certain of the
vertebrae to form a sacrum, and in living forms the centra are not
terminated by well-formed epiphyses[120].

The cervical vertebrae are much compressed, but they are never
ankylosed together. In _Manatus_ there are only six cervical
vertebrae. The caudal vertebrae have well-developed chevron bones. The
humerus is distinctly articulated to the radius and ulna, and these
two bones are about equally developed, and are often fused together.
There are no clavicles, and the pelvis is vestigial, consisting of a
pair of somewhat cylindrical bones suspended at some distance from the
vertebral column. In living forms there is no trace of a posterior
limb, but in _Halitherium_ there is a vestigial femur connected with
each half of the pelvis.


_Order 3._ CETACEA[121].

In these mammals the general form is more fish-like than is the case
even in the Sirenia. The skin is generally almost completely naked,
but hairs are sometimes present in the neighbourhood of the mouth,
especially in the foetus. In some Odontoceti vestiges of dermal
ossicles have been described, and in _Zeuglodon_ the back was probably
protected by dermal plates. The anterior limbs have the form of
flattened paddles, showing no trace of nails, the posterior limb bones
are quite vestigial or absent, and there is never any external sign of
the limb. Teeth are always present at some period of the life history,
but in the whalebone whales they are only present during foetal life,
their place in the adult animal being taken by horny plates of baleen.
In all living forms the teeth are simple and uniform structures
without enamel; they have single roots, and the alveoli in which they
are imbedded are often incompletely separated from one another. As in
some forms traces of a replacing dentition have been described, it has
been concluded that the functional teeth of Cetacea belong to the milk
dentition.

The texture of the bones is spongy. The cervical vertebrae are very
short, and though originally seven in number, are in many forms
completely fused, forming one solid mass (fig. 67). The odontoid
process of the axis is short and blunt, or may be completely wanting.
The lumbar and caudal vertebrae are large and numerous, and as
zygapophyses are absent, are very freely movable on one another;
zygapophyses are also absent from the posterior thoracic vertebrae.
The lumbar vertebrae are sometimes more numerous than the thoracic.
The epiphyses are very distinct, and do not unite with the centra till
the animal is quite adult. None of the vertebrae are united to form a
sacrum, but the caudal vertebrae have large chevron bones.

[Illustration FIG. 67. CERVICAL VERTEBRAE OF A CA'ING WHALE
(_Globicephalus melas_) × 1/4. (Camb. Mus.)

    1. centrum of seventh cervical vertebra.
    2. neural arch of seventh cervical vertebra.
    3. transverse process of atlas.
    4. foramen for exit of first spinal nerve.
    5. transverse process of axis.
    6. fused neural spines of atlas and axis.]

The skull is peculiarly modified; the bones forming the occipital
segment show a specially strong development, and the cranial cavity is
short, high, and almost spherical. The supra-occipital is very large
and rises up to meet the frontals, thus with the interparietal
completely separating the parietals from one another.

The frontals are expanded, forming large bony plates, which roof over
the orbits. The zygomatic process of the squamosal is extremely large
and extends forwards to meet the supra-orbital process of the frontal;
the zygomatic process of the jugal is on the contrary very slender.
The face is drawn out into a long rostrum, formed of the maxillae and
premaxillae surrounding the vomer and the mesethmoid cartilage. The
maxillae are specially large, and extend backwards so as to partially
overlap the frontals. The nasals are always small, and the anterior
nares open upwards between the cranium and rostrum. The periotics are
loosely connected with the other bones of the skull and the tympanics
are commonly large and dense. The mandible has hardly any coronoid
process, and the condyles are at its posterior end.

There are no clavicles, but the scapula and humerus are well
developed. The humerus moves freely in the glenoid cavity, but all the
other articulations of the anterior limb are imperfect; the various
bones have flattened ends, and are connected with one another by
fibrous tissue, which allows of hardly any movement. Frequently the
carpus is imperfectly ossified.

The number of digits in the manus is generally five, sometimes four,
and when there are four digits it is the third and not the first that
is suppressed. The number of phalanges in the second and third digits
almost always exceeds that which is normal in mammals, and the
phalanges are also remarkable for having epiphyses at both ends. The
pelvis is represented by two small bones which lie suspended
horizontally at some distance below the vertebral column; in some
cases vestiges of the skeleton of the hind limb are attached to them.

The Cetacea are divided into three suborders.


_Suborder_ (1). ARCHAEOCETI.

The members of this group are extinct; they differ from all living
Cetacea in having the dentition heterodont and in the fact that the
back was probably protected by dermal plates. The skull is elongated
and depressed, and the brain cavity is very small. The temporal fossae
are large, and there is a strong sagittal crest. The nasals and
premaxillae are a good deal larger than they are in living Cetacea, and
the anterior nares are usually far forward. The cervical vertebrae are
not fused with one another, and the lumbar vertebrae are unusually
elongated.

The limbs are very imperfectly known, but while the humerus is much
longer than in modern Cetaceans, it is nevertheless flattened
distally, indicating that the limb was paddle-like, and that there was
scarcely any free movement between the fore-arm and upper arm.

The best known genus is _Zeuglodon_, which is found in beds of Eocene
age in various parts of Europe, and in Alabama.


_Suborder_ (2). MYSTACOCETI OR BALAENOIDEA.

These are the Whalebone Whales or True Whales.

Calcified teeth representing the milk dentition occur in the foetus,
but the teeth are never functional, and always disappear before the
close of foetal life. There is a definite though small olfactory
fossa. The palate is provided with plates of baleen or whalebone. The
skull is symmetrical, and is extremely large in proportion to the
body. The nasals are moderately well developed, and the maxillae do not
overlap the orbital processes of the frontals. The lachrymals are
small and distinct from the jugals. The tympanics are ankylosed to the
periotics, and the rami of the mandible do not meet in a true
symphysis. The ribs articulate only with the transverse processes, and
the capitula are absent or imperfectly developed. Only one pair of
ribs meets the sternum, which is composed of a single piece.

The group includes among others the Right whale (_Balaena_),
Humpbacked whale (_Megaptera_), and Rorqual (_Balaenoptera_).


_Suborder_ (3). ODONTOCETI.

Teeth always exist after birth and baleen is never present. The teeth
are generally numerous, but are sometimes few and deciduous; the
dentition is homodont (except in _Squalodon_). The dorsal surface of
the skull is somewhat asymmetrical, there is no trace of an olfactory
fossa, the nasals are quite rudimentary, and the hind ends of the
maxillae cover part of the frontals; in all these respects the skull
differs from that of the Mystacoceti. The lachrymal may either be
united to the jugal or may be large and distinct. The tympanic is not
ankylosed to the periotic. The rami of the mandible are nearly
straight and become united in a long symphysis. Some of the ribs have
well developed capitula articulating with the vertebral centra. The
sternum is almost always composed of several pieces as in other
mammals, and several pairs of ribs are connected with it. There are
always five digits to the manus, though the first and fifth are
usually very little developed.

The suborder includes the Sperm Whale (_Physeter_), Narwhal
(_Monodon_), Dolphin (_Delphinus_), Porpoise (_Phocoena_), and many
other living forms as well as the extinct _Squalodon_ which differs
from the other members of the suborder in its heterodont dentition.

_Order 4._ UNGULATA.

This order includes a great and somewhat heterogeneous group of
animals, a large proportion of which are extinct. They all (except
certain extinct forms) agree in having the ends of the digits either
encased in hoofs or provided with broad flat nails. The teeth are
markedly heterodont and diphyodont, and the molars have broad crowns
with tuberculated or ridged surfaces. Clavicles are never present in
the adult except in a few generalised extinct forms such as
_Typotherium_, and it is only recently that vestigial clavicles have
been discovered in the embryo[122]. The scaphoid and lunar are always
distinct.

The order Ungulata may be subdivided into two main groups, Ungulata
vera and Subungulata.


_Section I._ UNGULATA VERA[123].

The cervical vertebrae except the atlas are generally opisthocoelous.
The feet are never _plantigrade_[124]. In all the living and the great
majority of the extinct forms the digits do not exceed four, the first
being suppressed. In the carpus the os magnum articulates freely with
the scaphoid, and is separated from the cuneiform by the lunar and
unciform. In the tarsus the cuboid articulates with the astragalus as
well as with the calcaneum, and the proximal surface of the astragalus
is marked by a pulley-like groove. All the bones of the carpus and
tarsus strongly interlock. These characters with regard to the carpus
and tarsus do not hold in _Macrauchenia_ and its allies. The humerus
never has an ent-epicondylar foramen.

The group is divided into two very distinct suborders:--


_Suborder_ (1). ARTIODACTYLA.

The Artiodactyla have a number of well marked characters, one of the
most obvious being the fact that many of the most characteristic forms
have large paired outgrowths on the frontal bones. These may be (1)
solid deciduous bony _antlers_, or (2) more or less hollow bony
outgrowths which are sheathed with permanently growing horn.

The premolar and molar teeth are usually dissimilar, the premolars
being one-lobed and the molars two-lobed; the last lower molar of both
the milk and permanent dentitions is almost always three-lobed.

The grinding surfaces of the molar teeth have a tendency to assume one
of two forms. In the Pigs and their allies the crowns are bunodont[1],
while in the more highly specialised Ruminants the crowns are
selenodont[125]. The nasals are not expanded posteriorly, and there is
no alisphenoid canal[126]. The thoraco-lumbar vertebrae are always
nineteen. The symphysis of the ischia and pubes is very elongated, and
the femur has no third trochanter. The limbs never have more than four
digits, and are symmetrical about a line drawn between the third and
fourth digits; the digits, on the other hand, are never symmetrical in
themselves. The astragalus has pulley-like surfaces both proximally
and distally, and articulates with the navicular and cuboid by two
nearly equal facets. The calcaneum articulates with the lower end of
the fibula when that bone is fully developed.

In the Artiodactyla are included the following living groups:--

_a._ Suina. Pigs and Hippopotami.

_b._ Tylopoda. Camels and Llamas.

_c._ Tragulina. Chevrotains.

_d._ Ruminantia or Pecora. Deer, giraffes, oxen, sheep and antelopes.


_Suborder_ (2). PERISSODACTYLA[127].

In this group there are never any bony outgrowths from the frontals.
The grinding teeth form a continuous series, the posterior premolars
resembling the molars in complexity, and the last lower molar
generally has no third lobe. The cervical vertebrae with the
exception of the atlas almost always have markedly opisthocoelous
centra, but in _Macrauchenia_ they are flat. The nasals are expanded
posteriorly, and an alisphenoid canal is present. The thoraco-lumbar
vertebrae are never less than twenty-two in number and are usually
twenty-three. The femur has a third trochanter (except in
_Chalicotherium_). The third digit of the manus and pes is symmetrical
in itself, and larger than the others, and in some cases the other
digits are quite vestigial. The number of the digits of the pes is
always odd. The astragalus is abruptly truncated distally, and the
facet by which it articulates with the cuboid, is much smaller than
that by which it articulates with the navicular. The calcaneum does
not articulate with the fibula, except in _Macrauchenia_. The group
includes many extinct forms, and the living families of the Tapirs,
Horses and Asses, and Rhinoceroses.


_Section II._ SUBUNGULATA.

In this group is placed a heterogeneous collection of animals, the
great majority of which are extinct. There is really no characteristic
which is common to them all, and which serves to distinguish them as a
group from the Ungulata vera. But the most distinctive character
common to the greatest number of them is to be found in the carpus,
whose bones in most cases retain their primitive relation to one
another, the os magnum articulating with the lunar and sometimes just
meeting the cuneiform, but in living forms at any rate not
articulating with the scaphoid. The feet frequently have five
functional digits, and may be plantigrade. The proximal surface of the
astragalus is generally flattened instead of being pulley-like as in
Ungulata vera.


_Suborder_ (1). TOXODONTIA.

This suborder includes some very aberrant extinct South American
ungulates, which have characters recalling the Proboscidea, both
groups of Ungulata vera, and the Rodentia. The limbs are
subplantigrade or digitigrade, and the digits are three, rarely five,
in number, the third being most developed. The carpus resembles that
of the Ungulata vera, in that the bones interlock and the magnum
articulates with the scaphoid. In the tarsus, however, the bones do
not interlock. The astragalus has a pulley-like proximal surface
(except in _Astrapotherium_, in which it is flat), and articulates
only with the navicular, not meeting the cuboid. The calcaneum has a
large facet for articulation with the fibula, as in Artiodactyla.
There is no alisphenoid canal, and the orbit is confluent with the
temporal fossa. Some of the forms (e.g. _Nesodon_) referred to this
group have the typical mammalian series of forty-four teeth, but in
others the canines are undeveloped. In _Toxodon_ all the cheek teeth
have persistent pulps, while in _Nesodon_ and _Astrapotherium_ they
are rooted. A clavicle is sometimes present (_Typotherium_), and the
femur sometimes has a third trochanter (_Typotherium_ and
_Astrapotherium_), sometimes is without one (_Toxodon_).

The remains of these curious Ungulates have been found in beds of late
Tertiary age in South America.


_Suborder_ (2). CONDYLARTHRA[128].

This group includes some comparatively small extinct ungulates, which
are best known from the Lower Eocene of Wyoming, though their remains
have also been found in deposits of similar age in France and
Switzerland. Their characters are little specialised, and they show
relationship on the one hand to the Ungulata vera and on the other to
the Hyracoidea. They also have characters allying them to the
Carnivora. They generally have the typical mammalian series of
forty-four teeth, the molars being brachydont and generally bunodont.
The premolars are more simple than the molars. The limbs are
plantigrade, and have five digits with rather pointed ungual
phalanges. The os magnum, as in living Subungulates, articulates with
the lunar, not reaching the scaphoid. The astragalus has an elongated
neck, a pulley-like proximal and a convex distal articular surface,
and does not articulate with the cuboid. The humerus has an
ent-epicondylar foramen, and the femur has a third trochanter. The
best known genus is _Phenacodus_; it is perhaps the most primitive
ungulate whose skeleton is thoroughly well known, and is of special
interest from the fact that it is regarded as the lowest stage in the
evolutionary series of the horse. Its remains are found in the Lower
Eocene of Wyoming.


_Suborder_ (3). HYRACOIDEA[129].

This group of animals is very isolated, having no very close allies,
either living or extinct. The digits are provided with flat nails,
except the second digit of the pes, which is clawed. Canine teeth are
absent, and the dental formula is usually given as _i_ 1/2, _c_ 0/0,
_pm_ 4/4, _m_ 3/3. The upper incisors are long and curved, and have
persistent pulps as in Rodents; their terminations are, however,
pointed, not chisel-shaped, as in Rodents. The lower incisors have
pectinated edges. The grinding teeth have a pattern much like that in
_Rhinoceros_. In the skull (fig. 83) the postorbital processes of the
frontal and jugal almost or quite meet. The jugal forms part of the
glenoid cavity for articulation with the mandible, and also extends
forwards so as to meet the lachrymal. There is an alisphenoid canal.
There are as many as twenty-one or twenty-two thoracic vertebrae, and
the number of thoraco-lumbar vertebrae reaches twenty-eight or thirty.
There are no clavicles, and the scapula has no acromion; the coracoid
process is, however, well developed. The ulna is complete. In the
manus the second, third and fourth digits are approximately equal in
size, the fifth is smaller, and the first is vestigial. The femur has
a slight ridge representing the third trochanter. The fibula is
complete, but is generally fused with the tibia proximally. There is a
complicated articulation between the tibia and astragalus, which has a
pulley-like proximal surface. In the pes the three middle digits are
well developed, but there is no trace of a hallux, and the fifth digit
is represented only by a vestigial metatarsal.

The only representatives of the suborder are some small animals
belonging to the genus _Procavia_ (_Hyrax_), which is found in Africa
and Syria; some of the species are by many authors placed in a
distinct genus _Dendrohyrax_.


_Suborder_ (4). AMBLYPODA[130].

This suborder includes a number of primitive extinct Ungulates, many
of which are of great size. Their most distinguishing characteristics
are afforded by the extremities. In the carpus the bones interlock a
little more than is the case in most Subungulata, and the corner of
the os magnum reaches the scaphoid, while the lunar articulates
partially with both magnum and unciform, instead of only with the
magnum. In the tarsus the cuboid articulates with both the calcaneum
and the astragalus, which is remarkably flat. The manus and pes are
short, nearly or quite plantigrade, and have the full number of
digits. The cranial cavity is singularly small. Canine teeth are
present in both jaws, and the grinding teeth have short crowns, marked
by =V=-shaped ridges. The pelvis is large, the ilia are placed
vertically, and the ischia do not take part in the ventral symphysis.

The best known animals belonging to this suborder are the
Uintatheriidae (Dinocerata)[131], found in the Upper Eocene of
Wyoming. They are as large as elephants, and are characterised by the
long narrow skull drawn out into three pairs of rounded protuberances,
by the strong occipital crest, and by the very large upper canines.


_Suborder_ (5). PROBOSCIDEA.

This suborder includes the largest of land mammals, the Elephants, and
certain of their extinct allies. The limbs are strong, and are
vertically placed; the proximal segment is the longest, and the manus
and pes are pentedactylate and subplantigrade. The digits are all
enclosed in a common integument, and each is provided with a broad
hoof. The vertebral centra are much flattened and compressed,
especially in the cervical region. The number of thoracic vertebrae is
very great, reaching twenty. The skull (figs. 96 and 97) is extremely
large, this being due to the great development of air cells, which
takes place in nearly all the bones of the adult skull. In the young
skull there are hardly any air cells, and the growth of the cranial
cavity does not by any means keep pace with the growth of the skull in
general. The supra-occipital is very large, and forms a considerable
part of the roof of the skull. The nasals and jugals are short, and
the premaxillae very large. The rami of the mandible meet in a long
symphysis, and the ascending portion is very high. Canine teeth are
absent, and the incisors have the form of ever-growing tusks composed
mainly of dentine; in living forms they are present in the upper jaw
only. The grinding teeth are large, and in living forms have a very
complex structure and mode of succession. In some of the extinct
forms, such as _Mastodon_ and especially _Dinotherium_, the teeth are
much more simple. In every case the teeth have the same general
structure, consisting of a series of ridges of dentine, coated with
enamel. In the more specialised forms the valleys between the ridges
are filled up with cement. The acromion of the scapula has a recurved
process, similar to that often found in rodents. Clavicles are absent.
The radius and ulna are not ankylosed, but are incapable of any
rotatory movement. All the bones of the extremities are very short and
thick; the scaphoid articulates regularly with the trapezoid and the
lunar with the magnum. The ilia are vertically placed, and are very
much expanded; the ischia and pubes are small, and form a short
symphysis. The femur has no third trochanter, and the tibia and fibula
are distinct. The fibula articulates with the calcaneum, and the
astragalus is very flat.

       *       *       *       *       *

Here brief reference may be made to the TILLODONTIA[132], a group of
extinct mammals found in the Eocene beds of both Europe and North
America. They seem to connect together the Ungulata, Rodentia, and
Carnivora.

The skull resembles that of bears, but the grinding teeth are of
Ungulate type, while the second incisors resemble those of rodents,
and have persistent pulps. The femur has a third trochanter, and the
feet resemble those of bears in being plantigrade and having pointed
ungual phalanges, differing, however, in having the scaphoid and lunar
distinct.


_Order 5._ RODENTIA.

The Rodents form a very large and well-defined group of mammals easily
distinguishable by their peculiar dentition. Canines are absent, and
the incisors are very large and curved, growing from persistent pulps.
They are rectangular in section and are much more thickly coated with
enamel on their anterior face than elsewhere; consequently, as they
wear down they acquire and retain a chisel-shaped (scalpriform) edge.
There is never more than one pair of incisors in the mandible, and
except in the Hares and Rabbits, there is similarly only a single pair
in the upper jaw. These animals are, too, the only rodents which have
well developed deciduous incisors. There is always a long diastema
separating the incisors from the grinding teeth. The grinding teeth,
which are arranged in a continuous series, vary in number from two to
six in the upper jaw, and from two to five in the lower jaw. The
number of premolars is always below the normal, often they are
altogether wanting, but generally they are 1/1. Sometimes the grinding
teeth form roots, sometimes they grow persistently.

The premaxillae are always large, and the orbits always communicate
freely with the temporal fossae. The condyle of the mandible is
elongated from before backwards, and owing to the absence of a
postglenoid process to the squamosal, a backward and forward motion of
the jaw can take place. The zygomatic arch is complete, but the jugal
is short and only forms the middle of it. The palate is small, being
sometimes, as in the hares, narrowed from before backwards, sometimes
as in the mole-rats (Bathyerginae) narrowed transversely.

The thoraco-lumbar vertebrae are usually nineteen in number. Clavicles
are generally present, and the acromion of the scapula is commonly
very long. The feet are as a rule plantigrade, and provided with five
clawed digits.

There are two main groups of Rodentia; the Duplicidentata, or Hares
and Rabbits, which have two pairs of upper incisors, whose enamel
extends round to the posterior surface; and the Simplicidentata, in
which there is only a single pair of upper incisors, whose enamel is
confined to the anterior surface. This group includes all the Rodents
except the Hares and Rabbits.


_Order 6._ CARNIVORA.

The living Carnivora form a natural and well-marked group, but as is
the case with so many other groups of animals, when their extinct
allies are included, it becomes impossible to readily define them.

The manus and pes never have less than four well-developed digits, and
these are nearly always provided with more or less pointed nails,
generally with definite claws. The hallux and pollex are never
opposable. The dentition is diphyodont and markedly heterodont. The
teeth are always rooted, except in the case of the canines of the
Walrus. The incisors are generally 3/3, and are comparatively small,
while the canines are large, pointed, and slightly recurved. The cheek
teeth are variable, and are generally more or less compressed and
pointed; sometimes their crowns are flattened and tuberculated, but
they are never divided into lobes by deep infoldings of enamel. The
squamosal is drawn out into a postglenoid process, and the mandible
has a large coronoid process. The condyle of the mandible is
transversely elongated, and the glenoid fossa is very deep; in
consequence of this arrangement the mandible can perform an up and
down movement only, any rotatory or back and fore movement being
impossible. The jugal is large, and the zygomatic arch generally
strong, while the orbit and temporal fossa are in most cases
completely confluent. The scapula has a large spine. The clavicle is
never complete and is often absent, this forming an important
distinction between the skeleton of a Carnivore and of any Insectivore
except _Potamogale_. The humerus often has an ent-epicondylar foramen,
and the radius and ulna, tibia and fibula are always separate. The
manus is often capable of the movements of pronation and supination,
and the scaphoid, lunar and centrale are in living forms always united
together.

The order Carnivora includes three suborders.


_Suborder_ (1). CREODONTA[133].

This suborder contains a number of extinct Carnivora, which present
very generalised characters.

The cranial cavity is very small; and the fourth upper premolar and
first lower molar are not differentiated as carnassial teeth[134], as
they are in modern Carnivora. The Creodonta also differ from modern
Carnivora in the fact that the scaphoid and lunar are usually
separate, and that the femur has a third trochanter. The feet are
plantigrade.

They resemble the Condylarthra, another very generalised group, in
having an ent-epicondylar foramen.

They occurred throughout the Tertiary period in both Europe and North
America, and have also been found in India. One of the best known
genera is _Hyaenodon_.


_Suborder_ (2). CARNIVORA VERA or FISSIPEDIA.

The skeleton is mainly adapted for a terrestrial mode of life, and the
hind limbs have the normal mammalian position. In almost every case
the number of incisors is 3/3. Each jaw always has one specially
modified _carnassial_ or sectorial tooth which bites like a scissors
blade against a corresponding tooth in the other jaw. In front of it
the teeth are always more or less pointed, while behind it they are
more or less broadened and tuberculated. In the manus the first digit,
and in the pes the first and fifth digits are never longer than the
rest, and the digits of both limbs are almost invariably clawed. Some
forms are plantigrade, some digitigrade, some subplantigrade. The
group includes all the ordinary terrestrial Carnivora, and is divided
into three sections:--

Æluroidea[135], including the cats, civets, hyaenas, and allied forms.

Cynoidea, including the dog tribe.

Arctoidea, including the bears, raccoons, weasels, and allied forms.


_Suborder_ (3). PINNIPEDIA[136].

In this suborder the limbs are greatly modified and adapted for a more
or less purely aquatic life, the proximal and middle segments of the
limbs are shortened, while the distal segment, especially in the leg,
is much elongated and expanded. There are always five well-developed
digits to each limb, and in the pes the first and fifth digits are
generally larger than the others. The digits generally bear straight
nails instead of claws, but even nails are sometimes absent. There is
no carnassial tooth, and the teeth in other ways differ considerably
from those of Carnivora vera. The incisors are always fewer than 3/3;
while the cheek teeth generally consist of four premolars and one
molar, all of very uniform character, being compressed with conical
crowns, and never more than two roots.

The suborder includes three families--Otariidae (Eared Seals),
Trichechidae (Walrus), and Phocidae (Seals).


_Order 7._ INSECTIVORA[137].

This order contains a large number of small generally terrestrial
mammals. The limbs are plantigrade or subplantigrade, and are
generally pentedactylate. All the digits are armed with claws, and the
pollex and hallux are not opposable. The teeth are diphyodont,
heterodont, and rooted. The cheek teeth have tuberculated crowns, and
there are never less than two pairs of incisors in the mandible; often
the incisors, canines, and premolars are not clearly differentiated
from one another, and special carnassial teeth are never found. The
cranial cavity is small, and the facial part of the skull is generally
much developed; often the zygomatic arch is incomplete. Clavicles are
well developed (except in _Potamogale_), and the humerus generally has
an ent-epicondylar foramen. The femur frequently has a ridge
representing the third trochanter. There are two suborders:


_Suborder_ (1). DERMOPTERA.

This suborder includes only a very aberrant arboreal genus
_Galeopithecus_, remarkable for its greatly elongated limb bones, and
peculiar dentition. The incisors of the lower jaw are deeply
pectinated or divided by several vertical fissures, the canines and
outer upper incisors have two roots. Ossified inter centra occur in
the thoraco-lumbar region of the vertebral column.


_Suborder_ (2). INSECTIVORA VERA.

This suborder includes all the ordinary Insectivora, such as moles,
shrews and hedgehogs. The upper and lower incisors are conical, not
pectinated.


_Order 8._ CHIROPTERA[138].

This order is perhaps the best marked and most easily defined of all
the orders of mammals. The anterior limbs form true wings and the
whole skeleton is modified in relation to flight.

The anterior limbs are vastly larger than the posterior; for all the
bones except the carpals are much elongated, and this applies
specially to the phalanges of all the digits except the pollex.

The pollex is clawed and so is sometimes the second digit; the other
digits of the manus are without nails or claws. The teeth are
divisible into the four usual types and the series never exceeds _i_
2/3 _c_ 1/1 _pm_ 3/3 _m_ 3/3 × 2, total 38. The milk teeth are quite
unlike the permanent teeth. The orbit is not divided by bone from the
temporal fossa. The vertebral column is short, and in old animals the
trunk vertebrae have a tendency to become partially fused together.
The cervical vertebrae are remarkably wide, and the development of
spinous processes is everywhere slight. The presternum has a prominent
keel for the attachment of the pectoral muscles. The clavicles are
very long and strong, and the scapula has a long spine and coracoid
process. The ulna is vestigial, consisting only of a proximal end
ankylosed to the radius. All the carpals of the proximal row--the
scaphoid, lunar and cuneiform--are united, forming a single bone. The
pelvis is very weak and narrow, and only in the Rhinolophidae do the
pubes meet in a symphysis. The anterior caudal vertebrae are
frequently united to the ischia. The fibula is generally vestigial,
and the knee joint is directed backwards instead of forwards. The pes
has five slender clawed digits, and the calcaneum is often drawn out
into a spur which helps to support the membrane connecting the hind
limbs with the tail.

There are two suborders of Chiroptera:

1. The MEGACHIROPTERA or Flying foxes, which almost always have smooth
crowns to the molar teeth, and the second digit of the manus clawed.

2. The MICROCHIROPTERA including all the ordinary bats which have
cusped molar teeth, and the second digit of the manus clawless.


_Order 9._ PRIMATES.

The dentition is diphyodont and heterodont, the incisors generally
number 2/2, and the molars, except in the Hapalidae (Marmosets), are
3/3. The cheek teeth are adapted for grinding, and the molars are more
complex than the premolars. A process from the jugal meets the
postorbital process of the frontal completing the postorbital bar.

The clavicle is well developed, and the radius and ulna are never
united. The scaphoid and lunar of the carpus, and commonly also the
centrale, remain distinct from one another. As a rule both manus and
pes have five digits, but the pollex may be vestigial. The pollex is
opposable to the other digits, and so is the hallux except in Man; the
digits are almost always provided with flat nails. The humerus has no
ent-epicondylar foramen and the femur has no third trochanter.

The order Primates is divisible into two suborders:


_Suborder_ (1). LEMUROIDEA.

The skull has the orbit communicating freely with the temporal fossa
beneath the postorbital bar (except in _Tarsius_). The lachrymal
foramen is external to the margin of the orbit. Both pollex and hallux
are well developed. In the pes the second digit is terminated by a
long pointed claw, and so is also the third in _Tarsius_. The lumbar
region of the vertebral column is long, sometimes including as many as
nine vertebrae. Besides the Lemurs the group includes the aberrant
_Tarsius_ and _Chiromys_.


_Suborder_ (2). ANTHROPOIDEA.

The skull has the orbit almost completely shut off from the temporal
fossa, and the lachrymal foramen is situated within the orbit. The
pollex is sometimes vestigial or absent. The second digit of the pes
has a flattened nail except in the Hapalidae, in which all the digits
of the pes except the hallux are clawed.

The Anthropoidea are divided into five families:

1. Hapalidae or Marmosets.

2. Cebidae or American Monkeys.

3. Cercopithecidae or Old World Monkeys.

4. Simiidae or Anthropoid Apes.

5. Hominidae or Men.


FOOTNOTES:

[108] This is Gadow's view; according to Huxley the quadrate forms the
malleus; according to Baur it forms the zygomatic process of the
squamosal, and according to Broom the interarticular mandibular
cartilage.

[109] According to Leche, _Morphol. Jahrb._ XIX. p. 502, the molar
teeth belong morphologically to the first series, i.e. they are milk
teeth without vertical successors.

[110] The researches of Bateson, _P.Z.S._ 1892, p. 102, have shown
that cases of individual variation in the number of teeth are common.

[111] Baur, however, suggests (_Anat. Anz._ vol. IV. 1889), that a
tibial sesamoid found in _Procavia_, many rodents, edentates and
_Ornithorhynchus_ is a vestigial tibiale, and that the astragalus is
the intermedium.

[112] This perforation of the acetabulum in _Echidna_ is a secondary
character occurring late in development, and consequently is not of
phylogenetic importance.

[113] See R. Owen, "Monograph of the Fossil Mammalia of the Mesozoic
Formation," _Pal. Soc. Mon._ 1871.

H.F. Osborn, "Structure and Affinities of Mesozoic Mammals," _J. of
Philad. Acad._ 1888, vol. IX.

O.C. Marsh, "Jurassic Mammals," _Amer. J. Sci._ 1878 _et seq._

[114] See Oldfield Thomas, _Brit. Mus. Cat. of Marsupialia and
Monotremata_ (1888).

[115] W. Kükenthal, _Anat. Anz._ VI. p. 364, 1891. C. Röse, _Anat.
Anz._ VII. p. 639.

[116] These bones however have no connection with the marsupium, being
nearly equally developed in both male and female. They are simply
sesamoid bones forming ossifications in the inner tendon of the
external oblique muscle, and are developed as supports for the
abdominal wall. Very similar structures have been independently
developed in various Amphibians, Reptiles and monodelphian Mammals.
See W. Leche, _Biol. Fören._ III. p. 120.

[117] See H. Gadow, _P.Z.S._ 1892, p. 361.

[118] See W.H. Flower, "On the Mutual Affinities of the Animals
composing the order Edentata," _P.Z.S._ 1882, p. 358. For the fossil
Edentates of N. America see E. Cope, _Amer. Natural._ 1889; for those
of S. America see various papers by F. Ameghino, H. Burmeister and R.
Owen. Also T.H. Huxley, "On the Osteology of Glyptodon," _Phil.
Trans._ 1865.

[119] See J.F. Brandt, _Symbolae Sirenologicae_, St Petersburg, 1846,
1861, 1868.

[120] Epiphyses are fully developed in _Halitherium_, and traces occur
in _Manatus_.

[121] See P.J. van Beneden and P. Gervais, _Ostéographie des Cétacés_,
1869-80.

[122] H. Wincza, _Morphol. Jahrb._ XVI., p. 647.

[123] See M. Pavlow, "Études sur l'histoire paléontologique des
Ongulés." _Bull. Soc. Moscou_, 1887--1890.

[124] In a _plantigrade_ animal the whole of the foot is placed on the
ground in walking. A _digitigrade_ animal places only its toes on the
ground. An intermediate condition is distinguished by the term
_subplantigrade_.

[125] See p. 345.

[126] See p. 401.

[127] See E.D. Cope, "The Perissodactyla," _Amer. Natural._, 1887.

[128] See E.D. Cope, "The Condylarthra," _Amer. Natural._, 1884, and
"Synopsis of the Vertebrates of the Puerco series," _Tr. Amer. Phil.
Soc._, 1888. O.C. Marsh, "A new order of extinct Eocene Mammals
(Mesodactyla)," _Amer. J. Sci._, 1892.

[129] See O. Thomas, "On the species of Hyracoidea," _P.Z.S._, 1892,
p. 50.

[130] See E.D. Cope, "The Amblypoda," _Amer. Natural._, 1884 and 1885.

[131] See O.C. Marsh, "The Dinocerata," _U.S. Geol. Survey_, 1884,
vol. X.

[132] See O.C. Marsh, _Amer. J. Sci._, 1875 and 1876.

[133] E.D. Cope, "The Creodonta," _Amer. Natural._, 1884. W.B. Scott,
"Revision of the N. American Creodonta," _P. Ac. Philad._, 1892.

[134] See next paragraph.

[135] St G. Mivart, _The Cat_, London, 1881.

[136] St G. Mivart, _P.Z.S._, 1885.

[137] St G. Mivart, "On the Osteology of Insectivora," _J. Anat.
Physiol. norm. path._, 1867 and 1868, and _P.Z.S._, 1871. G.E. Dobson,
_Monograph of the Insectivora_, London, 1882--90.

[138] See G.E. Dobson, _Brit. Mus. Catalogue of Chiroptera_, 1878. See
also other papers by the same author and by Oldfield Thomas.




CHAPTER XXI.

THE SKELETON OF THE DOG[139] (_Canis familiaris_).


I. EXOSKELETON.

The exoskeleton of the dog includes three sets of structures: 1.
hairs, 2. claws, 3. teeth. =Hairs= and =claws= are epidermal
exoskeletal structures, while =teeth= are partly of dermal, and partly
of epidermal origin.

1. =Hairs= are delicate epidermal structures which grow imbedded in
little pits or follicles in the dermis. Specially large hairs forming
the =vibrissae= or =whiskers= grow attached to the upper lip.

2. =Claws= are horny epidermal sheaths, one of which fits on to the
pointed distal phalanx of each digit. They are sharply curved
structures, and being in the dog non-retractile, their points are
commonly much blunted by friction with the ground. The claws of the
pollex, and of the hallux when it is present, however do not meet the
ground, and therefore remain comparatively sharp.

3. =Teeth=[140]. Although as regards their mode of origin, teeth are
purely exoskeletal or tegumentary structures, they become so
intimately connected with the skull that they appear to belong to the
endoskeleton.

Each tooth, as has been already described, consists of three distinct
tissues, dentine and cement of dermal origin, and enamel of epidermal
origin.

[Illustration FIG. 68. DENTITION OF A DOG (_Canis familiaris_) × 1/2.
(Camb. Mus.)

    _i_ 2. second incisor.
    _c._ canine.
    _pm_ 1, _pm_ 4. first and fourth premolars.
    _m_ 1. first molar.]

The teeth of the dog (fig. 68) form a regular series arranged along
the margins of both upper and lower jaws, and imbedded in pits or
=alveoli= of the maxillae, premaxillae, and mandibles. They are all
fixed in the bone by tapering roots, and none of them grow from
persistent pulps.

They are divisible into four distinct groups, the =incisors=,
=canines=, =premolars= and =molars=. There are three incisors, one
canine and four premolars on each side of each jaw. But while there
are three molars on each side of the lower jaw, the last is wanting in
the upper jaw. The dentition of the dog may then be represented by the
formula

    _i_ 3/3 _c_ 1/1 _pm_ 4/4 _m_ 2/3 × 2 = 42.

In each jaw there is one large specially modified tooth called the
=carnassial=, the teeth in front of this are more or less pointed and
compressed, while those behind it are more or less flattened and
tuberculated.

=Teeth of the upper jaw.=

The first and second =incisors= are small teeth with long conical
roots and somewhat chisel-shaped crowns. Surrounding the base of the
crown there is a rather prominent ridge, terminated laterally by a
pair of small cusps. This ridge, the =cingulum=, serves to protect the
edge of the gums from injury by the hard parts of food. The third
incisor is a good deal like the others but larger, and has the
cingulum well developed though not terminated by lateral cusps. All
the incisors are borne by the premaxillae, the remaining teeth by the
maxillae.

The =canine= is a large pointed tooth, slightly recurved and with a
long tapering root.

The =premolars= are four in number, and in all the cingulum is fairly
well seen. The first is a very small tooth with a single tapering
root, the second and third are larger and have two roots, while the
fourth, the =carnassial=, is much the largest and has three roots.
Each of the second, third and fourth premolars has a stout blade, the
middle portion of which is drawn out into a prominent cone; the
posterior part of the fourth premolar forms a compressed ridge, and
at the antero-internal edge of the tooth there is a small inner
tubercle.

The two =molar= teeth are of very unequal size. The first, which has
two anterior roots and one posterior, is wider than it is long, its
outer portion being produced into two prominent cusps, while its inner
portion is depressed. The second molar is a small tooth resembling the
first in its general appearance, but with much smaller outer cusps.

=Teeth of the lower jaw.=

The three =incisors= of the lower jaw have much the same character as
the first two of the upper jaw; while the =canine= is identical in
character with that of the upper jaw.

The four =premolars= gradually increase in size from the first to the
last, but none are very large. The first premolar is a single-rooted
tooth resembling that of the upper jaw; the second, third and fourth
are two-rooted, like the second and third of the upper jaw, which they
closely resemble in other respects.

The first =molar= forms the =carnassial= (fig. 84, V), and with the
exception of the canine, is much the largest tooth of the lower jaw;
it is a two-rooted tooth, with a long compressed bilobed blade, and a
posterior tuberculated talon or heel. The second molar is much
smaller, though likewise two-rooted, while the third molar is very
small and has only a single root. All the teeth except the molars are
preceded in the young animal by temporary =milk teeth=. These milk
teeth, though smaller, are very similar to the permanent teeth by
which they are ultimately replaced.


II. ENDOSKELETON.

1. THE AXIAL SKELETON.

This includes the vertebral column, the skull, and the ribs and
sternum.


A. THE VERTEBRAL COLUMN.

This consists of a series of about forty vertebrae arranged in
succession so that their centra form a continuous rod, and their
neural arches a continuous tube, surrounding a cavity, the =neural
canal=.

The vertebrae may be readily divided into five groups:--

1. The =cervical= or neck vertebrae.

2. The =thoracic= or chest vertebrae which bear ribs.

3. The =lumbar= vertebrae which are large and ribless.

4. The =sacral= vertebrae which are fused with one another and united
with the pelvis.

5. The =caudal= or tail vertebrae which are small.

Except in the sacral region the vertebrae are movably articulated to
one another, while their centra are separated from one another by
cartilaginous =intervertebral discs=.


GENERAL CHARACTERS OF A VERTEBRA.

Take as a type the =fourth lumbar vertebra=. It may be compared to a
short tube whose inner surface is smooth and regular, and whose outer
surface is thickened and drawn out in a variety of ways. The basal
part of the vertebra is the =centrum= or body which forms the
thickened floor of the neural canal. Its two ends are slightly convex
and are formed by the =epiphyses=, two thin plates of bone which are
at first altogether distinct from the main part of the centrum, but
fuse with it as the animal grows older; its sides are drawn out into a
pair of strong =transverse processes=, which project forwards,
outwards, and slightly downwards. The =neural arch= forms the sides
and roof of the neural canal, and at each end just above the centrum
bears a pair of =intervertebral notches= for the passage of the spinal
nerves, the posterior notches being considerably deeper than the
anterior. The neural arch is drawn out into a series of processes.
Arising from the centre of the dorsal surface is a prominent median
=neural spine= or =spinous process=, which projects upwards and
slightly forwards; its anterior edge is vertical, while its posterior
edge slopes gradually. At the two ends of the neural arch arise the
two pairs of =zygapophyses= or articulating surfaces, which
interlock with those of the adjacent vertebrae. The anterior or
=prezygapophyses= look inwards, and are large and concave; they are
borne upon a pair of large blunt outgrowths of the neural arch, the
=metapophyses=. The posterior or =postzygapophyses= are slightly
convex and look outwards and downwards; they are borne upon backwardly
projecting outgrowths of the neural arch. Lastly there are a pair of
minute projections arising from the posterior end of the neural arch,
below the postzygapophyses. These are the =anapophyses=. In young
individuals the development of all the processes of the various
vertebrae is less marked, and the epiphyses are obviously distinct.

[Illustration FIG. 69. A, ATLAS AND B, AXIS VERTEBRA OF A DOG (_Canis
familiaris_) (after VON ZITTEL).

    1. transverse process of atlas.
    2. vertebrarterial canal.
    3. foramen for exit of spinal nerve.
    4. neural spine.
    5. odontoid process.
    6. anterior articulating surface of centrum.
    7. centrum.
    8. transverse process of axis.
    9. postzygapophysis.]


THE CERVICAL VERTEBRAE.

These are seven in number, as in almost all mammals. They are
characterised by the fact that they have small ribs fused with them,
forming transverse processes perforated by canals through which the
vertebral arteries run.

The first, or =atlas= vertebra (fig. 69, A), differs much from all the
others; it is drawn out into a pair of wide wing-like transverse
processes (fig. 69, A, 1), and forms a ring surrounding a large
cavity. This cavity is during life divided into two parts by a
transverse ligament; the upper cavity is the true neural canal, while
the lower lodges the =odontoid process= of the second vertebra, which
is the detached centrum of the atlas. The neural arch is broad and
regular; it has no spinous process, and is perforated in front by a
pair of foramina for the passage of the first spinal nerves. The
mid-ventral portion of the atlas is rather thick, and bears a minute
backwardly-projecting hypapophysis. The bases of the broad transverse
processes are perforated by the =vertebrarterial canals= (fig. 69, A,
2). The atlas bears at each end a pair of large articulating surfaces;
those at the anterior end articulate with the condyles of the skull,
and are very deeply concave; those at the posterior end for
articulation with the axis, are nearly as large, but are flattened.
The atlas ossifies from three centres, one forming the mid-ventral
portion, the others the two halves of the remainder.

The second, or =axis= vertebra (fig. 69, B), also differs much from
the other cervicals. The long and broad centrum has a very flat dorsal
surface, and is produced in front into the conical =odontoid process=
(fig. 69, B, 5), and bears a pair of very large convex outwardly
directed surfaces for articulation with the atlas. At its posterior
end it is drawn out into a pair of small backwardly-directed spines,
the transverse processes; these are perforated at their bases by the
vertebrarterial canals. The neural arch is deeply notched in front and
behind for the passage of the spinal nerves, and is drawn out above
into a very long compressed neural spine (fig. 69, B, 4), which
projects a long way forwards, and behind becomes bifid and thickened,
bearing a pair of flat downwardly directed postzygapophyses. In the
young animal the odontoid process is readily seen to ossify from a
centre anterior to that forming the anterior epiphysis of the axis.

The remaining five cervical vertebrae, the third to the seventh
inclusive, have rather flattened wide centra, obliquely truncated at
either end. The neural spine progressively increases in size as the
vertebrae are followed back. The transverse processes vary
considerably; those of the third are divided into a thicker
backwardly-, and a more slender forwardly-projecting portion; those of
the fourth and fifth mainly extend downwards, and that of the sixth is
divided into a horizontal portion and a downwardly-projecting
=inferior lamella=. All the cervical vertebrae except the seventh have
the bases of the transverse processes perforated by the
vertebrarterial canals. The prezygapophyses in each case look upwards
and slightly inwards, while the postzygapophyses look downwards and
slightly outwards.


THE THORACIC VERTEBRAE.

The =thoracic vertebrae= are twelve or thirteen in number, and all
bear movably articulated ribs. As a group they are characterised by
their comparative shortness, and in the case of the first eight or
nine by the great length of the backwardly-sloping neural spine. The
posterior thoracic vertebrae approach in character the succeeding
lumbar vertebrae.

As type of the anterior thoracic vertebrae, take any one between the
second and sixth inclusive. The centrum is short, and has its
terminations vertically truncated. At the top of the centrum, at both
anterior and posterior ends on each side, is a demi-facet (fig. 70, A,
4), which, together with that on the adjacent vertebra, forms an
articulating surface for the capitulum of the rib. The neural arch is
small and deeply notched behind for the passage of the spinal nerve.
It is drawn out above into a very long neural spine (fig. 70, A, 1),
whose base extends back over the succeeding vertebra and bears the
downwardly-directed postzygapophyses (fig. 70, A, 6). The summit of
the neural arch is deeply notched in front, and on each side of the
notch are the prezygapophyses, which look almost vertically upwards.
The transverse processes are short and blunt, and are flattened below
(fig. 70, A, 3) for the articulation of the tubercula of the ribs.

[Illustration FIG. 70. A, SECOND THORACIC, AND B, SECOND LUMBAR
VERTEBRA OF A DOG (_Canis familiaris_) SEEN FROM THE RIGHT SIDE (after
VON ZITTEL).

    1. neural spine.
    2. centrum.
    3. transverse process bearing in A the facet for articulation with
        the tuberculum of the rib.
    4. facet for articulation with the capitulum of the rib.
    5. metapophysis.
    6. postzygapophysis.]

The posterior three or four thoracic vertebrae differ much from the
others. The centra are longer, the neural spines short and not
directed backwards, the articular facets for the heads of the ribs are
confined to the anterior end of the centrum of each vertebra, not
overlapping on to the preceding vertebra. The transverse processes are
small and irregular, and metapophyses and anapophyses are developed.
The prezygapophyses also look more inwards, and the postzygapophyses
more outwards than in the more typical thoracic vertebrae.


THE LUMBAR VERTEBRAE.

The =lumbar vertebrae= are seven in number, and their general
characteristics have been already described. As a group they are
characterised by their large size, and the great development of the
transverse processes, metapophyses and neural spines.


THE SACRAL VERTEBRAE.

Three vertebrae are commonly found fused together, forming the
=sacrum=; the divisions between the three being indicated by the
foramina for the exit of the spinal nerves.

Of these three vertebrae, the first is much the largest, and is firmly
united to the ilium on each side by a structure formed by the
transverse processes and expanded ribs. In the adult this structure
forms one continuous mass, but in the young animal a ventral portion
formed by the rib is clearly distinguishable from a dorsal portion
formed by the transverse process. All three have low neural spines.
The anterior sacral vertebra bears a large pair of prezygapophyses,
while the posterior one bears a small pair of postzygapophyses.


THE CAUDAL VERTEBRAE.

The =caudal vertebrae= are about nineteen in number. The earlier ones
have well-developed neural arches, transverse processes, and
zygapophyses, but as the vertebrae are followed back they gradually
lose all their processes, and the neural arch as well, becoming at
about the thirteenth from the end reduced to simple cylindrical
centra.


B. THE SKULL.

The =skull= consists of the following three parts: (_a_) the cranium,
with which are included the skeletal supports of the various special
sense organs, and the bones of the face and upper jaw; (_b_) the lower
jaw or mandible, which is movably articulated to the cranium, and
(_c_) the hyoid.


(_a_) THE CRANIUM.

The cranium is a compact bony box, forming the anterior expanded
portion of the axial skeleton. It has a longitudinal axis, the
=craniofacial= axis around which the various parts are arranged, and
this axis is a direct continuation of that of the vertebral column.
Similarly the cavity of the cranium is a direct continuation of the
spinal canal. The posterior part of the craniofacial axis, which has
relations only with the cranium, is called the =basicranial axis=.

In the dog as in the other types previously described, the skull in
its earliest stages is cartilaginous, containing no bone. In the
adult, however, the cartilage is to a great extent replaced by bone,
and in addition to this cartilage bone, membrane bone is largely
developed, and intimately united with the cartilage bone to form one
complete whole.

In the description of the dog's skeleton, as in those of the previous
types, the names of the membrane bones are printed in italics, while
those of the cartilage bones are printed in thick type.

Most of the numerous foramina perforating the skull walls will be
described after the bones have been dealt with.

For purposes of description the cranium may be further subdivided
into:--

1. The cranium proper or brain case.

2. The sense capsules.

3. The upper jaw.


1. THE CRANIUM PROPER OR BRAIN CASE.

Taking the membrane and cartilage bones together, they are seen to be
more or less arranged in three segments, which however must not be
regarded as homologous with the segments forming the vertebral column.

[Illustration FIG. 71. DIAGRAM OF THE RELATIONS OF THE PRINCIPAL BONES
IN THE MAMMALIAN SKULL (modified after FLOWER).

Cartilage is dotted. Cartilage bones are marked by dots and dashes,
membrane bones are left white.

    1. basi-occipital.
    2. exoccipital.
    3. supra-occipital.
    4. basisphenoid.
    5. alisphenoid.
    6. parietal.
    7. presphenoid.
    8. orbitosphenoid.
    9. frontal.
    10. periotic, immediately below which is the tympanic.
    11. lachrymal.
    12. ethmo-turbinal.
    13. maxillo-turbinal.
    14. nasal.
    15. mesethmoid.
    16. vomer.
    17. pterygoid.
    18. palatine.
    19. maxillae.
    20. premaxillae.
    21. squamosal.
    22. mandible.
    23. tympano-hyal.
    24. stylo-hyal.
    25. epi-hyal.
    26. basi-hyal. Between this and the epi-hyal is the cerato-hyal.
    27. thyro-hyal.
    28. jugal.

Nerve exits are indicated by Roman numerals.]

The =occipital segment= is the most posterior of the three, and
consists of four cartilage bones, which in the adult are commonly
completely fused together. They surround the great =foramen magnum=
(fig. 75, 2) through which the brain and spinal cord communicate.
Forming the lower margin of the foramen magnum is a large flat
unpaired bone, the =basi-occipital= (fig. 75, 5). Above this on each
side are the =exoccipitals=, whose sides are drawn out into a pair of
downwardly-directed =paroccipital processes=, which are applied to the
tympanic bullae[141]. The inner side of each exoccipital is converted
into the large rounded =occipital condyle= (fig. 72, 13) by which the
skull articulates with the atlas vertebra. The dorsal boundary of the
foramen magnum is formed by a large unpaired flat bone, the
=supra-occipital= (figs. 72 and 75, 1), which is continuous with a
small bone, the _interparietal_, prolonged forwards between the
parietal bones of the next segment.

In old animals the interparietal forms the hind part of a prominent
ridge running along the mid-dorsal surface of the skull and called the
=sagittal crest=, while the junction line of the occipital and
parietal segments forms a prominent =occipital crest=.

The plane in which the bones of the occipital segment lie is called
the occipital plane; the angle that it makes with the basicranial axis
varies much in different mammals.

The =parietal segment= consists of both cartilage and membrane bones.
It is formed of five bones, which are in contact with those of the
occipital segment on the dorsal and ventral surfaces, while laterally
they are separated by the interposition of the auditory bones, and to
some extent of the squamosal. The =basisphenoid= (fig. 75, 6), an
unpaired bone forming the ventral member of this segment, is the
direct continuation of the basi-occipital. It tapers anteriorly, but
is rather deep vertically, its upper or dorsal surface bearing a
depression, the =sella turcica=, which lodges the pituitary body of
the brain. From the sides of the basisphenoid arise the =alisphenoids=
(fig. 75, 11) a pair of bones of irregular shape generally described
as wing-like; each gives off from its lower surface a =pterygoid
plate=, which is united in front with the palatine, and below with the
pterygoid. The alisphenoids are united above with a pair of large
nearly square bones, the _parietals_ (fig. 73, 2), which meet one
another in the mid-dorsal line. The line of junction is frequently
drawn out into a strong ridge, which forms the anterior part of the
=sagittal crest=.

[Illustration FIG. 72. VERTICAL LONGITUDINAL SECTION TAKEN A LITTLE TO
THE LEFT OF THE MIDDLE LINE THROUGH THE SKULL OF A DOG (_Canis
familiaris_) × 3/5. (Camb. Mus.)

    1. supra-occipital.
    2. interparietal.
    3. parietal.
    4. frontal.
    5. cribriform plate.
    6. nasal.
    7. mesethmoid.
    8. maxillae.
    9. vomer.
    10. ethmo-turbinal.
    11. maxillo-turbinal.
    12. premaxillae.
    13. occipital condyle.
    14. basi-occipital.
    15. tympanic bulla.
    16. basisphenoid.
    17. pterygoid.
    18. palatine.
    19. alisphenoid.
    20. internal auditory meatus.
    21. tentorium.
    22. foramen lacerum posterius.
    23. floccular fossa.
    24. coronoid process.
    25. condyle.
    26. angle.
    27. mandibular symphysis.
    28. inferior dental foramen.
    29. stylo-hyal.
    30. epi-hyal.
    31. cerato-hyal.
    32. basi-hyal.
    33. thyro-hyal.
    XII. condylar foramen.]

The =frontal segment=, which surrounds the anterior part of the brain,
is closely connected along almost its whole posterior border with the
parietal segment.

Its base is formed by the =presphenoid= (fig. 75, 12), a very deep
unpaired bone, narrow and compressed ventrally, and with an irregular
dorsal surface. The presphenoid is continuous with a second pair of
wing-like bones, the =orbitosphenoids=. Each orbitosphenoid meets the
alisphenoid behind, but the relations of the parts in this region are
somewhat obscured by a number of large foramina piercing the bones,
and also by an irregular vacuity, the =foramen lacerum anterius= or
=sphenoidal fissure=, which lies between the orbitosphenoid and
alisphenoid, separating the lateral parts of the parietal and frontal
segments, in the same way as the space occupied by the auditory bones
separates the lateral parts of the occipital and parietal segments.
The orbitosphenoids pass obliquely forwards and upwards, and are
united above with a second pair of large membrane bones, the
_frontals_ (fig. 73, 3). The outer side of each frontal is drawn out
into a rather prominent rounded =postorbital process= (fig. 73, 10),
from which a ridge converges backwards to meet the sagittal crest. The
anterior part of the frontal is produced to form the long nasal
process, which is wedged in between the nasal and maxillae.

[Illustration FIG. 73. DORSAL VIEW OF THE CRANIUM OF A DOG (_Canis
familiaris_) × 2/3.

    1. supra-occipital.
    2. parietal.
    3. frontal.
    4. nasal.
    5. maxillae (facial portion).
    6. premaxillae.
    7. squamosal.
    8. jugal.
    10. postorbital process of frontal.
    11. infra-orbital foramen.
    12. anterior palatine foramen.
    13. lachrymal foramen.
    _i_ 1. first incisor.
    _c._ canine.
    _pm_ 4. fourth premolar.]

The cranial cavity is continuous in front with the =nasal= or
=olfactory cavities=, but the passage is partially closed by a screen
of bone, the =cribriform plate= (fig. 72, 5), which is placed
obliquely across the anterior end of the cranial cavity, and is
perforated by a number of holes through which the olfactory nerves
pass. The plane of the cribriform plate is called the =ethmoidal
plane=, and as was the case also with the occipital plane, the angle
that it makes with the basicranial axis varies much in different
mammals, and is of importance. The =olfactory fossa= in which lie the
olfactory lobes of the brain, is partially separated from the
=cerebral fossa=, or cavity occupied by the cerebral hemispheres, by
ridges on the orbitosphenoids and frontals. The presphenoid is
connected in front with a vertical plate formed partly of bone, partly
of unossified cartilage; this plate, the =mesethmoid= (fig. 72, 7),
separates the two olfactory cavities which lodge the olfactory organs.
Its anterior end always remains unossified, and forms the septal
cartilage of the nose.

The brain case may then, to use the words of Sir W.H. Flower, be
described as a tube dilated in the middle and composed of three bony
rings or segments, with an aperture at each end, and a fissure or
space at the sides between each of them.


2. THE SENSE CAPSULES.

Each of the three special sense organs, of hearing, of sight, and of
smell, is in the embryo provided with a cartilaginous or membranous
protecting capsule; and two of these, the auditory and olfactory
capsules, become afterwards more or less ossified, and intimately
related to the cranium proper.


(1) =Bones in relation to the Auditory capsules.=

These bones lie on each side wedged into the vacuity between the
lateral parts of the occipital and parietal segments; they are three
in number, the =periotic=, the _tympanic_ and the _squamosal_.

The =periotic= is the most important of them, as it replaces the
cartilaginous auditory capsule of the embryo, and encloses the
essential organ of hearing. It commences to ossify from three centres
corresponding to the pro-otic, epi-otic and opisthotic of lower
skulls, such as those of the Turtle and Crocodile.

These ossifications however very early combine to form a single bone,
the =periotic=, which nevertheless consists of two portions, the
=petrous= and the =mastoid=, differing considerably from one another.

[Illustration FIG. 74. DIAGRAM OF THE MAMMALIAN TYMPANIC CAVITY AND
ASSOCIATED PARTS (modified from LLOYD MORGAN).

    1. external auditory meatus.
    2. tympanic membrane.
    3. malleus.
    4. incus.
    5. lenticular.
    6. stapes.
    7. fenestra ovalis.
    8. fenestra rotunda.
    9. Eustachian tube.
    10. cavity occupied by the cochlea.
    11. cavity occupied by the membranous labyrinth.]

The =petrous portion= lies dorsally and anteriorly, and is much the
more important of the two, as it encloses the essential part of the
auditory organ. It forms an irregular mass of hard dense bone,
projecting into the cranial cavity, and does not appear on the
external surface at all. The =mastoid portion= lies ventrally and
posteriorly, is smaller, and formed of less dense bone than is the
petrous portion, from which it differs also in the fact that it
appears on the surface of the skull, just external to the exoccipital.
The petrous portion bears a ridge, which together with a ridge on the
supra-occipital, and the =tentorium= (fig. 72, 21), a transverse fold
of the dura mater[142], separates the large cerebral fossa from the
=cerebellar fossa=, which is much smaller than the cerebral fossa and
lies behind and partly beneath it. The plane of the tentorium is
called the =tentorial plane=, and the angles that it makes with the
basicranial axis and with the occipital and ethmoidal planes vary much
in different mammals.

The periotic has its inner surface marked by important depressions,
while both inner and outer surfaces are pierced by foramina. At about
the middle of its inner surface are seen two deep pits, one lying
immediately above the other. Of these the more ventral is a foramen,
the =internal auditory meatus= (fig. 72, 20), through which the VIIth
(facial) and VIIIth (auditory) nerves leave the cranial cavity, the
facial nerve passing through the bone and afterwards leaving the skull
by the =stylomastoid foramen= (fig. 75, VII), while the auditory
passes to the inner ear. The more dorsal of the two pits is not a
foramen but the =floccular fossa= (fig. 72, 23) which lodges the
floccular lobe of the cerebellum. In some skulls another wide and
shallow but fairly prominent depression is seen dorsal to and slightly
behind the floccular fossa, this also lodges part of the cerebellum.
Behind the internal auditory meatus, between the periotic and
exoccipital is seen the internal opening of the =foramen lacerum
posterius= (fig. 72, 22). The shape of this opening varies. The
ventro-anterior border of the periotic is marked by a deep notch, the
sides of which sometimes unite, converting it into a foramen.

On the outer side of the periotic, and clearly seen only after the
removal of the tympanic, are two holes, the =fenestra ovalis= and the
=fenestra rotunda=.

The _tympanic_ (figs. 72, 15 and 75, 4) is a greatly expanded
boat-shaped bone, which forms the auditory bulla and lies immediately
ventral to the periotic; it is separated from the periotic by the
=tympanic cavity= into which the fenestra rotunda and the fenestra
ovalis open.

There are several other openings into the tympanic cavity.

(_a_) On the external surface is a large oval opening, the =external
auditory meatus= bounded by a thickened rim.

(_b_) Into the outer and anterior part of the cavity the outer end of
the =Eustachian tube= opens; while the inner end passes through a
foramen (fig. 75, 22) just external to the foramen lacerum medium, on
its way to open into the pharynx.

(_c_) The internal carotid artery also enters the tympanic cavity by a
canal which commences in the foramen lacerum posterius, and passes
forwards to open on the inner side of the bulla. The artery then
passes forwards, and barely appearing on the ventral surface of the
cranium, enters the brain cavity through the foramen lacerum medium
(fig. 75, 9).

Immediately behind the tympanic, between it and the mastoid process of
the periotic and the paroccipital process of the exoccipital is the
=stylomastoid foramen= (fig. 75, VIII).

Within the tympanic cavity are four small bones, the =auditory
ossicles= (cp. fig. 74), called respectively the =malleus=, =incus=,
=lenticular= and =stapes=; these together form a chain extending from
the fenestra ovalis to the tympanic membrane.

The =malleus= has a somewhat rounded head (fig. 100, B, 1) which
articulates with the incus, while the other end of the bone is drawn
out into a long process, the =manubrium=, which lies in relation to
the tympanic membrane. The head is also more or less connected by a
thin plate of bone, the =lamella=, to another outgrowth, the
=processus longus=. The =incus= (fig. 100, B, 3) is somewhat
anvil-shaped, and is drawn out into a process which is connected with
the =lenticular=, a nodule of bone interposed between the incus and
the stapes, with which it early becomes united. The =stapes= (fig.
100, B, 2) is stirrup-shaped, consisting of a basal portion from which
arise two =crura=, which meet and enclose a space, the =canal=.

The _squamosal_ (fig. 73, 7) is a large bone occupying much of the
side wall of the cranial cavity, and articulating above with the
parietal, and behind with the supra-occipital, while in front it
overlaps the frontal and alisphenoid. But though it occupies so large
a space on the outer wall, it forms very little of the internal wall
of the skull, but is really like a bony plate attached to the outer
surface of the cranial wall. The squamosal is drawn out into a strong
forwardly-directed =zygomatic process= which meets the jugal or malar.
The ventral side of the zygomatic process is hollowed out, forming the
=glenoid fossa= (fig. 75, 8), a smooth laterally elongated surface
with which the lower jaw articulates, while the hinder edge of the
glenoid fossa is drawn out into a rounded =postglenoid process= (fig.
75, 23). The articulation is such as to allow but little lateral play
of the lower jaw.


(2) =Bones in relation to the Optic capsules.=

The only bone developed in relation to the optic capsule on each side
is the _lachrymal_. This is a small membrane bone lying between the
frontal and palatine behind, and the maxillae and jugal in front. It is
perforated by a prominent =lachrymal foramen= (fig. 73, 13) which
opens within the orbit.


(3) =Bones in relation to the Olfactory capsules.=

In connection with the =olfactory capsules=, five pairs of bones are
developed, two pairs being membrane bones, and three pairs cartilage
bones.

Of membrane bones, the _nasals_ (fig. 73, 4) are a pair of long narrow
bones, lying closely side by side, and forming the main part of the
roof of the olfactory chamber. Their posterior ends overlap the
frontals, and the outer margin of each is in contact with the nasal
process of the frontal, and with the maxillae and premaxillae.

Lying immediately ventral to the nasals, and on each side of the
perpendicular mesethmoid, are the =ethmoid= or =turbinal= bones,
which have a curious character, being formed of a number of delicate
plates intimately folded on one another. The posterior pair of these
bones, the =ethmo-turbinals= (fig. 72, 10), are the larger, and form a
mass of intricately folded lamellae attached behind to the cribriform
plate, and passing laterally into two thin plates of bone, which abut
on the maxillae. The uppermost lamella of each ethmo-turbinal is larger
than the others and more distinct. It is sometimes distinguished as
the =naso-turbinal=, and forms an imperfect lower boundary to a canal,
which is bounded above by the nasals. In front of and somewhat below
the ethmo-turbinals, lie another pair of bones of similar character,
the =maxillo-turbinals= (fig. 72, 11).

The last bone to be mentioned in connection with the olfactory
capsules is a membrane bone, the _vomer_ (fig. 72, 9). This is a
slender vertically-placed bone, whose anterior part lies between the
maxillo-turbinals, while behind it extends beyond the mesethmoid, so
as to underlie the anterior part of the presphenoid. The anterior part
of the vomer forms a kind of trough, while further back in the region
of the ethmo-turbinals it sends out a pair of strong lateral plates,
each of which, passing below the ethmo-turbinal, joins the side wall
of the nasal cavity, and forms a partition dividing the nasal cavity
into a lower =narial passage= and an upper =olfactory chamber=.


THE JAWS.

In the embryo both upper and lower jaws are formed of cartilaginous
bars, but in the adult not only has the cartilage entirely
disappeared, but even cartilage bone is absent, the jaws being formed
of membrane bone.


3. THE UPPER JAW.

[Illustration FIG. 75. VENTRAL VIEW OF THE CRANIUM OF A DOG (_Canis
familiaris_) × 3/5. (Camb. Mus.)

    1. supra-occipital.
    2. foramen magnum.
    3. occipital condyle.
    4. tympanic bulla.
    5. basi-occipital.
    6. basisphenoid.
    7. external auditory meatus.
    8. glenoid fossa.
    9. foramen lacerum medium and anterior opening of carotid canal.
    10. postglenoid foramen.
    11. alisphenoid.
    12. presphenoid.
    13. vomer.
    14. jugal.
    15. pterygoid.
    16. palatal process of palatine.
    17. maxillae (palatal portion).
    18. posterior palatine foramina.
    19. anterior palatine foramen.
    20. premaxillae.
    21. alisphenoid canal.
    22. Eustachian foramen.
    23. postglenoid process of squamosal.
    II. optic foramen.
    III, IV, V{1}, VI. foramen lacerum anterius.
    V{2}. foramen rotundum.
    V{3}. foramen ovale.
    VII. stylomastoid foramen.
    IX, X, XI. foramen lacerum posterius.
    XII. condylar foramen.
    _i_ 2. second incisor.
    _c._ canine.
    _pm_ 1, _pm_ 4. first and fourth premolars.
    _m_ 1. first molar.]

The bones of the upper jaw are closely connected with those of the
cranium proper and olfactory capsules. The most posterior of them is
the _pterygoid_ (fig. 75, 15), a thin vertically placed plate of bone,
which articulates above with the basisphenoid, the presphenoid, and
the strong pterygoid process of the alisphenoid. The ventral end of
the pterygoid is drawn out into a small backwardly-projecting =hamular
process=. In front the pterygoid articulates with the _palatine_, a
much larger bone, consisting of (1) a vertical portion, which passes
up to meet the orbitosphenoid and frontal, and sends inwards a plate
which meets the presphenoid and vomer, forming much of the roof of the
posterior part of the narial passage; and (2) a strong horizontal
portion, the =palatal process= (fig. 75, 16), which passes inwards and
meets its fellow in the middle line, forming the posterior part of the
bridge of bone supporting the hard palate. The palatal process is
continuous in front, with a large bone, the _maxillae_, which, like
the palatine, consists of vertical and horizontal portions. The
vertical, or =facial portion= (fig. 73, 5), is the largest, and
constitutes the main part of the side of the face in front of the
orbit, forming also the chief part of the outer wall of the nasal
cavity. It is continuous in front with the premaxillae, above with the
nasal and frontal, and behind with the lachrymal, jugal, and palatine.
The horizontal, or =palatal portion= (fig. 75, 17), forms the anterior
part of the bony plate supporting the hard palate, and meets its
fellow in a long straight symphysis. The junction line between the
palatal and facial portions is called the =alveolar border=, and
along it are attached the canine, premolar, and molar teeth.

The anterior part of the upper jaw on each side is formed by a small
bone, the _premaxillae_, which bears the incisor teeth. It, like the
maxillae, has a palatal portion (fig. 75, 20), which meets its fellow
in the middle line, and an ascending portion, which passes backwards
as the =nasal process=, tapering regularly and lying between the nasal
and the maxillae. The two premaxillae form the outer and lower borders
of the anterior nares. The last bone to be mentioned in connection
with the upper jaw and face is the _jugal_ or _malar_ (figs. 73, 8,
and 75, 14), a strong bone which forms the anterior half of the
zygomatic arch. It is firmly united in front to the maxillae, and
behind meets the zygomatic process of the squamosal, being drawn out
dorsally into a short =postorbital process= at the point of meeting.
This process lies immediately below the postorbital process of the
frontal, and if the two met, as they do in some mammals, they would
partially shut off the orbit from a larger posterior cavity, the
=temporal fossa=. In the living animal a ligament unites the two
postorbital processes.


(_b_) THE LOWER JAW OR MANDIBLE.

This consists of two elongated symmetrical halves, the =rami=, which
are united to one another at the median symphysis in front, while
behind they diverge considerably, and each articulates with the
glenoid surface of the corresponding squamosal. In young animals the
rami are united at the symphysis by fibrous tissue, but in old animals
they sometimes become fused together. The upper or alveolar border
bears the teeth, and behind them is drawn out into a high laterally
compressed =coronoid process= (fig. 72, 24), which is hollowed on its
outer surface. Immediately behind the coronoid process is the
transversely elongated =condyle= (fig. 72, 25), which fits into the
glenoid cavity in such a way as to allow free up and down movement of
the jaw, with but little rolling motion. The posterior end of the jaw
below the condyle forms a short rounded process, the =angle= (fig.
72, 26). Two prominent foramina are to be seen in the lower jaw. These
are firstly the =inferior dental foramen= (fig. 72, 28), which lies on
the inner surface below the coronoid process; through it an artery and
a branch of the fifth nerve enter to supply the teeth, and secondly
the =mental foramen=, which lies on the outer side near the anterior
end, and through which a branch of the same nerve emerges.


(_c_) THE HYOID.

The =Hyoid= of the dog consists of a transverse median piece, the
=basi-hyal=[1] (fig. 72, 32), from which arise two pairs of =cornua=.
The =anterior cornu= is much the longer of the two, and consists
principally of three short separate ossifications, placed end to end
and called respectively the =cerato-hyal=[143], =epi-hyal=, and
=stylo-hyal=. All of them are short rods of bone, contracted in the
middle, and expanded at the ends, where they are tipped with
cartilage. The cerato-hyal (fig. 72, 31) lies next to the basi hyal.
The stylo-hyal is terminated by a much smaller bone, the
=tympano-hyal=, which lies in a canal between the tympanic and
periotic, and is ankylosed to the periotic just to the anterior and
inner side of the stylomastoid foramen.

The =posterior cornu= of the hyoid is much smaller than the anterior;
it consists of a short bone, the =thyro-hyal= (fig. 72, 33), which
connects the basi-hyal with the thyroid cartilage of the larynx.

FORAMINA OF THE SKULL.

The foramina, or apertures perforating the walls of the skull, are
very numerous, and may either be due to holes actually penetrating the
bone, or may be small vacuities between the margins of two elsewhere
contiguous bones.

They may be divided into two groups, the first including

I. The holes through which the =twelve cranial nerves= leave the
cranial cavity.

_a._ The most anterior of these nerves, the olfactory, leaves the
skull by a number of small holes piercing the =cribriform plate= (fig.
72, 5).

_b._ The second, or optic, passes out by a large hole, the =optic
foramen= (fig. 75, II) piercing the orbitosphenoid. The optic foramen
is the most anterior of the three prominent holes seen within and
immediately behind the orbit.

_c._ The third, fourth, and sixth nerves, i.e. those supplying the eye
muscles, and with them the first or ophthalmic branch of the large
fifth or trigeminal nerve, pass out by a large hole, the =foramen
lacerum anterius= (fig. 75, III, IV, V{1}, VI), which, as has been
already mentioned, lies between the orbitosphenoid and alisphenoid.

_d._ Immediately behind the foramen lacerum anterius, the alisphenoid
is perforated by a prominent round hole, the =foramen rotundum= (fig.
75, V{2}), through which the second branch of the trigeminal nerve
passes out.

_e._ A quarter of an inch further back there is another prominent
hole, the =foramen ovale= (fig. 75, V{3}), through which the third
branch of the trigeminal nerve leaves the cranium.

_f._ The seventh or facial nerve, as already mentioned, leaves the
cranial cavity and enters the auditory capsule, through an opening in
the periotic called the =internal auditory meatus=, while it finally
leaves the skull by the =stylomastoid foramen= (fig. 75, VII), which
lies between the tympanic bulla, the paroccipital process, and the
mastoid portion of the periotic.

_g._ The eighth or auditory nerve on leaving the cranial cavity,
passes with the facial straight into the auditory capsule through the
=internal auditory meatus= (fig. 72, 20). It is then distributed to
the organ of hearing.

_h._ The ninth, tenth and eleventh nerves leave the skull through the
=foramen lacerum posterius= (fig. 75, IX, X, XI), a large space lying
between the auditory bones and the exoccipital.

_i._ Finally, the twelfth nerve, the hypoglossal, passes out through
the prominent =condylar foramen= (fig. 75, XII), which perforates the
exoccipital just behind the foramen lacerum posterius.


II. OTHER OPENINGS IN THE SKULL.

_a._ The =anterior narial opening= lies at the anterior end of the
skull, and is bounded by the premaxillae and nasals. In the natural
condition it is divided into two by a vertical partition, formed by
the =narial septum=, the anterior unossified part of the mesethmoid.

_b._ Penetrating the middle of the maxillae at the side of the face is
the rather large =infra-orbital foramen= (fig. 73, 11), through which
part of the second branch of the trigeminal nerve passes out from the
orbit to the side of the face.

_c._ Several foramina are seen perforating the anterior part of the
orbit. The most dorsal of these, perforating the lachrymal bone, is
the =lachrymal foramen= (fig. 73, 13). Lying below and slightly
external to this is a large foramen, through which part of the second
branch of the trigeminal enters on its way to the infra-orbital
foramen and so to the side of the face. Lastly, lying below these, and
perforating the palatine, are two closely apposed foramina, the
=internal orbital foramina=, through which part of the first or
ophthalmic branch of the trigeminal nerve leaves the orbit, passing
into the nasal cavity.

_d._ The anterior part of the palate between the premaxillae and the
maxillae is perforated by a pair of long closely apposed apertures, the
=anterior palatine foramina= (fig. 75, 19). They transmit part of the
trigeminal nerve.

_e._ Towards the posterior part of the palate are two pairs of small
=posterior palatine foramina= (fig. 75, 18). These perforate the
palatine and transmit branches of the trigeminal nerve and certain
blood-vessels.

_f._ The =posterior narial opening= is bounded chiefly by the
palatines.

_g._ The =alisphenoid canal= (fig. 75, 21) is a short canal
penetrating the base of the alisphenoid bone, and transmitting the
external carotid artery. It lies between the foramen rotundum and the
foramen ovale.

_h._ Between the auditory bulla and the foramen ovale are seen two
openings. The more external of these is the opening of the =Eustachian
canal= (fig. 75, 22), which communicates with the tympanic cavity. The
more internal is the =foramen lacerum medium= (fig. 75, 9), through
which the internal carotid enters the cranial cavity.

_i._ The =external auditory aperture= (fig. 75, 7) is a large opening
with rough edges at the outer side of the tympanic bulla.

_j._ Between it and the glenoid surface of the squamosal is the
=postglenoid= foramen (fig. 75, 10) through which a vein passes out.

_k._ Lastly, there is the great =foramen magnum= (fig. 75, 2), between
the occipital condyles. Through it the brain and spinal cord
communicate.


C. THE RIBS AND STERNUM.

[Illustration FIG. 76. STERNUM AND STERNAL RIBS OF A DOG (_Canis
familiaris_) × 1/2.

    1. presternum.
    2. first sternebra of mesosternum.
    3. last sternebra of mesosternum.
    4. xiphisternum. The flattened cartilaginous plate terminating the
        xiphisternum is not shown.
    5. first sternal rib.]

These, together with the thoracic vertebrae, form the skeletal
framework of the thorax. Each rib is a curved rod, which at its dorsal
end is movably articulated to the vertebra, and at its ventral end is
either connected with the sternum, or ends freely. In the dog there
are thirteen pairs of ribs, nine pairs of which are directly connected
with the sternum, while the remaining four end freely and are known as
=floating ribs=. Each rib is obviously divided into two parts, a
dorsal or =vertebral part=, and a ventral or =sternal part=. The
vertebral portion, which forms about two-thirds of the whole rib, is a
flattened, regularly curved rod, completely ossified. Its dorsal end
is rounded, forming the =head= or =capitulum=, which articulates with
a concave surface furnished partly by the corresponding vertebra and
partly by the vertebra next in front. The last three or four however
articulate with one vertebra only. A short way behind the capitulum on
the dorsal side of the rib is a rounded outgrowth, the =tubercle= or
=tuberculum=, by means of which the rib articulates with the
transverse process. The portion of the rib between the head and the
tubercle is known as the =neck=. The =sternal portion= of the rib
(fig. 76) is a short bar of calcified or imperfectly ossified
cartilage, about one-third of the length of the corresponding bony
portion. The anterior sternal ribs are somewhat more cartilaginous
than the posterior ones. The vertebral portions increase in length
from the first which is very stout, and has the capitulum and
tuberculum very distinct, to about the eighth or ninth; afterwards
they gradually diminish in size. The first nine to eleven have the
capitula and tubercula separate, afterwards they gradually merge
together.


THE STERNUM.

This is an elongated cylindrical structure lying in the mid-ventral
wall of the thorax, and is divided into eight segments or
=sternebrae=. The anterior segment, the =presternum= (fig. 76, 1) or
=manubrium sterni= is expanded in front; the next six segments, which,
together form the =mesosternum= are elongated, somewhat contracted in
the middle and expanded at the ends. The last segment or
=xiphisternum= (fig. 76, 4) is long and narrow, and terminates in a
flattened expanded plate of cartilage. The first pair of sternal ribs
articulate with the sides of the presternum, and the remaining pairs
between the successive sternebrae. Between the last sternebra and the
xiphisternum two pairs articulate. Development shows that the sternum
is formed by the union in the middle line of two lateral portions;
this can be well seen in the presternum and xiphisternum of the puppy,
but no traces of this median division remain in the adult dog.


2. THE APPENDICULAR SKELETON.

The appendicular skeleton consists of the bones of the anterior and
posterior limbs, and of their respective supports, the pectoral and
pelvic girdles.


THE PECTORAL GIRDLE.

The =pectoral girdle= lies external to the ribs, and has no bony
attachment to the axial skeleton. In almost all Mammalia it is, as
compared with that in Sauropsids, very incomplete; and in the dog it
is even more reduced than in the majority of Mammalia. The dorsal
portion or =scapula= is well developed, but the ventral portion is
almost entirely absent.

The =scapula= is somewhat triangular in shape, the apex being
directed downwards and forwards, and being expanded to form the
shallow =glenoid cavity= with which the head of the humerus
articulates. The inner surface of the scapula is nearly flat, while
the outer is drawn out into a very prominent ridge, the =spine=,
which, arising gradually near the dorsal end, runs downwards, dividing
the surface into two nearly equal parts, the =prescapular= and
=postscapular fossae=, and ends in a short blunt process, the
=acromion=. The anterior border of the scapula is somewhat curved, and
is called the =coracoid border=; it is terminated ventrally by a
slight blunt swelling, the =coracoid process=, which ossifies from a
different centre from the rest of the scapula, and is probably the
sole representative of the =coracoid=. The dorsal or =suprascapular
border= of the scapula is rounded, while the posterior or =glenoid
border= is nearly straight. The clavicle[144] or collar bone, which in
a large proportion of mammals is well seen, in the dog is very
imperfectly developed; it is short and broad, and is suspended in the
muscles, not reaching either the scapula or sternum.


THE ANTERIOR LIMB.

The anterior limb of the dog is divisible into the usual three
portions, the =brachium= or =upper arm=, the =antibrachium= or
=fore-arm=, and the =manus= or =wrist= and =hand=.

The =brachium= or =upper arm= includes only a single bone, the
=humerus=.

The =humerus= is a stout elongated bone, articulating by its large
proximal =head= (fig. 77, 1) with the glenoid cavity of the scapula,
and at its distal end by the =trochlea= with the bones of the
fore-arm. The head passes on its inner side into an area roughened for
the attachment of muscles and called the =lesser tuberosity= (fig. 77,
2); while in front it is divided by the shallow =bicipital groove=
from a large roughened area, the =greater tuberosity= (fig. 77, 3),
which is continued as a slight roughened ridge, extending about
one-third of the way down the outer side of the shaft. This ridge,
which in many animals is much more strongly developed than it is in
the dog, is called the =deltoid ridge=. The =trochlea= (fig. 77, 5) at
the distal end of the bone is a pulley-like surface, elevated at the
sides and grooved in the middle. It articulates with the radius and
ulna of the fore-arm. On each side of it are slight roughened
projections, the =internal= and =external condyles= (fig. 77, 7). In
the cat and many other animals there's a foramen, the =ent-epicondylar
foramen= above the internal condyle, but in the dog this is not
developed. Passing up the shaft from the external condyle is a slight
ridge, the =supinator= or =ectocondylar ridge=; this is better
developed in many mammals. Immediately above the trochlea in front and
behind are the deep =supra-trochlear fossae=, which communicate with
one another through the =supra-trochlear foramen= (fig. 77, 8). The
posterior of these, the =olecranon fossa=, is much the deeper, and
receives the olecranon process of the ulna when the arm is extended.
The head and tuberosities of the humerus ossify from one centre, the
shaft from a second, and the trochlea and condyles from a third.

The =fore-arm= or =antibrachium= contains two bones, the =radius= and
=ulna=; they are immovably articulated with one another, but not
fused. The pre-axial bone, the =radius= (fig. 77, B), which lies more
or less in front of the ulna, is external to the ulna at its proximal
end, and at its distal end is internal to that bone. It articulates
with the external portion of the trochlea, while the ulna articulates
with the internal portion. It is a straight bone with its distal end
slightly larger than its proximal end. The proximal end articulates
with the trochlea, the distal end with the bones of the carpus.

[Illustration FIG. 77. BONES OF THE LEFT UPPER ARM AND FORE-ARM OF A
DOG (_Canis familiaris_) × 1/2.

A, humerus (seen from the posterior side); B, radius, C, ulna, both
seen from the anterior side.

    1. head.
    2. lesser tuberosity.
    3. greater tuberosity.
    4. shaft of the humerus.
    5. trochlea.
    6. internal condyle.
    7. external condyle.
    8. supra-trochlear foramen.
    9. proximal end of the radius.
    10. shaft of the radius.
    11. olecranon.
    12. surface for articulation with the trochlea.
    13. surface for articulation with the radius.
    14. distal end of the ulna.]

The postaxial bone, the =ulna= (fig. 77, C), has the proximal end much
enlarged, forming the =olecranon= (fig. 77, 11), and tapers gradually
to the distal end. Near its proximal end the ulna is marked by a deep
=sigmoid notch=, which bears on its inner side a concave surface
(fig. 77, 12) for articulation with the trochlea. The pointed proximal
end of the sigmoid notch is called the =coronoid process=. Somewhat
in front of and below the sigmoid notch is a smaller hollow (fig. 77,
13), with which the radius articulates.

In the young animal the ends of both radius and ulna are seen to
ossify from centres different from those forming the shafts. The
epiphyses forming both ends of the radius, and the distal end of the
ulna are large, while that at the proximal end of the ulna is small,
and forms only the end of the olecranon.

The =Manus= is divided into

_a._ The =carpus= or =wrist=, formed of a group of small bones.

_b._ The =hand=, which includes firstly some elongated bones, the
=metacarpals=, forming what corresponds to the palm of the hand, and
secondly the phalanges, which form the =fingers=.

The =Carpus= or =wrist=. The carpus of the dog consists of seven small
bones, arranged in a proximal row of three, and a distal row of four.
It differs much from the simpler type met with in the newt. The
largest bone of the proximal row is the =scapho-lunar= (fig. 80, 1),
formed by the fused =scaphoid= (radiale), =lunar= (intermedium), and
=centrale=; it has a large convex proximal surface for articulation
with the radius, and articulates distally with the trapezium,
trapezoid, and magnum, and internally with the cuneiform. The
=cuneiform= (ulnare) (fig. 80, 2) has a posterior rounded surface
articulating with the ulna; it articulates in front with the unciform,
and internally with the =pisiform= (fig. 80, 7), which is a
comparatively large sesamoid bone on the ulnar side of the carpus.
Frequently also there is a small sesamoid bone on the radial side of
the carpus. The =trapezium= (carpale 1), =trapezoid= (carpale 2), and
=magnum= (carpale 3) (fig. 80, 5) are all small bones, and support
respectively the first, second, and third metacarpals. The =unciform=
(carpalia 4 and 5) (fig. 80, 6) is larger, and supports the fourth and
fifth metacarpals.

The hand has five =digits=, each consisting of an elongated
=metacarpal=, followed by =phalanges=, the last of which, the =ungual
phalanx=, is pointed and curved, and bears the claw. Each of the
metacarpals is seen in the young animal to have its distal end formed
by a prominent epiphysis, and each of the phalanges, except those
bearing the claws, has a similar epiphysis at its proximal end.

The =pollex= (fig. 80, A, I ) is far shorter than the other digits,
and normally does not touch the ground in walking. It has only two
phalanges, while each of the other digits has three. A pair of small
sesamoid bones are developed on the ventral or flexor side of the
metacarpo-phalangeal articulations of all the digits except the
pollex. Frequently similar sesamoid bones occur also on the dorsal
side of the phalangeal articulations.


THE PELVIC GIRDLE.

The =pelvic girdle= consists of two halves, which lie nearly parallel
to the vertebral column.

Each half is firmly united to its fellow in a ventral symphysis
behind, and is in front expanded and united to the sacrum. Each half
or =innominate bone= is seen in the young animal to consist of four
distinct parts, the =ilium= or dorsal element, the =pubis= or anterior
ventral element, the =ischium= or posterior ventral element, and a
small fourth part, the =acetabular= or =cotyloid= bone, wedged in
between the three others. These parts, though all distinct in the
young animal, are in the adult so completely fused that their
respective boundaries cannot be distinguished. At about the middle of
the outer surface of the innominate bone is a very deep cavity, the
=acetabulum= (fig. 78, A, 1) with which the head of the femur
articulates; all the bones except the pubis take part in its
formation.

The =ilium= is a rather long bone, expanded in front and contracted
behind; it forms about half the acetabulum. On its inner or =sacral
surface= (fig. 78, 4) is a large roughened patch for articulation
with the sacrum; its outer or =gluteal surface= is concave. The
posterior part of the bone is flattened below, forming the narrow
=iliac surface= (fig. 78, A, 5).

[Illustration FIG. 78. RIGHT INNOMINATE BONE, A, OF A FULL-GROWN
TERRIER, B, OF A COLLIE PUPPY. × 1.

A is seen from the ventral side, B from the inner or sacral side.

    1. acetabulum.
    2. thyroid foramen.
    3. supra-iliac border of ilium.
    4. sacral surface.
    5. iliac surface.
    6. acetabular border.
    7. pubic border.
    8. ischial border.
    9. ischium.
    10. tuberosity of ischium.
    11. ischial symphysis.
    12. pubis.
    13. pubic symphysis.
    14. cotyloid or acetabular bone.]

The =ischium= (fig. 78, 9) is a wide flattened bone forming the
posterior part of the innominate bone. It meets the pubis ventrally,
but is separated from it for the greater part of its length by the
large =obturator= or =thyroid foramen= (fig. 78, 2). At its posterior
end externally it bears a rather prominent roughened =ischial
tuberosity= (fig. 78, A, 10). The ischium meets its fellow in a
ventral symphysis, and forms about one-third of the acetabulum.

[Illustration FIG. 79. FRONT VIEW OF THE LEFT LEG BONES OF A DOG
(_Canis familiaris_) × 1/2.

A femur, B tibia, C fibula, D patella.

    1. head of femur.
    2. neck.
    3. great trochanter.
    4. shaft.
    5. external condyle.
    6. internal condyle.
    7. fabella.
    8. cnemial crest.]

The =pubis= (fig. 78, 12) is smaller than either the ischium or ilium;
it does not take part in the formation of the acetabulum, and like the
ischium, meets its fellow in a ventral symphysis. The =acetabular
bone= (fig, 78, B, 14) is small and triangular, and is wedged in
between the other three. It forms about one-sixth of the acetabulum.


THE POSTERIOR LIMB.

The =posterior limb=, like the anterior, is divisible into three
parts; these are the =thigh=, the =crus= or =shin=, and the =pes=.

The =thigh= contains only a single bone, the =femur=.

The =femur= is a long straight bone with a nearly smooth shaft and
expanded ends. The proximal end bears on its inner side the large
rounded =head= (fig. 79, A, 1) which articulates with the acetabulum.
External to the head and divided from it by a deep pit is a large
rough outgrowth, the =great trochanter= (fig. 79, 3). The deep pit is
the =trochanteric= or =digital fossa=. On the inner side below the
head is a smaller roughened surface, the =lesser trochanter=. The
lower or distal end of the bone bears two prominent rounded surfaces,
the =condyles=, which articulate with the tibia. They are separated
from one another by the deep =intercondylar notch=, which is continued
above and in front as a shallow groove, lodging a large sesamoid bone,
the =patella= or =knee-cap=. At the back of the knee-joint are a pair
of smaller sesamoids, the =fabellae= (fig. 79, 7).

In the young animal there are three epiphyses to the shaft of the
femur, one forming the head, one the great trochanter, and one the
distal end.

The =crus= or =shin= contains two bones, the =tibia= and =fibula=. The
=tibia= is a fairly thick straight bone, expanded at both ends,
especially at the head or proximal end. The proximal end is triangular
in cross section, and bears two facets for articulation with the
condyles of the femur. The anterior surface of the proximal end of the
tibia is marked by the strong =cnemial crest= (fig. 79, 8), which runs
some way down the shaft. The distal end of the tibia articulates with
the astragalus by an irregular, somewhat square surface.

The shaft of the tibia ossifies from one centre, the distal end from a
second, and the proximal end from two more.

[Illustration FIG. 80. A, RIGHT MANUS, B, RIGHT PES OF A DOG (_Canis
familiaris_) × 1/2 (after VON ZITTEL).

    1. bone representing the fused scaphoid, lunar and centrale.
    2. cuneiform.
    3. trapezium.
    4. trapezoid.
    5. magnum.
    6. unciform.
    7. pisiform.
    8. first metacarpal.
    9. fifth metacarpal.
    10. astragalus.
    11. calcaneum.
    12. navicular.
    13. middle cuneiform.
    14. external cuneiform.
    15. cuboid.
    16. first metatarsal.

The digits are numbered with Roman numerals.]

The =fibula= (fig. 79, C) is a distinct but very slender bone,
somewhat expanded at both ends. It lies external to the tibia and
articulates by its proximal end with the head of the tibia, and by its
distal end with the calcaneum. Its shaft and proximal end ossify from
one centre, and its distal end from a second.


The =Pes=.

The structure of the =pes= corresponds closely with that of the manus.
It is divided into:--

_a._ The =tarsus= or =ankle= formed of a group of small bones.

_b._ The =foot=, which includes, firstly, some elongated bones, the
=metatarsals=, forming what corresponds to the sole of the foot, and
secondly the =phalanges=, which form the toes.

The =Tarsus=. The tarsus of the dog consists of seven bones arranged
in two rows, of two and four respectively, with a =centrale= between
them. The two bones of the proximal row are the =astragalus= and
=calcaneum=.

The =astragalus= (fig. 80, 10) corresponds to the fused =tibiale= and
=intermedium= of the typical tarsus. Its proximal end is much wider
than its distal end, and forms a large rounded =condyle= articulating
with the tibia, while its posterior end meets the navicular. It lies
to the dorsal side of the foot.

The =calcaneum= (fibulare) (fig. 80, 11), the thickest bone in the
pes, lies somewhat behind, and to the outer side of the astragalus. It
articulates with the astragalus and fibula, and is drawn out behind
into a long rounded process, which forms the heel, and is in the young
animal terminated by an epiphysis. Between the proximal and distal
rows of tarsals is the =navicular= (centrale) (fig. 80, 12), a
somewhat flattened and square bone articulating with the astragalus.

The distal row of tarsals consists of four bones. The =internal
cuneiform= (tarsale 1) is a smooth flattened bone lying to the inner
side of the foot; it articulates with the first metatarsal and with
the navicular. The =middle cuneiform= (tarsale 2) (fig. 80, 13) is a
still smaller bone, lying external to the internal cuneiform. It
articulates with the second metatarsal and with the navicular. The
=external cuneiform= (tarsale 3) (fig. 80, 14) is a larger, somewhat
square bone lying external to the middle cuneiform. It articulates
with the third metatarsal and with the navicular. The =cuboid=
(tarsalia 4 and 5) (fig. 80, 15) is a considerably larger bone lying
to the outer side of the foot. It articulates with the fourth and
fifth metatarsals and with the calcaneum.

The pes has sometimes five digits, sometimes four, the hallux being
absent. Even when present the =hallux= (fig. 80, =B, I=) is commonly
much reduced, and may be quite vestigial, and represented only by a
small nodular metatarsal.

Each of the other digits consists of a long metatarsal, which in the
young animal has a prominent epiphysis at its distal end, and of three
phalanges. The proximal and middle phalanges have epiphyses at their
proximal ends, while the distal phalanx is without epiphyses and is
claw-shaped.


FOOTNOTES:

[139] W. Ellenberger and H. Baum, _Anatomie des Hundes_, Berlin, 1891.

[140] T.H. Huxley, "Dental and cranial characters of the Canidae,"
_P.Z.S._ 1880.

[141] See p. 392.

[142] The dura mater is a membrane which lines the cranial cavity and
is formed of tough connective tissue.

[143] These are not strictly homologous with the basi-hyal and
cerato-hyal of the Dogfish.

[144] See note to p. 25.




CHAPTER XXII.

GENERAL ACCOUNT OF THE SKELETON IN MAMMALIA.


THE EXOSKELETON AND VERTEBRAL COLUMN.


EPIDERMAL EXOSKELETON.

=Hair=, which forms the characteristic Mammalian exoskeleton, varies
much in different animals, and in different parts of the same animal.
A large proportion of mammals have the surface fairly uniformly
covered with hair of one kind only. In some forms however there are
two kinds of hair, a longer and stiffer kind alone appearing on the
surface, and a shorter and softer kind forming the under fur. In most
mammals hairs of a special character occur in certain regions, such as
above the eyes, on the margins of the eyelids, and on the lips and
cheeks, here forming the vibrissae or whiskers.

Sometimes as in _Hippopotamus_, _Orycteropus_ and the Sirenia, the
hair, though scattered over the whole surface, is extremely scanty,
while in the Cetacea it is limited to a few bristles in the
neighbourhood of the mouth, or may even be absent altogether in the
adult. In most mammals the hairs are shed and renewed at intervals,
sometimes twice a year, before and after the winter. The vibrissae or
large hairs which occur in many animals upon the upper lip, and the
mane and tail of Equidae are probably persistent.

In the hedgehogs, porcupines and _Echidna_ certain of the hairs are
modified and greatly enlarged, forming stiff spines. Similar spines
occur in the young of _Centetes_, and in _Acanthomys_ among the
Muridae.

Several other forms of epidermal exoskeleton are met with in mammals,
including:--

(_a_) =Scales=. These overlie the bony scutes of armadillos and occur
covering the tail in several groups of mammals, such as beavers and
rats. In the Manidae the body is covered by flat scales which overlap.

(_b_) The =horns= of Bovine Ruminants. These, which must on no account
be confused with antlers, are hollow cases of hardened epidermis
fitting on to bony outgrowths of the frontals. In almost every case
they are unbranched structures growing continuously throughout life,
and are very rarely shed entire. In the Prongbuck _Antilocapra_
however they are bifurcated and are periodically shed. Horns are
nearly always limited to a single pair, but the four-horned antelope
_Tetraceros_ has two pairs, the anterior pair being the smaller.

(_c_) The =horns of Rhinoceroses=. These are conical structures
composed of a solid mass of hardened epidermal cells growing from a
cluster of long dermal papillae. From each papilla there grows a fibre
which resembles a thick hair, and cementing the whole together are
cells which grow from the interspaces between the papillae. These
fibres differ from true hairs in not being developed in pits in the
dermis. Rhinoceros horns may be either one or two in number, and are
borne on the fronto-nasal region of the skull. They vary much in
length, the longest recorded having the enormous length of fifty-seven
inches.

(_d_) _Nails_, _hoofs_ and =claws=. In almost all mammals except the
Cetacea, these are found terminating the digits of both limbs. =Nails=
are more or less flattened structures, =claws= are pointed and
somewhat curved. In most mammals the nails tend to surround the ends
of the digits much more than they do in man. Sometimes the nail of one
digit differs from that of all the others; thus the second digit of
the pes in the Hyracoidea and Lemuroidea is terminated by a long claw,
the other digits having flat nails. In the Felidae the claws are
retractile, the ungual phalanx with claw attached folding back when
the animal is at rest into a sheath, above, or by the side of the
middle phalanx. In the Sloths and Bats enormously developed claws
occur, forming hooks by which the animals suspend themselves. In
_Notoryctes_ the third and fourth digits of the manus bear claws of
great size; similar claws occur in _Chrysochloris_, being correlated
in each case with fossorial habits. The nail at its maximum
development entirely surrounds the terminal phalanx of the digit to
which it is attached, and is then called a =hoof=. Hoofs are specially
characteristic of the Ungulata.

(_e_) =Spurs= and =beaks= are structures which are hardly represented
among mammals, while so characteristic of birds. They are however both
found in the Monotremata. In both _Echidna_ and _Ornithorhynchus_ the
male has a peculiar hollow horny spur borne on a sesamoid bone
articulated to the tibia. The jaws in _Ornithorhynchus_ are cased in
horny beaks similar to those of birds, and are provided with horny
pads which act as teeth.

(_f_) =Horny plates= of a ridged or roughened character occur upon the
anterior portion of the palate, and of the mandibular symphysis in all
three genera of recent Sirenia; also upon the toothless anterior
portion of the palate in Ruminants.

(_g_) The =baleen of whales= also belongs to the epidermal
exoskeleton. It consists of a number of flattened horny plates
arranged in a double series along the palate. The plates are somewhat
triangular in form and have their bases attached to the palate at
right angles to its long axis, while their apices hang downwards into
the mouth cavity. The outer edge of each plate is hard and smooth,
while the inner edge and apex fray out into long fibres which look
like hair. At the inner edge of each principal plate are subsidiary
smaller plates. The plates are formed of a number of fibres each
developed round a dermal papilla in the same way as are the fibres
forming the horns of _Rhinoceros_. Baleen and Rhinoceros horn likewise
agree in that the fibres are bound together by less hardened
epithelial cells, which readily wear away and allow the harder fibres
to fray out. The greatest development of baleen occurs in the Northern
Right whale, _Balaena mysticetus_, in which the plates number three
hundred and eighty or more on each side, and reach a length of ten or
twelve feet near the middle of the series.


DERMAL EXOSKELETON.

Mammals show two principal kinds of exoskeletal structures which are
entirely or partially dermal in origin, viz. the bony scutes of
armadillos, and teeth.

The =bony scutes of armadillos= are quadrate or polygonal in shape and
are in general aggregated together, forming several shields protecting
various regions of the body. The head is generally protected by a
_cephalic_ shield, the anterior part of the body by a _scapular_, and
the posterior by a _pelvic_ shield. The tail is also generally encased
in bony rings, and scutes are irregularly scattered over the surface
of the limbs. The mid-body region is protected by a varying number of
bands of scutes united by soft skin, so as to allow of movement.
Corresponding to each dermal scute is an epidermal plate. In
_Chlamydophorus_ the scutes are mainly confined to the posterior
region where they form a strong vertically-placed shield which
coalesces with the pelvis. The anterior part of the body is mainly
covered by horny epidermal plates with very little ossification
beneath. In the gigantic extinct Glyptodonts the body is covered with
a solid carapace formed by the union of an immense number of plates,
and there are no movable rings. The top of the head is defended by a
similar plate, the tail is generally encased in an unjointed bony
tube, and there is commonly a ventral plastron.

In _Phocaena phocaenoides_ the occurrence of vestigial dermal ossicles
has been described, and in _Zeuglodon_ the back was probably protected
by dermal plates.

TEETH[145].

Teeth are well developed in the vast majority of mammalia, and are of
the greatest morphological and systematic importance, many extinct
forms being known only by their teeth. Mammalian teeth differ from
those of lower animals in various well-marked respects. (1) They are
attached only to the maxillae, premaxillae and mandible, never to the
palatines, pterygoids or other bones. (2) They frequently have more
than one root. (3) They are always, except in some Odontoceti, placed
in distinct sockets. (4) They are hardly ever ankylosed to the bone.
(5) They are in most cases markedly heterodont. (6) They are commonly
developed in two sets, the milk dentition and permanent dentition.

It sometimes happens that teeth after being formed are reabsorbed
without ever cutting the gum. This is the case, for instance, with the
upper incisors of Ruminants.

The form of mammalian teeth varies much, some are simple conical
structures comparable to those of most reptiles, and these may either
have persistent pulps, as in the case of the upper canines of the
Walrus and the tusks of Elephants, or may be rooted as in most canine
teeth. Some teeth have chisel-shaped edges, and this may be their
original form, as in the human incisors, or may, as in those of
Rodents, be brought about by the more rapid wearing away of the
posterior edge, the anterior edge being hardened by a layer of enamel.
Then, again, the crown may, as in the majority of grinding teeth, be
more or less flattened. The various terms used in describing some of
the forms of the surface of grinding teeth are defined on page 345.

[Illustration FIG. 81. SKULL OF A YOUNG INDIAN RHINOCEROS (_R.
unicornis_), SHOWING THE CHANGE OF THE DENTITION × 1/7. (Brit. Mus.)

    1. nasal.
    2. frontal.
    3. parietal.
    4. zygomatic process of squamosal.
    5. jugal.
    _mI{1}._ milk incisor.
    _mc._ milk canine.
    _mpm{1}._ milk premolar.
    _I{1}._ first incisor.
    _c._ canine.
    _pm{2}_, _pm{3}_, _pm{4}_. 2nd, 3rd and 4th premolars.
    _m{1}_, _m{2}_. first and second molars.]

The teeth of the Aard Varks are compound, and differ completely from
those of all other mammals (see p. 425).

As a rule, the higher the general organisation of an animal the
better are its milk teeth developed, and the more do they form a
reproduction on a small scale of the permanent set. This fact is well
seen in the Primates, Carnivora and Ungulata. The method of notation
by which the dentition of any mammal can be briefly expressed as a
formula has been already described. The regular mammalian arrangement
of teeth for each side is expressed by the formula

    _i_ 3/3 _c_ 1/1 _pm_ 4/4 _m_ 3/3 × 2; total, 44.

MONOTREMATA. In _Echidna_ teeth are quite absent. In the young
_Ornithorhynchus_[146] functional molar teeth of a multi-tubercular
type resembling those of some Mesozoic mammalia are present, but in
the adult they disappear, their office being discharged by horny
plates.

MARSUPIALIA[147] have a heterodont dentition, which has generally been
regarded as almost monophyodont, the only tooth which has an obvious
deciduous predecessor being the last premolar. The researches of
Röse[148] and Kükenthal[149] tend to show that the teeth of Marsupials
are developed in the same way as in other mammals, and are diphyodont.
In the case of the premolars, teeth which are homologous with the
permanent teeth of other mammals begin to develop as lateral
outgrowths from the milk teeth, but afterwards become absorbed, so
that the teeth which actually persist belong to the milk series. The
last premolar, however, does as a rule develop and replace its milk
predecessor; sometimes, however, as in _Didelphys_, it takes its place
among the milk molars without replacing one of them.

The types of dentition characteristic of the different groups of
placental mammals may mostly be paralleled among the Marsupials. Thus
among the polyprotodont forms the Didelphyidae or opossums, and some
of the Dasyuridae, such as _Sarcophilus_ and _Thylacinus_, have a
typical carnivorous dentition with small incisors, large canines, and
molars with pointed compressed crowns. The dental formula of
_Thylacinus_, is _i_ 4/3 _c_ 1/1 _pm_ 3/3 _m_ 4/4, total 46.

In _Myrmecobius_ five or six molar teeth occur on each side, and the
total number of teeth reaches fifty-two or fifty-six. The teeth bear
rows of tubercles, and resemble those of the Multituberculate mesozoic
Mammals[150], more than do those of any other living form. Calcified
teeth have recently been described[151] in _Myrmecobius_ earlier than
the functional or milk set. This would relegate the milk teeth of
mammals in general to a second series, and the permanent teeth to a
third. In _Notoryctes_ the dental formula[152] is given as _i_ 3/2 _c_
1/1 _pm_ 2/3 _m_ 4/4, total 40. The canines are small, and the
anterior molars have strongly developed cusps, and much resemble those
of _Chrysochloris_ (Insectivora).

Among the diprotodont types the Phascolomyidae, or Wombats, have a
dentition recalling that of the Rodents. All the teeth grow from
persistent pulps, and the incisors have enamel only on the anterior
surface as in Rodents. The dental formula is _i_ 1/1 _c_ 0/0 _pm_ 1/1
_m_ 4/4, total 24. There are indications of a vestigial second pair of
incisors.

The Macropodidae, or Kangaroos, have a herbivorous dentition with the
formula _i_ 3/1 _c_ (0--1)/0 _pm_ 2/2 _m_ 4/4. The incisors are sharp
and cutting, and are separated by a long diastema or gap from the
molars, which have their crowns marked by ridges or cusps. There are
indications of several vestigial incisors.

_Coenolestes_, a remarkable form recently described from America,
belongs to the diprotodont section, and is the only living member of
the section known outside the Australian region[153]. An exceptional
dentition is seen in the case of the extinct _Thylacoleo_, in which
the functional teeth are reduced to two pairs; one pair of large
cutting incisors and one of compressed sharp-edged premolars.

EDENTATA. Some Edentata, viz. the ant-eaters (Myrmecophagidae) are, as
far as is known, absolutely toothless at all stages of their
existence; being the only mammals except _Echidna_ in which no tooth
germs have been discovered; others, viz. the Manidae, though showing
foetal tooth germs, are quite toothless in post-foetal life; others,
viz. some of the armadillos, have the largest number of teeth met with
in land mammals. The teeth are homodont except in the Aard Varks, and
grow from persistent pulps. In the sloths (Bradypodidae) and the
Megatheriidae, there are five pairs of teeth in the upper and four in
the lower jaw. The teeth of sloths consist of a central axis of
vasodentine, surrounded firstly by a thin coating of hard dentine, and
secondly by a thick coating of cement.

In no living Edentate have the teeth any enamel; it has, however, been
described as occurring in certain early Megatheroid forms from S.
America[154], and an enamel organ has also been discovered in an
embryo _Dasypus_[155]. In the Armadillos (Dasypodidae) the number of
teeth varies from 8/8 or 7/7 in _Tatusia_, to upwards of 25/25 in
_Priodon_, which therefore may have upwards of a hundred teeth, the
largest number met with in any land mammal. In _Tatusia_ all the teeth
except the last are preceded by two-rooted milk teeth. The Aard Varks
are diphyodont, and milk teeth are also known in a species of
_Dasypus_, but with these exceptions Edentates are, as far as is
known, monophyodont. In _Glyptodon_ the teeth are almost divided into
three lobes by two deep grooves on each side.

The Aard Varks (Orycteropodidae) are quite exceptional as regards
their teeth, which are cylindrical in shape, and are made up of a
number of elongated denticles fused together. Each denticle contains a
pulp cavity from which a number of minute tubes radiate outwards.
These teeth are diphyodont and somewhat heterodont, eight to ten pairs
occur in the upper jaw and eight in the lower, but they are not all in
place at one time. The last three teeth in each jaw are not preceded
by milk teeth[156].

SIRENIA. The teeth of Sirenia show several very distinct types, the
least modified being that of the extinct Halitheriidae, which have
large incisors in the upper jaw, and five or six pairs of tuberculated
grinding teeth in each jaw, the anterior ones being preceded by milk
teeth.

In both the living genera the dentition is monophyodont. In _Manatus_
the dentition is _i_ 2/2 _pm_ and _m_ 11/11. The incisors are
vestigial, and disappear before maturity. The grinding teeth have
square enamelled crowns marked by transverse tuberculated ridges. They
are not all present in the jaw at the same time. In _Halicore_ the
upper jaw bears a pair of straight tusklike incisors; in the male
these have persistent pulps and project out of the mouth; in the
female they soon cease to grow and are never cut. They are separated
by a long diastema from the grinding teeth which have tuberculated
crowns and are 5/5 or 6/6 in number, but are not all in place at once.
Several other pairs of slender teeth occur in the young animal, but
are absorbed or fall out before maturity. In _Rhytina_ teeth are
altogether absent.


CETACEA.

_ARCHAEOCETI._ _Zeuglodon_ has the following dentition, _i_ 3/3 _c_
1/1 _pm_ and _m_ 5/5, total 36. The incisors and canines are simple
and conical; the cheek teeth are compressed and have serrated cutting
edges like those in some seals.

In the _MYSTACOCETI_, or whalebone whales, calcified tooth germs
probably belonging to the milk dentition are present in the embryo,
but they are never functional, and are altogether absent in the adult.
The anterior of these germs are simple, the posterior ones are
originally complex, but subsequently split up into simple teeth like
those of the anterior part of the jaw. Hence according to Kükenthal,
who described these structures, the Cetacean dentition was originally
heterodont.

In the living _ODONTOCETI_ the dentition is homodont and monophyodont.
In some cases traces occur of a replacing dentition which never comes
to maturity, and renders it probable that the functional teeth of the
Odontoceti are really homologous with the milk teeth of other mammals.
Some of the dolphins afford the apparently simplest type of mammalian
dentition known. The teeth are all simple, conical, slightly recurved
structures, with simple tapering roots and without enamel. The
dentition is typically _piscivorous_, being adapted for seizing active
slippery animals such as fish. The prey is then swallowed entire
without mastication. Sometimes the teeth are excessively numerous,
reaching two hundred or more (fifty to sixty on each side of each jaw)
in _Pontoporia_. This multiplication of teeth is regarded by Kükenthal
as due to the division into three parts of numbers of trilobed teeth
similar to those of some seals.

In the Sperm whale, _Physeter_, the lower jaw bears a series of twenty
to twenty-five stout conical recurved teeth, while in the upper jaw
the teeth are vestigial and remain imbedded in the gum. An extinct
form, _Physodon_, from the Pliocene of Europe and Patagonia is allied
to the Sperm whale, but has teeth in both jaws. In the Grampus _Orca_,
the teeth number about 12/12, and are very large and strong. In some
forms the teeth are very much reduced in number; thus in _Mesoplodon_
the dentition consists simply of a pair of conical teeth borne in the
mandible. In the Narwhal _Monodon_ the dentition is practically
reduced to a single pair of teeth, which lie horizontally in the
maxillae, and in the female normally remain permanently in the alveoli.
In the male the right tooth remains rudimentary, while the left is
developed into an enormous cylindrical tusk marked by a spiral groove.
Occasionally both teeth develop into tusks, and there is reason for
thinking that two-tusked individuals are generally or always female.
In the extinct _Squalodon_ the dentition is decidedly heterodont, and
the molars have two roots. The dental formula is

    _i_ 3/3 _c_ 1/1 _pm_ 4/4 _m_ 7/7, total 60.

It is probable that the homodont condition of modern Odontoceti is not
primitive, but due to retrogressive evolution.


UNGULATA.

Just as in the Cetacea a piscivorous dentition is most typically
developed, so the Ungulata are, as a group, the most characteristic
representatives of a _herbivorous_ dentition in its various forms.


UNGULATA VERA.

_ARTIODACTYLA_. As regards the living forms, the Artiodactyla can be
readily divided into two groups, namely those with bunodont and those
with selenodont teeth. It has, however, been shown that selenodont
teeth always pass through an embryonic bunodont stage[157]. The
bunodont type is best seen in Pigs and Hippopotami and such extinct
forms as _Hyotherium_. In _Hippopotamus_ the dental formula is _i_
(2-3)/(1-3) _c_ 1/1 _pm_ 4/4 _m_ 3/3.

The incisors and canines of _Hippopotamus_ are very large and grow
continuously. The genus _Sus_, which affords a good instance of an
_omnivorous_ type of dentition, has the regular unmodified Mammalian
dental formula _i_ 3/3 _c_ 1/1 _pm_ 4/4 _m_ 3/3, total 44. The
canines, specially in the male, are large and have persistent pulps,
and the upper canines do not have the usual downward direction but
pass outwards and upwards. In the Wart Hog, _Phacochaerus_, they are
enormously large, but a still more extraordinary development of teeth
is found in _Babirussa_. In the male _Babirussa_ the canines, which
are without enamel, are long, curved and grow continuously. Those of
the upper jaw never enter the mouth, but pierce the skin of the face
and curve backwards over the forehead. The dental formula of
_Babirussa_ is _i_ 2/3 _c_ 1/1 _pm_ 2/2 _m_ 3/3, total 34.

The Wart Hog has a very anomalous dentition, for as age advances all
the teeth except the canines and last molars show signs of
disappearing; both pairs of persisting teeth are however very large.

Various extinct Ungulata such as _Anoplotherium_ have teeth which are
intermediate in character between the bunodont and selenodont types.
_Anoplotherium_ has the regular mammalian series of forty-four teeth.
The crowns of all the teeth are equal in height, and there is no
diastema--an arrangement found in no living mammal but man.

We come now to the selenodont Artiodactyla.

The Tylopoda--camels (Camelidae) and Llamas (Aucheniidae) when young
have the full number of incisors, but in the adult the two upper
middle ones are lost. The molars are typically selenodont and
hypsodont[158]. In the Camel the dental formula is _i_ 1/3 _c_ 1/1
_pm_ 3/2 _m_ 3/3, total 34. The upper incisors, canines and first
premolars of the Camel are very small teeth, and the first premolar
is separated by a long diastema from the others.

The Tragulina or Chevrotains have no upper incisors, while the canines
are largely developed, especially in the male.

The Ruminantia or Pecora are very uniform as regards their dentition.
The upper incisors are always absent, for though their germs are
developed they are reabsorbed without ever becoming visible, and as a
rule the upper canines are absent too, while the lower canines are
incisiform. The grinding teeth are typically selenodont, and in the
lower jaw form a continuous series separated by a wide diastema from
the canines. The dental formula is usually

    _i_ 0/3 _c_ 0-1/1 _pm_ 3/3 _m_ 3/3.

The canines are largely developed in the male Muskdeer (_Moschus_) and
in _Hydropotes_.

_PERISSODACTYLA_. The premolars and molars have a very similar
structure and form a continuous series of large square teeth with
complex crowns. The crowns are always constructed on some modification
of the bilophodont[159] plan, as is easily seen in the case of the
forms with brachydont teeth, but in animals like the Horse, in which
the teeth are very hypsodont, this arrangement is hard to trace. All
four premolars in the upper jaw are preceded by milk teeth, while in
Artiodactyla the first has no milk predecessors.

In the Tapiridae the grinding teeth are brachydont and the lower ones
are typically bilophodont. The last two upper molars have the
transverse ridges united by an outer longitudinal ridge. The dentition
is _i_ 3/3 _c_ 1/1 _pm_ 4/3 _m_ 3/3, total 42.

In some of the extinct Perissodactyles such as _Lophiodon_[160], the
dentition is brachydont and bilophodont, the grinding teeth in general
resembling the posterior upper molars of the Tapir. The same type of
brachydont tooth is seen in _Palaeotherium_ but the transverse ridges
are crescentic instead of straight, and are separated from one another
by shallow valleys without cement. Some of the Palaeotheridae have the
regular series of forty-four teeth.

A complete series of forms is known showing how from the simple
brachydont teeth of the Palaeotheridae, were derived the complicated
hypsodont teeth of the Equidae. The increase in depth of the tooth was
accompanied by increase in the depth and complexity of the enamel
infoldings, and of the cement filling them.

Both upper and lower grinding teeth of the Equidae are much
complicated by enamel infoldings, but their derivation from the
bilophodont type can still be recognised. The diastema in front of the
premolars is longer in the living Equidae than in their extinct
allies. In the adult horse the dental formula is _i_ 3/3 _c_ 1/1 _pm_
3/3 _m_ 3/3, total 40, with often a vestigial first upper premolar
(fig. 82, _pm_ 1). The last molar is not more complex than the others,
and in the female the canine is quite vestigial. The incisors are
large and adapted for cutting and have the enamel curiously folded in
forming a deep pit. The milk dentition is _di_ 3/3 _dc_ 0/0 _dpm_ 3/3,
total 24. The last milk premolar is not more complex than the premolar
that succeeds it. The horse affords an excellent instance of a
typically _herbivorous_ type of dentition, the cutting incisors,
reduced canines and series of large square flat-crowned grinding teeth
being most characteristic.

In _Rhinoceros_ the grinding teeth are much like those of _Lophiodon_,
having an outer longitudinal ridge from which two crescentic
transverse ridges diverge. The upper premolars are as complex as the
molars, and there are no canines; in some species incisors also are
absent. The dental formula is

    _i_ (0--2)/(0--1) _c_ 0/(0--1) _pm_ 4/4 _m_ 3/3.

[Illustration FIG. 82. PALATAL ASPECT OF THE CRANIUM AND MANDIBLE OF A
DONKEY (_Equus asinus_) × 1/5. (Camb. Mus.)

    1. supra-occipital.
    2. occipital condyle.
    3. basi-occipital.
    4. vacuity representing the confluent foramen lacerum posterius
       and foramen lacerum medium.
    5. auditory bulla.
    6. glenoid surface.
    7. vomer.
    _i_ 1, _i_ 3. first and third incisors.
    _c._ canine.
    _pm_ 1, _pm_ 2. first and second premolars.
    _m_ 1. first molar.]

Among the Titanotheriidae _Palaeosyops_[161] has very brachydont teeth
whose crowns have been described as _buno-selenodont_, the inner pair
of columns being bunodont, the outer, selenodont. Similar grinding
teeth occur in _Chalicotherium_. Some of the Titanotheriidae have the
regular mammalian series of forty-four teeth.


SUBUNGULATA.

_TOXODONTIA._ _Nesodon_ has the regular dental formula; its grinding
teeth are rooted and the upper ones resemble those of Rhinoceros. The
second upper and third lower incisors form ever-growing tusks. There
is a marked difference between the deciduous and permanent dentition.
_Astrapotherium_ likewise has large rooted cheek teeth of a
rhinocerotic type, and each jaw bears a pair of permanently growing
tusks, those of the lower jaw being the canines. The dental formula is

    _i_ 1/3 _c_ 0/1 _pm_ 2/1 _m_ 3/3, total 28.

In _Toxodon_ the upper incisors and molars are large and curved and
all the teeth have persistent pulps. In _Typotherium_ there are no
tusks, but the upper incisors are chisel-like, recalling those of
Rodents.

The _CONDYLARTHRA_ have brachydont, generally bunodont teeth, with the
premolars simpler than the molars. They generally have the regular
dental formula.

_HYRACOIDEA._ The dental formula of _Procavia_ is usually given as _i_
1/2 _c_ 0/0 _pm_ 4/4 _m_ 3/3, total 34; in young individuals however
there occur a second pair of upper incisors which early fall out. The
upper incisors resemble those of Rodents in being long and curved and
growing from persistent pulps. They are however triangular in
transverse section, not rectangular, having two antero-lateral faces
covered with enamel and a posterior face without enamel. Their
terminations are pointed, not chisel-shaped as in Rodents. The lower
incisors (fig. 83, _i_ 1) are pectinate or partially divided by
vertical fissures, and the grinding teeth are of the rhinocerotic
type.

[Illustration FIG. 83. SKULL OF _Procavia (Dendrohyrax) dorsalis_ ×
2/3. (Camb. Mus.)

    1. nasal.
    2. parietal.
    3. external auditory meatus.
    4. paroccipital process of the exoccipital.
    5. jugal.
    6. lachrymal foramen.
    _i_ 1. first incisor.]

_AMBLYPODA._ Two of the best known forms belonging to this extinct
group differ much as regards dentition. For while _Coryphodon_ has the
regular dental formula, and the canines of both jaws of moderate size,
in _Uintatherium_ the dentition is very specialised, there are no
upper incisors, and the upper canines form a pair of enormous tusks.
The grinding teeth form a continuous series marked by =V=-shaped ridges
and the dental formula is _i_ 0/3 _c_ 1/1 _pm_ 3/3 _m_ 3/3 total 34.

_PROBOSCIDEA._ The incisors are composed entirely of dentine and have
the form of conical tusks projecting greatly from the mouth. In
living forms they are confined to the upper jaw, in some species of
the extinct _Mastodon_ however they occur in the lower jaw also. In
_Dinotherium_ they are probably absent from the upper jaw, but form a
pair of downwardly and backwardly-directed tusks growing from the
elongated symphysis of the mandible.

The grinding teeth in the various Proboscidea show a very remarkable
series of modifications. In _Dinotherium_ they are bilophodont or else
are marked by three straight transverse ridges. The dental formula is
_i_ 0?/1 _c_ 0/0 _pm_ 2/2 _m_ 3/3, and the teeth have the normal
method of succession. In _Mastodon_ as in _Dinotherium_ the grinding
teeth are marked by transverse ridges, but the ridges are subdivided
into conical or mammillary cusps, and similar cusps often occur
between the ridges. These cusps are covered with very thick enamel and
the spaces between them are not filled up with cement. There are six
of these grinding teeth for each side of each jaw but only three are
in place at once. The first three are milk teeth as they may be
succeeded vertically by others.

In the true Elephants the number and depth of the enamel folds is much
increased, and the spaces between the folds are filled up with cement.
A very complete series of extinct forms is known with teeth
intermediate in character between those of _Mastodon_ and those of the
Mammoth and living elephants. The dental formula of _Elephas_ is

    _di_ 1/0 _i_ 1/0 _c_ 0/0 _dm_ 3--4/3--4 _m_ 3/3.

Sir W.H. Flower describes[162] the mode of succession of teeth in
Elephants as follows: "As regards the mode of succession that of
modern Elephants is as before mentioned very peculiar. During the
complete lifetime of the animal there are but six molar teeth on each
side of each jaw with occasionally a rudimentary one in front,
completing the typical number of seven. The last three represent the
true molars of ordinary mammals, those in front appear to be milk
molars which are never replaced by permanent successors, but the whole
series gradually moves forwards in the jaw, and the teeth become worn
away and their remnants cast out in front while development of others
proceeds behind. The individual teeth are so large and the processes
of growth and destruction by wear take place so slowly, that not more
than one or portions of two teeth are ever in place and in use on each
side of each jaw at one time, and the whole series of changes
coincides with the usual duration of the animal's life. On the other
hand the _Dinotherium_, the opposite extreme of the Proboscidean
series, has the whole of the molar teeth in place and use at one time,
and the milk molars are vertically displaced by premolars in the
ordinary fashion. Among Mastodons transitional forms occur in the mode
of succession as well as in structure, many species showing a vertical
displacement of one or more of the milk molars, and the same has been
observed in one extinct species of Elephant (_E. planifrons_) as
regards the posterior of these teeth."

In the TILLODONTIA the grinding teeth are of Ungulate type, while the
second incisors are large and grow from persistent pulps, so as to
resemble those of Rodents.

RODENTIA have a most characteristic and very constant dentition, the
common dental formula being

    _i_ 1/1 _c_ 0/0 _pm_ (0--1)/(0--1) _m_ 3/3, total 18 or 20.

The incisors always have chisel-like edges and persistent pulps, and
are separated by a wide diastema from the premolars. Canines are
always absent, and there are generally three grinding teeth not
preceded by milk teeth; their surface may be grooved, or may be
bunodont. Teeth are most numerous in the Duplicidentata (Hares and
Rabbits), in which the formula is _i_ 2/1 _c_ 0/0 _pm_ 3/2 _m_ 3/3,
total 28, and fewest in Hydromys and certain other forms, in which
the formula is _i_ 1/1 _c_ 0/0 _pm_ 0/0 _m_ 2/2, total 12. The hares
and rabbits are the only rodents which have well developed deciduous
incisors, though a vestigial milk incisor has been described in the
Mouse (_Mus musculus_). The last upper molar of _Hydrochaerus_ is very
complicated, its structure approaching that of the teeth of Elephants.

[Illustration FIG. 84. CARNASSIAL OR SECTORIAL TEETH OF CARNIVORA
(from FLOWER).

_Upper sectorial teeth_ of I. _Felis_, II. _Canis_, III. _Ursus_. 1.
anterior, 2. middle, 3. posterior cusp of blade, 4. inner lobe
supported on distinct root, 5. inner lobe posterior in position and
without distinct root, characteristic of the Ursidae.

_Lower sectorial teeth._ 1. _Felis_, 2. _Canis_, 3. _Herpestes_. 1.
anterior, 2. posterior lobe of blade, 3. inner tubercle, 4. heel.]

CARNIVORA have the teeth rooted and markedly diphyodont and
heterodont. The canines are greatly developed, and the incisors are
small.

In _CARNIVORA VERA_ the incisors are almost always 3/3. The fourth
upper premolar and first lower molar are differentiated as carnassial
teeth (see p. 436), and retain fundamentally the same characters
throughout the suborder. The upper carnassial (fig. 84, I. II. III.)
consists of a more or less compressed, commonly trilobed blade borne
on two roots, with an inner tubercle borne on a third root. The lower
carnassial has only two roots; its crown consists of a bilobed blade
with generally an inner cusp, and a heel or talon (fig. 84, 4) behind
the blade.

The most thoroughly carnivorous type of dentition is seen in the
Æluroidea, and especially in the cat tribe (Felidae). In the genus
_Felis_ the dental formula is _i_ 3/3 _c_ 1/1 _pm_ 3/2 _m_ 1/1, total
30. The incisors are very small, so as not to interfere with the
action of the large canines, the lower carnassial is reduced to simply
the bilobed blade (fig. 84, IV), and the cheek teeth are greatly
subordinated to the carnassial. The extinct _Machaerodus_ has the
upper canines comparable in size to those of the Walrus.

The Civets and Hyaenas have a dentition allying them closely to the
cats. The hyaena-like _Proteles_ has, however, the grinding teeth
greatly reduced.

In the Cynoidea[163] the general dentition is _i_ 3/3 _c_ 1/1 _pm_ 4/4
_m_ 2/3, total 42. This differs from the regular mammalian dentition
only in the absence of the last upper molar. The upper carnassial
tooth (fig. 84, II.) consists of a larger middle and smaller posterior
lobe with hardly any trace of an anterior lobe. The lower carnassial
(fig. 84, V.) is typical, consisting of a bilobed blade with inner
cusp and posterior talon.

The dentition of the Cynoidea is most closely linked with that of the
Arctoidea by means of fossil forms.

[Illustration FIG. 85. MANDIBLE OF ISABELLINE BEAR (_Ursus
isabellinus_) × 1/2. (Camb. Mus.)

    1. condyle.
    2. coronoid process.
    _i_ 1. first incisor.
    _c._ canine.
    _pm_ 1, _pm_ 2. first and second premolars.
    _m_ 1. first molar. The dotted
        line is pointing to the posterior
        half of the tooth.
        This specimen has only
        three premolars, there
        should be four.]

In the Arctoidea the dentition is not so typically carnivorous as in
the Æluroidea and Cynoidea. In the bears, Ursidae, the molars have
broad flat tuberculated crowns (fig. 85). The dental formula in
_Ursus_ is _i_ 3/3 _c_ 1/1 _pm_ 4/4 _m_ 2/3, total 42. The upper
carnassial (fig. 84, III.) differs from that of the Æluroidea and
Cynoidea in having no inner lobe supported on a third root. In the
large group of Mustelidae there are generally two molars in the lower
and one in the upper jaw. The grinding teeth commonly have large,
flattened, more or less tuberculated crowns, and the upper molar may
be as large or much smaller than the carnassial.

In the _CREODONTA_ there are no specially differentiated carnassial
teeth.

[Illustration FIG. 86. LEFT MANDIBULAR RAMUS OF THE SEA LEOPARD
(_Ogmorhinus leptonyx_) WITH THE ROOTS OF THE TEETH EXPOSED × 1/3.
(Camb. Mus.)

    1. condyle.
    2. coronoid process.
    _i_ 3. third incisor.
    _c._ canine.
    _pm_ 1, _pm_ 4. first and fourth premolars.
    _m._ molar.]

In the _PINNIPEDIA_ the dentition differs considerably from that of
the Carnivora vera. The milk dentition is always vestigial, and the
teeth are frequently absorbed before birth. There are four premolars
and one molar, forming an uniform series of cheek teeth, all of which
except in the Walrus have compressed and pointed, never flattened,
crowns. There is no special carnassial tooth, and the incisors are
always fewer than 3/3. In _Otaria_ the dentition is

    _i_ 3/2 _c_ 1/1 _pm_ 4/4 _m_ 1 or 2/1, total 34 or 36.

In the Walrus the upper canines form immense tusks. The other teeth
are all small and one-rooted, and the molars have flat crowns. In the
true seals the dentition is strikingly piscivorous, the cheek teeth
often having accessory cusps (fig. 86).

The INSECTIVORA are diphyodont and heterodont, having well-developed
rooted teeth. The canines are usually weak, the incisors pointed, and
those of the two jaws often meet like a pair of forceps. The crowns of
the molars are characteristically studded with short cusps. Some
genera, such as _Gymnura_ and the mole, _Talpa_, have the regular
mammalian dentition. In the hedgehog, _Erinaceus_, the dentition is

    _i_ 3/2 _c_ 1/1 _pm_ 3/2 _m_ 3/3, total 36.

In the genus _Sorex_ (Shrews) the teeth differ in the following two
marked respects from those of most other Monodelphia, (1) they are
monophyodont, (2) the lower incisors sometimes become fused to the
jaws. Most Insectivora have square molar teeth, but in _Potamogale_,
_Chrysochloris_, _Solenodon_ and the Centetidae the molar teeth are
triangular in section. Four molars occur in _Centetes_.

In the aberrant genus _Galeopithecus_ the dentition is _i_ 2/3 _c_ 1/1
_pm_ 2/2 _m_ 3/3, total 34. The upper incisors are placed at some
distance from the anterior end of the jaw, and the outer upper
incisors and canines of both jaws have two roots,--a very unusual
character. The lower incisors are deeply grooved or pectinated in the
same way as are the lower incisors of _Procavia_. The upper incisors
and canines of both jaws bear many cusps, and are very similar in
appearance to the cheek teeth of some Seals.

The dentition of the CHIROPTERA is diphyodont and heterodont, and the
dental formula never exceeds

    _i_ 2/3 _c_ 1/1 pm 3/3 _m_ 3/3, total 38.

The milk teeth are very slender and have sharp recurved cusps; they
are quite unlike the permanent teeth. The permanent teeth are of two
types. In the Insectivorous forms the molar teeth are cusped, and
resemble those of Insectivora. In the blood-sucking Vampire bat
_Desmodus_, the teeth are peculiarly modified; the canines and the
single pair of upper incisors are much enlarged and exceedingly sharp,
while all the other teeth are much reduced in size.

In the Frugivorous bats the molar teeth have nearly always smooth
crowns. The dental formula in the chief genus _Pteropus_ is _i_ 2/2
_c_ 1/1 _pm_ 3/3 _m_ 2/3, total 34.

The PRIMATES have a diphyodont and heterodont dentition, generally of
an omnivorous type, with cheek teeth adapted for grinding. The
incisors are generally 2/2, and the molars, except in the Hapalidae,
are 3/3. In the Lemurs the upper canines are large, and the lower
incisors slender and directed almost horizontally forwards. The Aye
Aye, _Chiromys_, has the following singular dentition: _i_ 1/1 _c_ 0/0
_pm_ 1/0 _m_ 3/3, total 18. The incisors much resemble those of
rodents having persistent pulps, and enamel only on the anterior face.

In Man and in the Anthropoid and Old World Apes the dental formula is
always _i_ 2/2 _c_ 1/1 _pm_ 2/2 _m_ 3/3, total 32.

In the Cebidae there is an extra premolar in each jaw bringing the
number up to 36. In the Hapalidae, as in the Cebidae, there is a third
premolar, but the molars are reduced to 2/2. Man is the only Primate
that has the teeth arranged in a continuous series. In all the others
there is a gap or diastema of larger or smaller size between the
incisors and canines. In all except man also the canines are enlarged,
especially in the males.

The Exoskeletal structures of mammals may be summarised in the
following table:

    I. _Epidermal exoskeletal structures._

      1. Hairs (_a_) ordinary hair,
               (_b_) vibrissae and bristles,
               (_c_) spines of hedgehog, porcupine, _Echidna_,
                      _Centetes_, _Acanthomys_.
      2. Scales { of Manidae,
                { on tails of rats, beavers, &c.
      3. Horns of Rhinoceros.
      4. Horns of Bovine Ruminants.
      5. Nails, claws, hoofs.
      6. Spurs of male _Ornithorhynchus_ and _Echidna_.
      7. Horny beak and teeth of _Ornithorhynchus_.
      8. Horny pads on jaws of Sirenians and Ruminants.
      9. Baleen of whales.
      10. Enamel of teeth.

    II. _Dermal exoskeletal structures._

      1. Dentine and cement of teeth.
      2. Bony scutes of Armadillos.


ENDOSKELETON.

VERTEBRAL COLUMN.

CERVICAL VERTEBRAE.

The cervical vertebrae of all mammals have certain characters in
common. However long the neck may be, the number of cervical
vertebrae, with very few exceptions, is seven. Movable ribs are
generally absent, and if present are small and do not reach the
sternum. The transverse processes are generally wide but not long, and
are perforated near the base by the vertebrarterial canals, through
which the vertebral arteries pass; they generally bear downwardly
directed inferior lamellae which are sometimes as in the seventh human
cervical seen to ossify from centres distinct from those forming the
rest of the transverse process, and are really of the nature of ribs.
The atlas and axis always differ much from the other vertebrae.

We may pass now to the special characters of the cervical vertebrae in
the different groups. In MONOTREMES and MARSUPIALS the number of
cervical vertebrae is always seven. With the exception of the atlas of
_Echidna_ the cervical vertebrae of Monotremes are without
zygapophyses. In Monotremes the transverse processes ossify from
centres distinct from that forming the body, and remain suturally
connected with the rest of the vertebra until the adult condition is
reached. The method of the ossification of the atlas in Marsupials
varies considerably, thus in some forms such as the Wombats
(_Phascolomys_) there is an unossified gap in the middle of the
inferior arch of the atlas, which may remain permanently open; in
_Thylacinus_ this gap is filled up by a distinct heart-shaped piece of
bone, while in _Didelphys_ and _Perameles_ the atlas is ossified below
in the same way as in other mammals. In _Notoryctes_ the second to
sixth cervical vertebrae are ankylosed together.

The cervical vertebrae of the EDENTATA have some remarkable
peculiarities. In the three-fingered Sloth, _Bradypus_, there are nine
cervical vertebrae, all except the last of which have their transverse
processes perforated by the vertebrarterial canals. In a two-fingered
sloth, _Choloepus hoffmanni_, there are only six cervical vertebrae.
In the Megatheriidae, Anteaters (Myrmecophagidae), Pangolins
(Manidae), and Aard Varks (Orycteropodidae), the cervical vertebrae
are normal, but in the Armadillos (Dasypodidae), and still more in the
Glyptodonts, several of them are commonly fused together. The fusion
affects not only the centra, but also the neural arches, so that the
neural canals form a continuous tube.

In the Glyptodonts there is a complex joint at the base of the neck to
allow the partial retraction of the head within the carapace. This
arrangement recalls that in Tortoises.

As a rule the SIRENIA possess seven short cervical vertebrae, not
fused together and not presenting any marked peculiarities. In
_Manatus_ however there are only six cervical vertebrae and they are
very variable.

[Illustration FIG. 87. CERVICAL VERTEBRAE OF A YOUNG FIN WHALE
(_Balaenoptera musculus_) × 1/10. (Camb. Mus.)

    1. surface on the atlas for articulation
        with the occipital condyle of the skull.
    2. foramen for exit of the first spinal nerve.
    3. upper transverse process.
    4. lower transverse process.

In the fresh specimen these two transverse processes are united by
cartilage, in adult individuals the whole transverse process is
ossified.

    5. epiphyses of centrum.
    6. neural spine.]

In the CETACEA there are invariably seven cervical vertebrae, but they
are always very short and are frequently even before birth fused
together by their centra into one continuous mass (see fig. 67).
Sometimes the last one or two are free. In the Rorquals
(_Balaenoptera_) however, the cervical vertebrae are quite separate
and distinct (fig. 87), and in the fluviatile Odontoceti,
_Platanista_, _Inia_, and _Pontoporia_, and also in _Beluga_ and
_Monodon_, though very short they are free. In _Physeter_ the first
vertebra is free while the others are fused. An odontoid process is
not commonly present even in Cetaceans with free cervical vertebrae,
but a very short one occurs in the Rorquals. The cervical vertebrae of
Rorquals give off on each side two transverse processes (fig. 87, 3
and 4) which enclose between them a wide space. These processes are
not completely ossified till the animal is adult.

[Illustration FIG. 88. ATLAS (B) AND AXIS (A) VERTEBRAE OF AN OX (_Bos
taurus_) × 1/4. (Camb. Mus.)

    1. neural canal.
    2. transverse process.
    3. surfaces for articulation with the occipital condyles of the skull.
    4. spout-like odontoid process.
    5. hypapophysis.
    6. anterior opening of the vertebrarterial canal.
    7. foramen for the exit of the second spinal nerve.
    8. neural spine.
    9. postzygapophysis.]

In all UNGULATA the number of cervical vertebrae is seven. Among the
Artiodactyla two forms of the odontoid process of the axis occurs; in
the Suina and Tragulina it is conical, in the Ruminantia and Tylopoda
it is spout-like (fig. 88, 4). The atlas in the Suina and to a less
extent in the Ruminantia has long flattened transverse processes, and
the remaining cervical vertebrae are opisthocoelous. Those of the
Giraffe and Llama (fig. 103) are noticeable for their great length. In
the Tylopoda the posterior half of the vertebrarterial canal is
confluent with the neural canal.

The Perissodactyla have remarkably opisthocoelous cervical vertebrae.
Those of _Macrauchenia_ have the posterior half of the vertebrarterial
canal confluent with the neural canal as in Tylopoda. In the
Proboscidea they are short flattened discs slightly opisthocoelous;
the axis and seventh vertebra and to a less extent the sixth have high
neural spines.

In the RODENTIA the atlas generally has broad wing-like transverse
processes, and the axis a large and long neural spine, while the
odontoid process is much developed. In the Jerboas (_Dipus_) all the
cervical vertebrae except the atlas are fused together, a condition
recalling that in armadillos.

In the CARNIVORA the wings of the atlas are well developed (fig. 69,
A, 1), and it is deeply cupped for articulation with the condyles of
the skull. The axis has a long odontoid process and a high compressed
neural spine (fig. 69, B, 4). The third to sixth cervical vertebrae
have large transverse processes with prominent perforated inferior
lamellae, whose ventral margins in the third and fourth vertebrae
diverge as they pass backwards, while in the fifth they are parallel
and in the sixth convergent. The transverse processes of the seventh
vertebra have no inferior lamellae and are not perforated.
Metapophyses are often developed.

In the INSECTIVORA the cervical vertebrae vary considerably. The
neural spines except in the case of the axis are generally very small
and in the Shrews and Moles the neural arches are exceedingly slender.

In the CHIROPTERA all the cervical vertebrae are broad and short with
slender neural arches.

PRIMATES. In Man the cervical vertebrae have short blunt transverse
processes and small often bifid neural spines. The neural and
vertebrarterial canals are large. The atlas forms a ring surrounding a
large cavity, and has a very slender inferior arch and small
transverse processes. Traces of a pro-atlas have been described in
_Macacus_ and _Cynocephalus_. The axis has a prominent spine and
odontoid process and short transverse processes. In most Primates the
cervical vertebrae are very similar to those of man, but the inferior
lamellae of the transverse processes are better developed. In the
Anthropoid Apes the neural spines are as a rule much elongated.


THORACO-LUMBAR, OR TRUNK VERTEBRAE.

In the MONOTREMATA there are nineteen thoraco-lumbar vertebrae,
sixteen (_Echidna_) or seventeen (_Ornithorhynchus_) of which bear
ribs. The transverse processes are very short and do not articulate
with the ribs, which are united to the centra only.

In the MARSUPIALIA there are always nineteen thoraco-lumbar vertebrae,
thirteen of which generally bear ribs. The lumbar vertebrae frequently
have large metapophyses and anapophyses, these being specially well
seen in the Kangaroos and Koala (_Phascolarctus_).

The EDENTATA are very variable as regards their trunk vertebrae. The
two genera of Sloths differ much as regards the number, for while
_Bradypus_ has only nineteen, fifteen or sixteen of which bear ribs,
_Choloepus_ has twenty-seven, twenty-four of which are thoracic, and
bear ribs. In _Bradypus_ a small outgrowth from the transverse process
articulates with the neural arch of the succeeding vertebra. In both
genera the neural spines are all directed backwards.

In the Megatheriidae as in the sloths the neural spines are all
directed backwards, and in the lumbar region additional articulating
surfaces occur, better developed than are those in _Bradypus_.

In the ant-eaters (Myrmecophagidae) there are seventeen or eighteen
thoraco-lumbar vertebrae, all of which except two or three bear ribs.
The posterior thoracic and anterior lumbar vertebrae articulate in a
very complex fashion, second, third, and fourth pairs of zygapophyses
being progressively developed in addition to the ordinary ones, as the
vertebrae are followed back.

In the Armadillos the lumbar vertebrae have long metapophyses which
project upwards and forwards and help to support the carapace. In
_Glyptodon_ almost all the thoraco-lumbar vertebrae are completely
ankylosed together.

In the Manidae there are no additional zygapophyses but the normal
ones of the lumbar and posterior thoracic regions are very much
developed, the postzygapophyses being semi-cylindrical and fitting
into the deep prezygapophyses of the succeeding vertebra.

In the SIRENIA the number of lumbar vertebrae is very small; in the
dugong there are nineteen thoracic and four lumbar, and in the manatee
seventeen thoracic and two lumbar.

In the CETACEA the number of thoracic vertebrae varies from nine in
_Hyperoödon_ to fifteen or sixteen in _Balaenoptera_, and the number
of lumbar vertebrae from three in _Inia_ to twenty-four or more in
_Delphinus_. The lumbar vertebrae are often very loosely articulated
together and the zygapophyses sometimes as in the Dolphins are placed
high up on the neural spines. The centra are large, short in the
anterior region but becoming longer behind. The epiphyses are
prominent, and so are the neural spines and to a less extent the
metapophyses. The transverse processes are well developed, anteriorly
they arise high up on the neural arch, but when the vertebral column
is followed back they come gradually to be placed lower down, till in
the lumbar region they project from the middle of the centra. This can
be well traced in the Porpoise (_Phocaena_). In the Physeteridae the
transverse processes of the anterior thoracic vertebrae are similar to
those of most Cetacea, but when followed back, instead of shifting
their position on the vertebrae, they gradually disappear, and other
processes gradually arise from the point where the capitulum of the
rib articulates.

UNGULATA. In the Ungulata vera the thoraco-lumbar vertebrae are
slightly opisthocoelous. The anterior thoracic vertebrae commonly have
exceedingly high backwardly-projecting neural spines (fig. 89, 1); but
those of the lumbar and posterior thoracic vertebrae often point
somewhat forwards so that the spines all converge somewhat to a point
called the _centre of motion_ (cp. fig. 101). In the Artiodactyla
there are always nineteen thoraco-lumbar vertebrae, and in the
Perissodactyla twenty-three.

_Procavia_ sometimes has thirty thoraco-lumbar vertebrae, a greater
number than occurs in any other terrestrial mammal; twenty-two of
these are thoracic and eight lumbar. In _Phenacodus_ the convergence
of the neural spines to a centre of motion is well seen.

[Illustration FIG. 89. FIRST AND SECOND THORACIC VERTEBRAE OF AN OX
(_Bos taurus_) × 1/3. (Camb. Mus.)

    1. neural spine.
    2. neural canal.
    3. prezygapophysis.
    4. facet for articulation with the tuberculum of the rib.
    5. facet for articulation with the capitulum of the rib.
    6. postzygapophysis.
    7. foramen for exit of spinal nerve.]

In the Proboscidea there are twenty-three thoraco-lumbar vertebrae, of
which nineteen or twenty bear ribs.

In the RODENTIA there are generally nineteen thoraco-lumbar vertebrae
but occasionally the number rises as high as twenty-five. In the Hares
(Leporidae) the number is nineteen, twelve or thirteen of which are
thoracic. The anterior thoracic vertebrae have short centra and high
backwardly-directed neural spines, the lumbar vertebrae have large
forwardly-and downwardly-directed transverse processes with expanded
ends. Metapophyses, anapophyses and hypapophyses are all present. In
the Agouti (_Dasyprocta_) the convergence of the neural spines to a
centre of motion is very strongly marked.

In the CARNIVORA the trunk vertebrae are nearly always twenty or
twenty-one in number; in the genera _Felis_ and _Canis_ thirteen of
these are thoracic and seven lumbar. The anterior thoracic vertebrae
have long backwardly-projecting neural spines, while the posterior
thoracic and lumbar vertebrae have shorter and thicker neural spines
which project slightly forwards. In the Pinnipedia there is no change
in the direction of the neural spines, and anapophyses are but little
developed.

In the INSECTIVORA the number of trunk vertebrae varies much from
nineteen--thirteen thoracic and six lumbar--in _Tupaia_, to
twenty-four--nineteen thoracic and five lumbar--in _Centetes_. The
development of the various processes varies in accordance with the
habits of the animals, being great in the active forms, slight in the
slowly moving or burrowing forms. In _Talpa_ and _Galeopithecus_ the
intervertebral discs of the thoraco-lumbar region instead of being
cartilaginous have ossified forming inter centra, a condition met with
in very few mammals.

In the CHIROPTERA there are seventeen or eighteen thoraco-lumbar
vertebrae, eleven to fourteen of which may bear ribs. The development
of processes is slight.

Among PRIMATES the number of trunk vertebrae is generally nineteen, of
which twelve to fourteen bear ribs; in man and the Gorilla and
Chimpanzee the number is, however, seventeen, and in the Orang
(_Simia_) sixteen. In some of the Lemuroidea there are as many as
twenty-three or twenty-four. In most cases the neural spines converge
more or less to a centre of motion, and this is especially marked in
some of the Lemurs; it does not occur in man and the anthropoid apes.


SACRAL AND CAUDAL VERTEBRAE.

At the posterior end of the trunk in all mammals a certain number of
vertebrae are found fused together forming the sacrum. But of these
only two or three answer to the definition of true sacral vertebrae in
being united to the ilia by small ribs. The others which belong to the
caudal series may be called pseudosacral vertebrae. In different
individuals of the same species it sometimes happens that different
vertebrae are attached to the pelvis and form the sacrum. Sometimes
even different vertebrae are attached to the pelvis at successive
periods in the life history of the individual. This is owing to a
shifting of the pelvis and has been especially well seen in man. In
young human embryos the pelvis is at a certain stage attached to
vertebra 30, but as development goes on it becomes progressively
attached to the twenty-ninth, twenty-eighth, twenty-seventh,
twenty-sixth and twenty-fifth vertebrae. As the attachment to these
anterior vertebrae is gained, the attachment to the posterior ones
becomes lost, so that in the adult the pelvis is generally attached to
vertebrae 25 and 26. But there are no absolutely pre-determined sacral
vertebrae, as sometimes the pelvis does not reach vertebra 25,
remaining attached to vertebrae 26 and 27; sometimes it becomes
attached even to vertebra 24. This shifting of the pelvis is seen in
_Choloepus_ in a more marked degree even than in man.

Of the MONOTREMATA, _Ornithorhynchus_ has two sacral vertebrae
ankylosed together, while _Echidna_ has three or four[164].

In MARSUPIALIA as a rule only one vertebra is directly united to the
ilia, but one or two more are commonly fused to the first. In the
Wombats there may be as many as four or five vertebrae fused together
in the sacral region. In _Notoryctes_ there is extensive fusion in the
sacral region, six vertebrae, owing mainly to the great development of
their metapophyses, being united with one another, and with the ilia,
and the greater part of the ischia.

In most EDENTATA there is an extensive fusion of vertebrae in the
sacral region. This is especially marked in the Armadillos and
Megatheriidae, and to a less extent in the Sloths and Aard Varks.

In the SIRENIA the vestigial pelvis is attached by ligament to the
transverse processes of a single vertebra, which hence may be regarded
as sacral.

In CETACEA there is no sacrum, the vestigial pelvis not being
connected with the vertebral column.

In most UNGULATA the sacrum consists of one large vertebra united to
the ilia, and having a varying number of smaller vertebrae fused with
it behind.

The same arrangement obtains in most RODENTIA, but in the Beavers
(Castoridae) all the fused vertebrae are of much the same size, the
posterior ones having long transverse processes which nearly meet the
ilia.

In CARNIVORA there may be two sacral vertebrae as in the Hyaena, three
as in the Dog, four or five as in Bears and Seals.

In INSECTIVORA from three to five are united, while in many CHIROPTERA
all the sacral and caudal vertebrae have coalesced. Among PRIMATES, in
Man and Anthropoid Apes there are usually five fused vertebrae forming
the sacrum, but of these only two or three are connected to the ilia
by ribs. In most of the other Anthropoidea there are two or three
fused vertebrae, and in the Lemuroidea two to five.

FREE CAUDAL VERTEBRAE. The free caudal vertebrae vary greatly in
number and character. When the tail is well developed, the anterior
vertebrae are comparatively short and broad, with well-developed
neural arches and zygapophyses; but as the tail is followed back, the
centra gradually lengthen and become cylindrical, and at the same time
the neural arches and all the processes gradually become reduced and
disappear, so that the last few vertebrae consist of simple rod-like
centra. Chevron bones are frequently well-developed.

Of the MONOTREMES _Echidna_ has twelve caudal vertebrae, two of which
bear irregular chevron bones. In _Ornithorhynchus_ there are twenty or
twenty-one caudal vertebrae with well-developed hypapophyses, but no
chevron bones.

In MARSUPIALS there is great diversity as regards the tail. In the
Wombat and Koala the tail is small and without chevron bones. In most
other Marsupials it is very long, having sometimes as many as
thirty-five vertebrae in the prehensile-tailed opossums. In the
Kangaroos the tail is very large and stout. Chevron bones are almost
always present, and in _Notoryctes_ are large and expanded.

Most EDENTATES have large tails with well-developed chevron bones. The
length of the tail varies greatly from the rudimentary condition in
Sloths to that in the Pangolins, one of which has forty-six to
forty-nine caudal vertebrae--the largest number in any known mammal.
Chevron bones are much developed, sometimes they are Y-shaped,
sometimes as in _Priodon_, they have strong diverging processes. The
caudal vertebrae of Glyptodonts, though enclosed in a continuous bony
sheath, have not become ankylosed together.

The SIRENIA have numerous caudal vertebrae with wide transverse
processes. In the CETACEA also the tail is much developed, and the
anterior vertebrae have large chevron bones and prominent straight
transverse processes; the posterior caudal vertebrae, which in life
are enclosed in the horizontally expanded tail fin, are without
transverse processes.

In UNGULATA the tail is simple, formed of short cylindrical vertebrae,
which in living forms are never provided with chevron bones. The
number of caudal vertebrae varies from four, sometimes met with in
_Procavia_, to thirty-one in the Elephant. The tail is exceedingly
long in _Anoplotherium_ and in _Phenacodus_, in which there are thirty
caudal vertebrae.

In RODENTIA the tail is variable. In the Hares, Guinea pig (_Cavia_)
and _Capybara_ it is very small, in _Pedetes_ and the Beaver it is
very long and has well-developed chevron bones.

Most of the CARNIVORA except the Bears and Seals have very long tails,
the greatest number of vertebrae, thirty-six, being met with in
_Paradoxurus_. Bears have only eight to ten caudal vertebrae. Chevron
bones are not often much developed.

In INSECTIVORA the tail is very variable as regards length, the number
of vertebrae varying from eight in _Centetes_ to forty-three in
_Microgale_.

In CHIROPTERA the tail is sometimes quite rudimentary, and as in
_Pteropus_, composed of a few coalesced vertebrae, sometimes it is
formed of a large number of slender vertebrae.

In PRIMATES also the tail is very variable. In Man all the four caudal
vertebrae are rudimentary and are fused together, forming the
_coccyx_. In the Anthropoid apes, too, there are only four or five
caudal vertebrae. In many monkeys of both the eastern and western
hemispheres the tail is very long, having thirty-three vertebrae in
_Ateles_, in which genus it is also prehensile. Chevron bones are
present in all Primates with well-developed tails. In the Lemuroidea
the number of caudal vertebrae varies from seven to twenty-nine.


FOOTNOTES:

[145] See W.H. Flower, "Remarks on the homologies and notation of the
teeth in Mammalia," _J. Anat. and Physiol. norm. path._, Vol. III., p.
262; R. Owen, _Odontography_, London, 1840--45; C.S. Tomes, _Manual of
Dental Anatomy_, London, 1876. See also H.F. Osborn, "Recent
researches on succession of teeth in Mammals," _Amer. Natural._,
XXVII., p. 493, and "Rise of Mammalia in N. America," _Stud. Biol.
Lab. Columb. Coll._, Zool. I., no. 2.

[146] See E.B. Poulton, _P.R.S._, Feb. 1888, and _Quart. J. Micr.
Sci._, Vol. XXIX. 1889; also Oldfield Thomas, _P.R.S._, XLVI. (1889).

[147] W.H. Flower, _Phil. Trans._, vol. 156, pp. 631--641, 1867; also
Oldfield Thomas, _Phil. Trans._, pp. 443--462, 1887.

[148] C. Röse, _Anat. Anz._ VII., p. 639.

[149] W. Kükenthal, _Anat. Anz._ VI., p. 364.

[150] See p. 348.

[151] W. Leche, _Morph. Jahrb._ XX., pp. 113--142 (1893).

[152] E.C. Stirling, _P.Z.S._ 1891, p. 327.

[153] O. Thomas, _P.Z.S._, 1895, p. 870.

[154] F. Ameghino, _Bull. Ac. Argen._ XII. p. 437. According to H.
Burmeister, _Annal. Mus. Buenos Aires_, III. 401 (1891), enamel does
not occur, osteodentine having been mistaken for it.

[155] E. Ballowitz, _Arch. Mikr. Anat._ XL. p. 133.

[156] See Oldfield Thomas, _P.R.S._, vol. XLVII., p. 246 (1890).

[157] J. Taeker, "_Fur Kenntniss der Odontogenese bei Ungulaten_."
Dorpat, 1892.

[158] See p. 345.

[159] See p. 345.

[160] According to H.F. Osborn, _Amer. Natural._, XXVI. p. 763, a
number of not very closely allied forms have been included under
_Lophiodon_.

[161] C. Earle, _J. Ac. Philad._, vol. IX., 1892, p. 267.

[162] _Encyclopaedia Britannica_, article _Mammalia_, p. 424.

[163] See T.H. Huxley, "The dental and cranial characters of the
Canidae," _P.Z.S._, 1880, p. 238.

[164] G.B. Howes, _Journ. of Anat. and Phys._ XXVII., p. 544.




CHAPTER XXIII.

GENERAL ACCOUNT OF THE SKELETON IN MAMMALIA (CONTINUED).


THE SKULL AND APPENDICULAR SKELETON.


THE SKULL.

MONOTREMATA. In both genera the cranium is thin-walled, has a fairly
large cavity, and is very smooth and rounded externally. The sutures
between many of the bones early become obliterated in a manner
comparable to that in birds, and the facial portion of the skull is
much prolonged.

In _Echidna_ the face is drawn out into a gradually tapering rostrum,
formed mainly by the premaxillae, maxillae and nasals. The zygomatic
arch is very weak, and the palate extends very far back. The tympanic
forms a slender ring. The mandible is extremely slight, with no
ascending portion, and but slight traces of the coronoid process and
angle. The hyoid has a wide basi-hyal and stout thyro-hyals, while the
anterior cornua are slender, and include ossified epi-hyals and
cerato-hyals.

In _Ornithorhynchus_ the zygomatic arch is much stouter than in
_Echidna_. The face is produced into a wide beak, mainly supported by
the premaxillae, between whose diverging anterior ends there is a
dumb-bell-shaped bone. The maxillae are flattened below, and each
bears a large horny tooth, which meets a corresponding structure borne
on a surface near the middle of the mandible. The mandible is
considerably stouter than in _Echidna_, but the angle and coronoid
process are but little developed. The infra-orbital foramen and the
inferior dental and mental foramina of the mandible are all very
large.

[Illustration FIG. 90. HALF FRONT VIEW[165] OF THE SKULLS OF A
TASMANIAN WOLF (_Thylacinus cynocephalus_) (to the left) × 3/8; AND OF
A HAIRY-NOSED WOMBAT (_Phascolomys latifrons_) (to the right) × 3/8.
(Camb. Mus.)

    1. premaxillae.
    2. nasal.
    3. frontal.
    4. infra-orbital foramen.
    5. lachrymal.
    6. jugal.
    7. coronoid process of the mandible.
    8. lachrymal foramen.
    i. 1. first upper incisor.
    C. canine.]

MARSUPIALIA. The skulls of the various types of the Marsupials
frequently bear a strong superficial resemblance to those of some of
the different groups of placental mammals. Thus the skull of the
Dasyuridae resembles that of the Carnivora, the resemblance being most
marked between the skulls of _Thylacinus_ and the dog. The skull of
_Notoryctes_ is strongly suggestive of that of an Insectivore, and
that of other Marsupials such as the wombat, recalls equally the
characteristic features of a Rodent's skull. But, however much they
may differ from one another, the skulls of all Marsupials agree in the
following respects. (1) The brain cavity, and especially the cerebral
fossa, has a very small comparative size. (2) The nasals are always
large, and the mesethmoid is extensively ossified, and terminated by a
prominent vertical edge. (3) Processes from the jugal and frontal in
living forms never meet and enclose the orbit, but the zygomatic arch
is always complete. (4) The jugal always extends back to form part of
the glenoid fossa. (5) The lachrymal canal opens either external to or
upon the margin of the orbit, and the nasal processes of the
premaxillae never quite reach the frontals. (6) The posterior part of
the palate is commonly pierced by large oval vacuities. (7) The
tympanic is small and never fused to the bones of the cranium. (8) The
carotid canal perforates the basisphenoid and not the tympanic bulla.
(9) The optic foramen and sphenoidal fissure are confluent. (10) In
every case except _Tarsipes_ the angle of the mandible is more or less
inflected.

The skull of the extinct _Thylacoleo_ differs from that of all other
Marsupials in the fact that the postorbital bar is complete. The hyoid
is constructed on much the same plan in all Marsupials. It consists of
a small basi-hyal, a pair of broad cerato-hyals, and a pair of strong
thyro-hyals. The epi-hyals and stylo-hyals are generally unossified.

EDENTATA. In Sloths (Bradypodidae) the sutures become early
obliterated, the cranial portion of the skull is rather high, and the
facial portion very short. The lachrymal is very small, and its canal
opens outside the orbit. The zygomatic arch is incomplete, and the
jugal (fig. 91, 5) is curiously forked, but in a manner differing in
the two genera. The premaxillae are very small,--in _Bradypus_ quite
vestigial. The mandible is well developed, the angle being specially
marked in _Bradypus_. In _Choloepus_ the symphysial part is drawn out
in a somewhat spout-like manner (fig. 91, 6). In both genera the
thyro-hyals are ankylosed with the basi-hyal.

[Illustration FIG. 91. SKULL OF A TWO-FINGERED SLOTH (_Choloepus
didactylus_) × 1/2. (Camb. Mus.)

    1. anterior nares.
    2. postorbital process of the frontal.
    3. coronoid process.
    4. angle of the mandible.
    5. jugal.
    6. spout-like prolongation of the mandible.]

In _Megatherium_ the general appearance of the skull is distinctly
sloth-like, but the facial portion is more elongated, partly owing to
the development of a prenasal bone, and the zygomatic arch is
complete. The mandible is very deep in the middle, and is drawn out
into a long spout-like process in front.

Anteaters (Myrmecophagidae) have a much modified skull, and this is
especially the case in the Great Anteater, _Myrmecophaga_. The skull
is smooth and evenly-rounded, in these respects recalling that of
_Echidna_, but it is longer and tapers much more gradually than in
_Echidna_. The occipital condyles are remarkably large. The premaxillae
are small, and the long rostrum is chiefly composed of the maxillae and
nasals with the mesethmoid and vomer. The zygomatic arch is
incomplete, and there is no trace of a separation between the orbit
and the temporal fossa. The palate is much elongated, the pterygoids
meeting in the middle line just like the palatines. The mandible is
very long and slender, there being no definite coronoid process, and a
short and slight symphysis. The hyoid arch is noticeable for the
length of the anterior cornu.

In the Armadillos (Dasypodidae) the skull varies a good deal in shape,
but the facial portion is always tapering and depressed. The zygomatic
arch is complete. In _Dasypus_ and _Chlamydophorus_ the tympanic bulla
is well ossified.

In the Glyptodontidae the skull is very short and deep; the zygomatic
arch is complete, and has a long downwardly projecting maxillary
process. The mandible is massive, and has a very high ascending
portion.

In the Manidae the skull is smooth and rounded, the zygomatic arch is
incomplete, and the orbit is inconspicuous. The palate is long and
narrow, but the pterygoids do not take part in its formation. The
mandible is slightly developed and has no angle or coronoid process.

In _Orycteropus_ the zygomatic arch is complete, and there is a small
postorbital process to the frontal. The mandible is well-developed,
having a coronoid process and definite ascending portion, and the
hyoid is well ossified.

SIRENIA. The skull, and especially the brain case of all Sirenia, is
remarkable for the general density of the component bones, which,
though often very thick, are without air sinuses. It is noticeable
also for the roughness of the bones, and the irregular manner in which
they are united together.

[Illustration FIG. 92. LATERAL VIEW OF THE SKULL OF _Rhytina stelleri_
× 1/8. (Brit. Mus.)

    1. frontal.
    2. parietal.
    3. zygomatic process of the squamosal.
    4. squamosal.
    5. exoccipital.
    6. occipital condyle.
    7. pterygoid process of the alisphenoid.
    8. jugal.
    9. premaxillae.
    10. angle of the mandible.
    11. maxillae.]

The cranial cavity is decidedly small, the reduction being specially
noticeable in the cerebral fossa, which is not much larger than the
cerebellar fossa. The foramen magnum is large, and the dorsal surface
of the cranium narrow. The zygomatic arch is very strongly developed,
the squamosal (fig. 92, 4) being especially prominent, and being drawn
out not only into the zygomatic process, but also into a large
post-tympanic process which articulates with the exoccipital. At the
side of the skull between the squamosal, supra-occipital and
exoccipital, there is a wide vacuity in the cranial wall, partially
filled up by the very large periotic, which is ankylosed to the
tympanic, but is not united to any other bones of the skull. The
foramen lacerum medium is confluent with the foramen lacerum anterius,
and the two together form an enormous vacuity on the floor of the
skull, bounded chiefly by the exoccipital, basi-occipital, alisphenoid
and squamosal. The jugal (fig. 92, 8) is large and in _Manatus_ sends
up a strong process, which nearly or quite meets the postorbital
process of the frontal, completing the orbit. In the other Sirenia the
orbit is completely confluent with the very large temporal fossa. The
lachrymal in _Manatus_ is very small, but is larger in _Halicore_. The
premaxillae (fig. 92, 9) are large, but smaller in _Manatus_ than in
the other genera, in all of which they are curiously bent down in
front. Their upper margin forms the anterior border of a very large
aperture lying high on the roof of the skull and extending back for a
considerable distance. This aperture is formed by the union of the two
anterior nares. The nasals are quite vestigial or absent, and the
narial aperture is bounded above by the frontals; in its floor are
seen the slender vomer and large mesethmoid. The palate is long and
narrow, and formed mainly by the maxillae; behind it there is a large
irregular process formed by the union of the palatine, pterygoid, and
pterygoid plate of the alisphenoid. The mandible is very massive and
has a very high ascending portion, a rounded angle (fig. 92, 10), and
a prominent coronoid process; the two rami are firmly ankylosed
together. The hyoid consists principally of the broad flat basi-hyal;
the anterior cornua are but slightly ossified, while the thyro-hyals
are not ossified at all.

CETACEA. The skull in all Cetacea, especially in the Odontoceti, is a
good deal modified from the ordinary mammalian type.

In the _ARCHAEOCETI_ this modification is less marked than in either
of the other suborders. The nasals and premaxillae are a good deal
larger than they are in living forms, and the anterior nares are
placed further forward. The maxillae do not extend back over the
frontals, and there is a well-marked sagittal crest.

In the _MYSTACOCETI_ the skull is always quite bilaterally
symmetrical, and is not so much modified from the ordinary mammalian
type as in the Odontoceti. The parietals are not, as in the
Odontoceti, separated by a wide interparietal, but meet; they are,
however, hidden under the very large supra-occipital. The nasals are
developed to a certain extent, and the nares, though placed very far
back and near the top of the head, terminate forwardly-directed narial
passages. Turbinal bones are also developed to some extent; this fact,
and the occurrence of a definite though small olfactory fossa
constituting important distinctions from the Odontoceti. The maxillae
are large, but do not extend back to cover the frontals as in the
Odontoceti. The zygomatic process of the squamosal is very large. The
mandibular rami are not compressed, but are rounded and arched
outwards, and never meet in a long symphysis.

_ODONTOCETI._ The skull departs widely from the ordinary mammalian
type. The following description will apply to any of the following
genera of the Delphinidae, _Phocaena_, _Globicephalus_,
_Lagenorhynchus_, _Delphinus_, _Tursiops_, _Prodelphinus_, _Sotalia_.

The upper surface of the skull is more or less asymmetrical. The
cerebral cavity is high, short and broad; and formed mainly by the
cerebral fossa, the olfactory fossa being entirely absent. The
supra-occipital (fig. 93, 3) is very large, and forms much of the
posterior part of the roof of the skull. It has the interparietal
(fig. 93, 7) fused with it, and completely separates the two
parietals. The frontal (fig. 93, 10) is large and laterally expanded,
forming the roof of the orbit, but is almost completely covered by an
extension of the maxillae. The zygomatic arch is very slender, and is
mainly formed by a rod-like process from the jugal (fig. 93, 15), the
zygomatic process of the squamosal being short and stout.

The nasal passages are peculiarly modified, instead of passing
horizontally forwards above the roof of the mouth, they pass upwards
and even somewhat backwards towards the top of the skull (fig. 93,
23). They are bounded laterally by two processes from the premaxillae,
the left of which is shorter than the right. The nasal cavities are
narrow and without turbinals and the nasals (fig. 93, 19) are almost
as much reduced as in Sirenia.

[Illustration FIG. 93. A, LATERAL VIEW, AND B, LONGITUDINAL SECTION OF
THE SKULL OF A YOUNG CA'ING WHALE (_Globicephalus melas_) × 1/6.
(Brit. Mus.)

    1. basi-occipital.
    2. exoccipital.
    3. supra-occipital.
    4. basisphenoid.
    5. alisphenoid.
    6. parietal.
    7. interparietal.
    8. presphenoid.
    9. orbitosphenoid.
    10. frontal.
    11. mesethmoid.
    12. tympanic.
    13. periotic.
    14. squamosal.
    15. jugal.
    16. vomer.
    17. palatine.
    18. pterygoid.
    19. nasal.
    20. maxillae.
    21. premaxillae.
    22. mandible.
    23. anterior nares.]

In front of the nasal openings the face is prolonged as a narrow beak
or rostrum of varying length, formed by the maxillae and premaxillae
surrounding the vomer and large mesethmoid (fig. 93, 11), which sends
forwards a long partially cartilaginous process, and is fused behind
with the presphenoid (fig. 93, 8). The basi-occipital (fig. 93, 1) too
is fused with the basisphenoid. The foramen rotundum is confluent with
the sphenoidal fissure, and the foramen ovale with the foramen lacerum
medium and the foramen lacerum posterius. The palate is mainly formed
by the maxillae; the premaxillae and palatines (fig. 93, 17), though
both meet in symphyses, forming very little of it. The pterygoids vary
in size in the different genera, sometimes as in _Lagenorhynchus_ and
_Delphinus_ meeting in the middle line, sometimes as in _Phocaena_ and
_Globicephalus_ (fig. 93, 18) being widely separated. The tympanic and
periotic are not fused together, and the periotic has generally no
bony union with the rest of the skull. The mandible is rather slightly
developed, with the rami straight, compressed and tapering to the
anterior end. The condyle is not raised at all above the edge of the
ramus; the angle is rounded and the coronoid process is very small.
_Platanista_ has a curiously modified skull; the rostrum and mandible
are exceedingly long and narrow, and arising from the maxillae are two
great plates of bone which nearly meet above.

In the Physeteridae the skull is raised into a very prominent crest at
the vertex behind the nares. In front of this in _Hyperoödon_ a pair
of ridges occur, formed by outgrowths from the maxillae. In the old
male these ridges reach an enormous size and almost meet in the middle
line. In _Physeter_, the Sperm whale, these ridges are not developed;
the maxillae and premaxillae unite with the other bones of the crest
enclosing an enormous half basin-shaped cavity, at the base of which
are the very asymmetrical anterior narial apertures.

In all living Cetacea the hyoid has the same general shape, consisting
firstly of a crescentic bone formed by the fusion of the thyro-hyals
with the basi-hyal, and secondly of the anterior cornu formed
principally by the strong stylo-hyal.

UNGULATA. None of the distinctive characters separating the Ungulata
from the other groups of mammals are drawn from the skull. But in the
Ungulata vera as opposed to the Subungulata a distinguishing feature
is found in the fact that the lachrymal and jugal form a considerable
part of the side of the face, and that the jugal always forms the
anterior part of the zygomatic arch, the maxillae taking no part in it.


UNGULATA VERA.

_ARTIODACTYLA._ The skull in Artiodactyla differs from that in
Perissodactyla in the fact that the posterior end of the nasal is not
expanded and there is no alisphenoid canal.

The skulls in the different groups of Artiodactyla differ considerably
from one another.

[Illustration FIG. 94. A, CRANIUM AND B, MANDIBLE OF A PIG (_Sus
scrofa_) × 1/5. (Camb. Mus.)

    1. jugal.
    2. postorbital process of the frontal.
    3. zygomatic process of the squamosal.
    4. supra-occipital.
    5. glenoid cavity.
    6. occipital condyle.
    7. foramen magnum.
    8. paroccipital process of the exoccipital.
    9. tympanic bulla.
    10. pterygoid.
    11. anterior palatine foramen.
    12. palatal plate of maxillae.
    13. coronoid process.
    14. mandibular condyle.
    _i_ 1, _i_ 2, _i_ 3. first, second, and third incisors.
    _c._ canine.
    _pm_ 1, _pm_ 2, _pm_ 3, _pm_ 4. first, second, third, and fourth
      premolars.
    _m_ 1, _m_ 2, _m_ 3. first, second, and third molars.]

The skull of the Pig[166] will be described as illustrative of the
skull in the Suina. In the Pig as in most Artiodactyla the face is
bent sharply down on the basicranial axis, the commencement of the
vomer being situated below the mesethmoid instead of in front of it as
in most skulls. The occipital region of the skull is small, and the
line of junction of the supra-occipital and parietals is raised into a
prominent occipital crest. The parietal completely fuses at an early
stage with its fellow, and the exoccipital is drawn out into a long
paroccipital process (fig. 94, A, 8). The frontal is large and broad
and drawn out into a small postorbital process. The lachrymal too is
large and takes a considerable part in forming the side of the face in
front of the orbit, as does also the jugal, though to a less extent.
The face is long and tapers much anteriorly. The nasals are long and
narrow, as are the nasal processes of the premaxillae, which do not
however reach the frontals. A prenasal ossicle is developed in front
of the mesethmoid. The palate is long and narrow, the pterygoid (fig.
94, A, 10) is small, but the pterygoid process of the alisphenoid is
prominent. The squamosal is small and has the tympanic fused with it;
the tympanic is dilated below, forming a bulla (fig. 94, A, 9) filled
with cancellous bone, and above forms the floor of a long
upwardly-directed auditory meatus. The mandible has a high ascending
portion and a small coronoid process (fig. 94, B, 13). The hyoid
differs from that of most Ungulates, the stylo-hyal being very
imperfectly ossified.

[Illustration FIG. 95. MANDIBLE OF A HIPPOPOTAMUS (_H. amphibius_) ×
1/7. (Camb. Mus.)

The second incisor of the left side is missing and the crowns of the
grinding teeth are much worn.

    1. condyle.
    2. coronoid process.
    3. mental foramina.
    _i_ 1, _i_ 2. first and second incisors.
    _c._ canine.
    _pm_ 3. third premolar.
    _m_ 1, _m_ 3. first and third molar.]

In _Hippopotamus_ the skull though essentially like that of the pig is
much modified in detail. The brain cavity is very small, while the
jaws are immensely developed. The face contracts in front of the
orbits and then expands again greatly, to lodge the enormous incisor
and canine teeth. The postorbital bar is complete or nearly so, and
the orbits project curiously outwards and slightly upwards; the
lachrymal is thin and much dilated. The squamosal is drawn out into a
postglenoid process, and the hamular process of the pterygoid is
prominent. The tympanic bulla is filled with cancellous bone. The
mandible is enormously large, the symphysis is long, the angle much
expanded and drawn out into a process which projects outwards and
forwards.

Among extinct forms related to the Suina, _Cyclopidius_ is noticeable
for having large vacuities in the lachrymo-nasal region, while
_Cotylops_ has the postorbital bar complete; both these forms are from
the North American Miocene.

In the Tylopoda and Tragulina the skull resembles in most respects
that of the Ruminants, shortly to be described; but it is allied to
that of the Suina in having the tympanic bulla filled with cancellous
bone. The tympanic bulla is better developed in the Tragulina than in
most Ungulates.

Among Ruminants, the Bovidae, that large group including the Oxen,
Sheep, and Antelopes, as a rule have the face bent on the basicranial
axis much as in the Suina. The parietals are generally small and early
coalesce, the frontals are large and are usually drawn out into horn
cores, which are however absent in the skulls of some domestic
varieties of sheep and oxen, and also in some of the earlier extinct
forms of Bovidae. These horn cores are formed internally of cancellous
bone, and on them the true epidermal horns are borne. In young animals
there is a distinct interparietal, but this early fuses with the
supra-occipital, and in the oxen also with the parietals. The
occipital crest is generally well marked, but in the genus _Bos_
becomes merged in a very prominent straight ridge running between the
two horn cores; this ridge, which contains air cells communicating
with those in the horn cores, is not nearly so well marked in _Bison_.
There is often, as in _Gazella_, a vacuity on the side of the face
between the nasal, frontal, lachrymal, and maxillae, but this is not
found in oxen or sheep. The premaxillae are small, the nasals are long
and pointed, and the turbinals are much developed. The Saiga antelope
has a curiously specialised skull; the nasals are absent or have
coalesced with the frontals and the anterior nares are enormously
large. In all Ruminants the lachrymal is large and forms a
considerable part of the side of the face; it often bears a
considerable depression, the _suborbital_ or _lachrymal fossa_, well
seen in most of the smaller antelopes. The postorbital bar is
complete, and the orbit is prominent and nearly circular. The
palatines and pterygoids are moderately large, and the pterygoids have
a backwardly-projecting hamular process. The squamosal is small, but
has a postglenoid process. The tympanic is not fused to the periotic
and has a small bulla not filled with cancellous bone. There is a
large paroccipital process to the exoccipital and the mandible has a
long slender coronoid process.

In the Cervidae and Giraffidae the face is not bent down on the
basicranial axis as it is in the Bovidae. The frontals are drawn out,
not into permanent horn cores as in the Bovidae, but into short
outgrowths, the pedicels, upon which in the Cervidae long antlers are
annually developed. These _antlers_ are outgrowths of bone, and are
covered during development by vascular integument, which dries up and
peels off when growth is complete. Every year they are detached, by a
process of absorption at the base, and shed. They may occur in both
sexes, as in the Reindeer, but as a rule they are found only in the
male. They are generally more or less branched, and are sometimes of
enormous size and weight, as in the extinct _Cervus megaceros_. In
young animals they are always simple, but become annually more and
more complicated as the animal grows older.

In the Giraffe the frontals bear a small pair of bony cores, which are
at first distinct, but subsequently become fused to the skull. In the
allied _Sivatherium_, a very large form from the Indian Pliocene, the
skull bears two pairs of bony outgrowths, a pair of short conical
outgrowths above the orbits, and a pair of large expanded outgrowths
on the occiput.

The opening of the lachrymal canal is commonly double and the
lachrymal fossa is large in the Cervidae and the Giraffidae except
_Sivatherium_. The vacuity between the frontal, lachrymal, maxillae,
and nasal is specially large.

The hyoid of Ruminants is noticeable for the development of the
anterior cornua, which include stout and short cerato-hyals and
epi-hyals, long and strong stylo-hyals and large tympano-hyals which
are more or less imbedded in the tympanics.

_PERISSODACTYLA._ In the skull of Perissodactyles an alisphenoid canal
is found and the nasals are expanded behind. Among the living animals
belonging to this group the skull least modified from the ordinary
type is that in _Rhinoceros_. In this form the skull is considerably
elongated, the facial portion being very large. The occipital region
is elevated, but the cranial cavity is small, the boundary line
between the occipital and parietal regions being drawn out into a
prominent crest, which is occupied by air cells. There is no
postorbital process to the frontal, and the orbit is completely
confluent with the temporal fossa. The nasals are fused together and
are very strongly developed, extending far forwards, sometimes
considerably beyond the premaxillae. In some extinct species, such as
_Elasmotherium_ and the Tichorhine Rhinoceros, _R. antiquitatis_, the
mesethmoid is ossified as far forwards as the end of the nasals. The
nasals are arched and bear one or two roughened surfaces to which the
great nasal horns are attached. The premaxillae are very small and the
pterygoids are slender. The palate is long, narrow, and deeply
excavated behind. The postglenoid process of the squamosal is well
developed, and generally longer than the paroccipital process of the
exoccipital. The tympanic and periotic are both small and are fused
together. The condyle of the mandible is very wide, the angle rounded,
and the coronoid process moderately developed.

In the Titanotheriidae, a family of extinct Perissodactyla from the
Miocene of North America, the occipital region is much elevated, as is
also the fronto-nasal region, the nasals (perhaps only in the male)
bearing a pair of blunt bony outgrowths. Between these two elevated
regions the skull is much depressed. The cranial cavity is very small,
the orbit confluent with the temporal fossa, and the zygomatic arch
massive.

In _Tapirus_ the orbit and temporal fossa are confluent. The nasals
are small, wide behind and pointed in front, and are supported by the
mesethmoid; the anterior nares are exceedingly large and their lateral
boundaries are entirely formed by the maxillae. The postglenoid and
post-tympanic processes of the squamosal are large. The periotic is
not fused to the squamosal or to the small tympanic. The mandible is
large and has the angle much developed and somewhat inflected.

_Palaeotherium_, which lived in early Tertiary times, has a skull much
like that of the Tapir, especially as regards the nasal bones.

In the Horse and its allies (Equidae) the facial portion of the skull
is very large as compared with the cranial portion, the nasals and
nasal cavities being specially large. In the living species of the
genus _Equus_ there is no fossa between the maxillae and lachrymal, but
it occurs in some extinct species. The lachrymal and jugal form a
considerable part of the side of the face; and the orbit though small
is complete and prominent. The postorbital bar is formed by a strong
outgrowth from the frontal, which unites with a forward extension of
the squamosal. The squamosal may extend forwards and form part of the
wall of the orbit, a very unusual feature, as in most mammals the
squamosal stops before the postorbital bar. The palate is narrow and
excavated behind as in _Rhinoceros_; the palatines take very little
part in its formation. The glenoid surface for the articulation of the
mandible is very wide. The squamosal gives rise to small postglenoid
and post-tympanic processes, and the exoccipital to a large
paroccipital process. The tympanic and periotic are ankylosed
together, but not to any other bones.

In the SUBUNGULATA, the lachrymal and jugal do not form any
considerable part of the side of the face, and the maxillae commonly
takes part in the formation of the zygomatic arch.

_TOXODONTIA._ The skull in the Toxodontia shows several Artiodactyloid
features, while the manus and pes are of a more Perissodactyloid type.
The Artiodactyloid features are (1) the absence of an alisphenoid
canal, (2) the fact that the palate is not excavated behind, and that
the palatines form a considerable part of it, and (3) the fusion of
the tympanic to the squamosal and exoccipital, forming the floor of an
upwardly directed auditory meatus. The frontal has a fairly well
developed postorbital process, but the orbit is confluent with the
temporal fossa. The premaxillae is well developed, as is the
paroccipital process of the exoccipital, especially in _Typotherium_.
The mandible has a rounded angle and a coronoid process of moderate
size. In _Typotherium_ the ascending portion is very massive.

_CONDYLARTHRA._ As far as is known the skull of these generalised
Ungulates is depressed, and is frequently marked by a strong sagittal
crest. The cranial cavity is small, the cerebral fossa in _Phenacodus_
being exceptionally small. The orbit is completely confluent with the
temporal fossa.

_HYRACOIDEA._ The skull of _Procavia_ resembles that of
Perissodactyles more than that of any other Ungulates, but differs
strongly in the comparatively small size of its facial portion. The
posterior portion of the cranium is rather high, the occipital plane
being nearly vertical. There is a small interparietal. The nasals are
wide behind, and the zygomatic arch is strongly developed, its most
anterior part being formed by the maxillae. The jugal and parietal give
rise to postorbital processes which sometimes meet, but as a rule the
orbit is confluent with the temporal fossa; it is very uncommon for
the parietal to give rise to a postorbital process, and even in
_Procavia_ the frontal often forms part of the process. The
alisphenoid canal, and postglenoid and paroccipital processes are well
developed. The tympanic bulla is large and the periotic and tympanic
are fused together, but not as a rule to the squamosal. The ascending
portion of the mandible is very high and broad, the angle rounded and
the coronoid process moderate in size. The hyoid is singular, there is
a large flat basi-hyal prolonged laterally into two broad flattened
thyro-hyals. Articulating with its anterior end are two large
triangular cerato-hyals, which are drawn out into two processes
meeting in the middle line.

_AMBLYPODA._ In the Uintatheriidae (Dinocerata) the skull has a very
remarkable character, being long and narrow and drawn out into three
pairs of rounded protuberances, a small pair on the nasals, a larger
pair on the maxillae in front of the orbits, and the largest pair on
the parietals. The cranial cavity, and especially the cerebral fossa,
is extraordinarily small. The orbit is not divided behind from the
temporal fossa. The mandible has a prominent angle, and a long curved
coronoid process; its symphysial portion bears a curious flattened
outgrowth to protect the great upper canines.

In _Coryphodon_ the skull is of a more normal character, being without
the conspicuous protuberances. The cranial cavity though very small is
not so small as in _Uintatherium_.

[Illustration FIG. 96. SKULL OF A YOUNG INDIAN ELEPHANT (_Elephas
indicus_), SEEN FROM THE RIGHT SIDE, THE ROOTS OF THE TEETH HAVE BEEN
EXPOSED. × 1/8. (Camb. Mus.)

    1. exoccipital.
    2. parietal.
    3. frontal.
    4. squamosal.
    5. jugal.
    6. premaxillae.
    7. maxillae.
    9. supra-occipital.
    13. basi-occipital.
    14. postorbital process of the frontal.
    15. lachrymal.
    16. pterygoid process of the alisphenoid.
    _i_ 1. incisor.
    _mm_ 3., _mm_ 4. third and fourth milk molars.
    _m_ 1. first molar.]

_PROBOSCIDEA._ The character of the skull in the young elephant
differs much from that in the old animal. In very young individuals
the skull is of a normal character, and the cranial cavity is
distinctly large in proportion to the bulk of the skull. But as the
animal gets older, while its brain does not grow much, the size of its
trunk and especially of its tusks increases greatly; and consequently
the skull wall is required to be of very great superficial extent in
order to afford space for the attachment of the muscles necessary for
the support of these heavy weights. This increase in superficial
extent is brought about without much increase in weight of bone by the
development of an enormous number of air cells in nearly all the bones
of the skull; sometimes, as in the case of the frontal, separating the
inner wall of the bone from the outer, by as much as a foot. This
development of air cells is accompanied by the obliteration of the
sutures between the various bones. The most noticeable point with
regard to the cranial cavity is the comparatively large size of the
olfactory fossa. The supra-occipital (figs. 96 and 97, 9) is
large--exceedingly large in the adult skull; the parietals (figs. 96
and 97, 2) are also very large. The frontals send out small
postorbital processes, but these do not meet processes from the small
jugal, which forms only the middle part of the slender zygomatic
arch, the anterior part being formed by the maxillae. The lachrymal
(fig. 96, 15) is small and lies almost entirely inside the orbit. The
anterior narial aperture (fig. 97, 8) is wide and directed upwards,
opening high on the anterior surface of the skull. It is bounded above
by the short thick nasals and below by the premaxillae. The narial
passage is freely open, maxillo-turbinals not being developed. The
palatine is well developed, the pterygoid is small and early fuses
with the pterygoid process of the alisphenoid. The tympanic is united
with the periotic but not with the squamosal, and forms a large
auditory bulla. There are no paroccipital or postglenoid processes.
The exoccipital is not perforated by the condylar foramen,--a very
exceptional condition.

[Illustration FIG. 97. LONGITUDINAL SECTION TAKEN RATHER TO THE RIGHT
OF THE MIDDLE LINE OF THE SKULL OF A YOUNG INDIAN ELEPHANT (_E.
Indicus_) × 1/8. (Camb. Mus.)

    8. anterior nares.
    10. periotic.
    11. palatine.
    12. pterygoid.
    17. nasal.

Other numbers as in Fig. 96.]

The mandible has a high ascending portion, is rounded off below and
has no angle. The symphysial portion is long, narrow, and spout-like,
and the coronoid process is small. The thyro-hyals are ankylosed with
the basi-hyal, which is connected with the large forked stylo-hyals by
ligament only.

RODENTIA. The cranial cavity is depressed, elongated, and rather
small, and the cerebral fossa lies entirely in front of the cerebellar
fossa. The occipital plane is vertical or directed somewhat backwards,
and the supra-occipital does not form much of the roof of the cranium.
The paroccipital processes of the exoccipitals are generally of
moderate size; in the Capybara (_Hydrochaerus_), however, they are
very long, and are laterally compressed and directed forwards. The
parietals are small, and often become completely fused together; there
is sometimes a small interparietal. The frontals in most genera have
no trace of a postorbital process; in Squirrels, Marmots and Hares,
however, one occurs, but in no case does it meet a corresponding
process from the zygomatic arch, so the orbit and temporal fossa are
completely confluent. In Hares the postorbital process of the frontal
is much flattened, and has an irregular margin. The temporal fossa is
always small, and in _Lophiomys_ is arched over by plates arising
respectively from the parietal and jugal; a secondary roof is thus
partially developed in a manner unique among mammals, but carried to a
great extent in many Chelonia. The nasal bones and cavities are large,
attaining their maximum development in the Porcupines (fig. 98, 1).
The premaxillae is always very large, and sends back a long process
which meets the frontal. The vomer is occasionally found persisting in
two separate halves, a feature recalling the arrangement in
Sauropsids. In many Rodents there is an enormous vacuity at the base
of the maxillary portion of the zygomatic arch. It is sometimes as
large as the orbit, and attains its maximum development in the
Capybara and other Hystricomorpha; in the Marmots, Beavers, and
Squirrels (Sciuromorpha), and in the Hares it is undeveloped. In
_Lagostomus_ the maxillae bears an upwardly directed plate of bone,
shutting off from this vacuity a space which is the true infra-orbital
foramen.

[Illustration FIG. 98. HALF FRONT VIEW OF THE SKULL OF A PORCUPINE
(_Hystrix cristata_) × 1/2. (Camb. Mus.)

    1. nasal.
    2. maxillo-turbinals.
    3. infra-orbital vacuity.
    4. maxillae.
    5. premaxillae.
    6. jugal.
    _i_ 1. upper incisor.]

The zygomatic arch is always complete, and in many cases the jugal
extends back to form part at least of the glenoid surface for
articulation with the mandible. In _Coelogenys_ the jugal and
maxillary portion of the zygomatic arch is greatly expanded and
roughened, and the maxillary portion encloses a large cavity. The
palate in Rodents is narrow, and the space between the incisor and
molar teeth passes imperceptibly into the sides of the face. The
anterior palatine foramina form long, rather narrow slits in this
region. The bony palate between the grinding teeth is sometimes as in
the Hares very short, sometimes as in the Capybara very long. The
maxillae extends back beneath the orbit to unite with the squamosal.
The pterygoid is always small, but sometimes has a well-marked hamular
process which in _Hystrix_, _Lagostomus_, and some other genera unites
with the tympanic bulla. The periotic is large, and fused with the
tympanic, which forms a prominent bulla, and is generally drawn out
into a tubular meatus. The bulla attains its maximum development in
_Chinchilla_ and _Dipus_.

The mandible is narrow and rounded in front, the two halves meeting in
a long symphysis. The angle is generally drawn out into a long
backwardly-projecting process, which is often pointed and directed
upwards. In the Hares the angle is rounded. The coronoid process is
never large.

There are a number of points in which the skull of the Duplicidentata
(Hares and Rabbits) differs from that of other Rodents. (_a_) The
sutures between the basi-occipital and basisphenoid, and between the
basisphenoid and presphenoid remain open throughout life. (_b_) Much
of the maxillae forming the side of the face in front of the orbit is
fenestrated. (_c_) The optic foramina are united to form a single
hole, much as in birds. (_d_) The coronoid process is slightly
differentiated from the ascending portion of the mandible. The first
two of these points have been thought to indicate degradation of the
hares and rabbits as compared with higher mammals.

CARNIVORA[167]. It is characteristic of the skull in Carnivora that
the glenoid fossa is deep, and the postglenoid process (fig. 75, 23)
well developed. The condyle of the mandible is much elongated
transversely. The orbit and temporal fossa in the great majority of
forms communicate freely, the postorbital bar being incomplete.

_CARNIVORA VERA._ The axis of the facial portion of the skull is a
direct continuation of that of the cranial portion. The cranial
cavity though rather depressed is large, and generally long, though in
Cats it is comparatively short and wide. The occipital plane is nearly
vertical, and the exoccipitals are developed into fairly prominent
paroccipital processes. The interparietal is commonly distinct, and
the parietals unite in a long sagittal suture, which is often
developed into a crest. The nasals (fig. 73, 4) are well developed,
especially in Cats, and the nasal processes of the premaxillae do not
nearly reach the frontals. A considerable part of the palate is formed
by the palatine, and the maxillary portion is pierced by rather long
anterior palatine foramina. The pterygoid has a hamular process. The
zygomatic arch is strong, especially in Cats. Postorbital processes
are developed on the frontal (fig. 73, 10) and jugal, but never form a
complete postorbital bar. A carotid canal is well seen in the Ursidae,
and to a less extent in the Felidae; in the Canidae there is an
alisphenoid canal (fig. 75, 21).

The auditory bulla differs a good deal in the different groups. In the
Bears (Ursidae) it is not much inflated, and is most prominent along
its inner border; it is not closely connected with the paroccipital
process. In the Cats it is very prominent, and its cavity is almost
divided by a septum into two parts, the inner of which contains the
auditory ossicles. The paroccipital process is closely applied to the
bulla. In the Dogs the bulla is intermediate in character between that
of the Cats and that of the Bears; it is partially divided by a
septum, and is moderately expanded.

The mandible is well developed with a prominent angle (fig. 72, 26),
and a large coronoid process. The hyoid consists of a broad basi-hyal,
a long many-jointed anterior cornu and short thyro-hyals (fig. 72,
33).

The skull in the _CREODONTA_ is in most respects allied to that of the
Canidae, but presents some ursine affinities. The tympanic bulla is
fairly prominent, but has no well-developed septum. The cranial cavity
is very small and narrow, the zygomatic arch standing away from it.
The temporal fossa is of great size.

In the _PINNIPEDIA_ the cranial cavity is large and rounded. The skull
is much compressed in the interorbital region, and in correlation with
this compression the ethmo-turbinals are little developed, while the
maxillo-turbinals are large. The orbit is large, and the temporal
fossa smaller than in the Carnivora vera. In the Walrus (_Trichechus_)
the anterior part of the face is distorted by the development of the
huge canines. The Otariidae have an alisphenoid canal. The tympanic
bulla is small in _Otaria_, large in the Phocidae, and flattened in
the Walrus. The hyoid is similar to that in Carnivora vera.

INSECTIVORA. The skull varies much in the different members of the
order Insectivora, but the following points of agreement are found.
The cranial cavity is of small size, and is never much elevated. The
facial part of the skull is generally considerably elongated, and the
nasals and premaxillae are well developed. The zygomatic arch is
usually slender or incomplete, and the coronoid process and angle of
the mandible are commonly prominent.

In some Insectivora, such as _Galeopithecus_, _Tupaia_, and
_Macroscelides_, the skull shows a higher type of structure than is
met with in most members of the order. In these genera the cranial
cavity is comparatively large, and the occipital plane is nearly
vertical. The zygomatic arch is fairly strong, and the frontal and
jugal give rise to postorbital processes which nearly or quite
(_Tupaia_) meet. The tympanic bulla is well developed, and produced
into a tubular auditory meatus, this being specially well marked in
_Macroscelides_.

In the other Insectivora the cranial cavity is of smaller comparative
size, and the orbit and temporal fossa are completely confluent, often
without any trace of a postorbital bar. The occipital plane commonly
slopes forwards. In the Hedgehogs (Erinaceidae) and Centetidae the
tympanic is very slightly developed, forming a small ring. The
zygomatic arch of Hedgehogs and _Gymnura_ is very slender, the jugal
being but little developed and the squamosal and maxillae meeting one
another; in the Centetidae the jugal is absent and the arch is
incomplete.

The Moles (Talpidae) have an elongated, depressed and rounded skull
with a very slender zygomatic arch formed by the squamosal and
maxillae. The nasals are fused together, and the mesethmoid is ossified
very far forwards. In the Shrews (Soricidae) there is no zygomatic
arch; the tympanic is ring-like, and the angle of the mandible is very
prominent. The hyoid has a transversely extended basi-hyal, a long
anterior cornu with three ossifications, and thyro-hyals which are
sometimes fused to the basi-hyal.

CHIROPTERA. In the frugivorous Flying Foxes (Pteropidae) the skull is
elongated, and the cranial cavity is large and arched, though
considerably contracted in front. There are commonly strong sagittal
and supra-orbital crests. The parietals take a great part in the
formation of the walls of the cranial cavity, the supra-occipital and
frontals being small. The frontal is drawn out into a long postorbital
process, but the zygomatic arch, which is slender, and formed mainly
by the squamosal and maxillae, gives rise to only a small postorbital
process, so that the orbit and temporal fossa are confluent. There is
no alisphenoid canal, and the tympanics are very slightly connected
with the rest of the skull. The mandible has a large coronoid process,
a rounded angle, and a transversely expanded condyle.

In Insectivorous Bats the skull is generally shorter and broader than
in the Pteropidae. The cranial cavity is large and rounded, and has
thin smooth walls. The zygomatic arch is slender, and postorbital
processes are not generally well developed. The premaxillae is
generally small, sometimes absent. The tympanics are ring-like and are
not connected with the surrounding bones. The angle of the mandible is
distinct. The hyoid in most respects resembles that of the
Insectivora.

PRIMATES. The characters of the skull differ greatly in the two
suborders of Primates, the Anthropoidea and the Lemuroidea.

In the _LEMUROIDEA_ the general relative proportions of the cranium
and face are much as in most lower mammals, and the occipital plane
forms nearly a right angle with the basicranial axis. The postorbital
processes of the frontals are commonly continued as a pair of ridges
crossing the roof of the cranium and meeting the occipital crest.
Though the postorbital bar is complete, the orbit and temporal fossa
communicate freely below it. The lachrymal canal opens outside the
orbit, and the lachrymal forms a considerable part of the side of the
face. The tympanic is developed into a large bulla. The hyoid
apparatus much resembles that of the Dog.

In the _ANTHROPOIDEA_ the skull differs greatly from that in the
Lemuroidea. The cranial portion of the skull is very large as compared
with the facial portion, though the comparative development varies,
some monkeys, such as the baboons (Cynocephali) having the facial
portion relatively large. The comparative size of the jaws does not
vary inversely with the general development of the animal, some of the
Cercopithecidae having comparatively larger jaws than some of the
Cebidae. The great size of the cranial part of the skull is mainly due
to the immense development of the cerebral fossa, which commonly
completely overlaps the olfactory fossa in front, and the cerebellar
fossa behind. This development also has the effect of making the
ethmoidal and occipital planes lie, not at right angles to the
basicranial axis, but almost in the same straight line with it. This
is, however, not always the case, as the Howling Monkey (_Mycetes_)
and also some of the very highest monkeys, the Gibbons (_Hylobates_),
have the occipital plane nearly vertical to the basicranial axis. In
adult Man the basi-occipital, exoccipitals and supra-occipital
coalesce, forming the so-called occipital bone; while the
basisphenoid, presphenoid, alisphenoids, orbitosphenoids and
pterygoids form the sphenoid bone. The roof of the skull is partly
formed by the large supra-occipital and frontals, but mainly by the
parietals (fig. 99, 1), which in Man are of enormous extent.

[Illustration FIG. 99. HALF FRONT VIEW OF THE SKULLS, _A_ OF AN OLD,
_B_ OF A YOUNG GORILLA (_Gorilla savagei_) × 1/4. (Camb. Mus.)

    1. parietal.
    2. sagittal crest.
    3. frontal.
    4. supra-orbital ridge.
    5. squamosal.
    6. maxillae.
    7. external auditory meatus.]

In Man and in most monkeys, at any rate when young (fig. 99, B), the
roof of the skull is smooth and rounded, but in many forms, such as
the Baboons, in the adult the supra-orbital and occipital ridges are
much developed. In the Gorilla this is also the case with the sagittal
crest (fig. 99, A, 2). The bones of the upper surface of the cranium
interlock with wavy outlines. The nasals vary much in length, being
much shorter in man than in most monkeys; they commonly become early
fused together, as do also the frontals. The vomer is well developed,
and the ethmo-turbinal always forms part of the boundary of the orbit.
There are frequently, as in many Lemuroidea, a pair of more or less
well-marked ridges, crossing the roof of the skull from the
postorbital processes of the frontals to the occipital crest. The
orbit is completely encircled by bone, and the alisphenoid assists the
jugal and frontal in shutting it off from the temporal fossa, leaving
however a communication between the two as the sphenomaxillary
fissure. In most cases the frontals meet one another in the middle
line between the mesethmoid and orbitosphenoid, but in Man, Simia, and
some Cebidae this does not take place. In nearly all Cebidae the
parietal and jugal meet one another, separating the frontal and
alisphenoid on the skull wall; in Man and all Old World monkeys, on
the other hand, the alisphenoid and frontal meet and separate the
jugal and parietal. The premaxillae nearly always send back processes
which meet the nasals. The palate is rather short and both the
palatine and the premaxillae take a considerable part in its formation.
The pterygoid plate of the alisphenoid is decidedly large, and there
is no alisphenoid canal. There is never any great development either
of the paroccipital process of the exoccipital, or of the postglenoid
process of the squamosal. The periotic and tympanic are always fused
together; in Cebidae they form a small bulla, but a bulla is not
developed in any Old World forms. The periotic is large, especially
the mastoid portion, which forms a distinct portion of the skull wall
between the squamosal and exoccipital. In Man and still more in Old
World monkeys, the external auditory meatus is drawn out into a
definite tube, whose lower wall is formed by the tympanic; in the
Cebidae the tympanic is ring-like. The perforation of the periotic by
the carotid canal is always conspicuous.

The mandible is rather short and broad, and the angle formed by the
meeting of the two rami is more obtuse than in most mammals. The
coronoid process is fairly well developed, and the angle is more or
less rounded. In most Primates the condyle is considerably widened,
but this is not the case in Man. In _Mycetes_ the mandible is very
large, its ascending portions being specially developed. The hyoid of
Primates is remarkable for the large expanded basi-hyal, which is
generally concave above and convex below. The anterior cornu is never
well ossified, but the thyro-hyal is always strong. In _Mycetes_ the
basi-hyal is enormously large, forming a somewhat globular thin-walled
capsule.


AUDITORY OSSICLES.

[Illustration FIG. 100. MALLEUS, STAPES AND INCUS OF _A._ MAN. _B._
DOG. _C._ RABBIT. (After DORAN) x 1.

    1. head of malleus.
    2. canal of stapes.
    3. incus.
    4. processus longus (or gracilis).
    5. manubrium of malleus.
    6. processus brevis.
    7. lamella.]

There are in mammals four auditory ossicles forming a chain extending
from the fenestra ovalis to the tympanic membrane. Three of these, the
=malleus=, =incus= and =stapes=, are always distinct, while the
fourth, the =lenticular=, is smaller than the others and is sometimes
not distinct. The names are derived from human anatomy and indicate in
the case of the first three a more or less fanciful resemblance
respectively to a hammer, an anvil and a stirrup. The ossicles are
homologous as a whole to the hyomandibular of fishes and to the
columellar chain of Sauropsids and Amphibians. The malleus is
homologous to the extra-columella of Crocodiles and the stapes to the
columella. The =malleus= when typically developed consists of a
rounded _head_ (fig. 100, 1) which bears a surface articulating with
the incus, and a short _neck_ continued into a process, the
_manubrium_ (fig. 100, 5), which comes into relation with the tympanic
membrane. From the junction of the neck and manubrium two processes
are given off, a _processus longus_ or _gracilis_ (fig. 100, 4), which
in the embryo is continuous with Meckel's cartilage, and a _processus
brevis_ (fig. 100, 6). The =incus= generally consists of a more or
less anvil-shaped portion which articulates with the malleus, and of a
process which is connected with the stapes by the small =lenticular=.
The =stapes= is generally stirrup shaped, consisting of a basal
portion from which arise two _crura_ separated by a space the canal
through which a branch of the pharyngeal artery runs The lenticular is
frequently cartilaginous and sometimes is not developed at all.

The above is the arrangement of the auditory ossicles met with in the
higher Mammalia, but in the lower Mammalia the characters approach
more nearly to those met with in Sauropsids.

In MONOTREMES the ossicles, though distinctly mammalian in character,
show a very low type of development. The incus is articulated, or
often fused, with an outgrowth from the head of the malleus. The
stapes is very much like a reptilian columella, having a single crus
with no perforation.

In MARSUPIALS the ossicles are of a low type, but not so low as the
rest of the skeleton might have led one to expect, and all or almost
all the points showing a low grade of development may be paralleled
among the Monodelphia. The lowest Marsupials as regards the ossicles
are the Peramelidae, whose ossicles are of a frail papery consistence.
The Didelphyidae on the other hand have the most highly developed
ossicles, the malleus much resembling that of many Insectivores, and
the stapes having two definite crura separated by a canal.

In EDENTATES the character of the ossicles varies much. In Sloths the
stapes approaches that of Sauropsids in its narrowness and the slight
trace of a canal; this character is however still more marked in
_Manis_, whose stapes is as Sauropsidan as that of Monotremes, and
consists of a nearly circular basal plate bearing a column which does
not show any sign of division into crura. The stapes of other
Edentates, such as ant-eaters, aard varks, and most armadillos, is of
a high type and has well-developed crura. _Priodon_ has a lower type
of stapes than _Dasypus_ and _Tatusia_.

The ossicles of the SIRENIA differ widely from those of all other
mammals in their great density and clumsy form.

In CETACEA the ossicles are solid, though not so solid as in Sirenia,
and their details vary much. The malleus is always firmly fused to the
tympanic by means of the processus longus, and the manubrium is very
little if at all developed. The incus has the stapedial end greatly
developed, and the stapes has very thick crura with hardly any canal.
The ossicles of the Mystacoceti are apparently less specialised than
are those of the Odontoceti.

The auditory ossicles of the UNGULATA do not present any characters
common to all the members of the group.

Among Ruminants they are chiefly remarkable for the development of a
broad lamellar expansion between the head and the processus longus of
the malleus. In some cases the malleus of the foetus differs
strikingly from that of the adult. Among Perissodactyla the Rhinoceros
and Tapir have the malleus of a low type, recalling those of
Marsupials; while in the Horse the head is well developed, and the
malleus is of a higher type.

The ossicles of _Procavia_, which recall those of the Equidae, are
chiefly remarkable for the small size of the body of the incus. In
Elephants the ossicles are large and massive.

In the RODENTIA (fig. 100, C) the malleus is generally characterised
by a very broad manubrium. In many genera such as _Bathyergus_, and
most of the Hystricomorpha such as _Hystrix_, _Chinchilla_ and
_Dasyprocta_, the malleus and incus are ankylosed together.

CARNIVORA. In Carnivora vera the most striking feature of the malleus
is the occurrence of a broad lamellar expansion between the head and
neck and the processus longus. This however does not occur in some
Viverridae. In the Carnivora vera the incus and stapes are small as
compared with the malleus, but in the Pinnipedia they are large. In
the Pinnipedia the auditory ossicles have a very dense consistence,
and except in the Otariidae are very large. The stapes frequently has
no canal, or only a very small one.

In INSECTIVORA the characters of the auditory ossicles are very
diverse. Many forms such as shrews, moles, hedgehogs, and the
Centetidae have a low type of malleus resembling that of Edentates.
_Chrysochloris_ has very extraordinary auditory ossicles. The head of
the malleus is drawn out into a great club-shaped process, the incus
is long and narrow, and differs much from the ordinary type.

In CHIROPTERA the ossicles and especially the malleus much resemble
those of shrews. The stapes is always normal in character, never
becoming at all columelliform.

PRIMATES. In Man and the Anthropoid Apes the malleus has a rounded
head, a short neck, and the manubrium, a processus longus and a
processus brevis. The incus consists of an anvil-shaped portion from
which arises a long tapering process. The stapes has diverging crura
and consequently a wide canal. The crura in other monkeys do not
diverge so much as in man and anthropoid apes. The New World monkeys
have no neck to the malleus.


THE STERNUM[168].

In MONOTREMES and most MARSUPIALS the sternum does not present any
characters of special importance. The presternum is strongly keeled in
_Notoryctes_.

The sternum in EDENTATES is very variable: in the Sloths it is very
long, the mesosternum of _Choloepus_ having twelve segments. In the
ant-eaters and armadillos the presternum is broad and sometimes as in
_Priodon_ strongly keeled. In _Manis macrura_ the xiphisternum is
drawn out into a pair of cartilaginous processes about nine inches
long.

In the SIRENIA the sternum is simple and elongated, and of fairly
equal width throughout, in the adult it shows no sign of segmentation.
Its origin from the union of two lateral portions can be well seen in
_Manatus_.

Two distinct types of sternum are met with in the CETACEA. In the
Odontoceti the sternum consists of a broad presternum followed by
three or four mesosternal segments, but with no xiphisternum.
Indications of the original median fissure can be traced, and are very
evident in _Hyperoödon_. In the Mystacoceti, on the other hand, the
sternum consists simply of a broad flattened presternum which is
sometimes more or less heart-shaped, sometimes cross-shaped. Only a
single pair of ribs are united to it.

The sternum in UNGULATA is generally long and narrow and formed of six
or generally seven segments. The presternum is as a rule small and
compressed, often much keeled, especially in the horse and tapir. The
segments of the mesosternum gradually widen as followed back and the
xiphisternum is often terminated by a cartilaginous plate.

In the RODENTIA the sternum is long and narrow and generally has a
large presternum, and a xiphisternum terminated by a broad
cartilaginous plate.

In the CARNIVORA, too, the sternum (fig. 76) is long and narrow and
formed of eight or nine pieces, all of nearly the same size. The
xiphisternum generally ends in an expanded plate of cartilage.

In INSECTIVORA the sternum is well developed but variable. The
presternum is commonly large and is sometimes as in the Hedgehog
(_Erinaceus_) bilobed in front, sometimes as in the Shrew (_Sorex_)
trilobed. It is especially large in the Mole (_Talpa_) and is expanded
laterally and keeled below.

In the CHIROPTERA the presternum is strongly keeled and so is
sometimes the mesosternum.

Among PRIMATES, in Man and the Anthropoid Apes the sternum is rather
broad and flattened; the mesosternum consists of four segments which
are commonly fused together and the xiphisternum is imperfectly
ossified.


THE RIBS.

Free ribs are borne as a rule only by the thoracic vertebrae; ribs may
be found in other regions, especially the cervical and sacral, but
these are almost always ankylosed to the vertebrae. As a general rule
the first thoracic rib joins the presternum, while the succeeding ones
are attached between the several segments of the mesosternum. Some of
the posterior ribs frequently do not reach the sternum; they may then
be attached by fibrous tissue to the ribs in front, or may end freely
(_floating ribs_). There are generally thirteen pairs of ribs, and in
no case do they have uncinate processes.

In MONOTREMES (fig. 102, B) each rib is divided not into two but into
three parts, an intermediate portion being interposed between the
vertebral and sternal parts. The sternal ribs are well ossified, and
some are very broad and flat. The intermediate portions are
unossified, those of the anterior ribs are short and narrow, but they
become longer and wider further back.

In MARSUPIALS there are almost always thirteen pairs of ribs, whose
sternal portions are very imperfectly ossified. _Notoryctes_ has
fourteen pairs of ribs, eight of which are floating: the first rib is
very stout, and is abruptly bent on itself to join the sternum. It has
no distinct sternal portion. All the other ribs are slender.

Of the EDENTATES the Sloths have very numerous ribs; twenty-four pairs
occur in _Choloepus_, and half of these reach the sternum. In the
Armadillos there are only ten or twelve pairs of ribs, but the sternal
portions are very strongly ossified. The first rib is remarkably broad
and flat, and is not divisible into vertebral and sternal portions.

In the SIRENIA there are a very large number of ribs noticeable for
their great thickness and solidity, but not more than three are
attached to the sternum.

CETACEA. In the Whalebone whales the ribs are remarkable for their
very loose connection both with the vertebral column and with the
sternum. The capitula are scarcely developed, and the attachment of
the tubercula to the transverse processes is loose. The first rib is
the only one connected with the sternum. In the Toothed whales the
anterior ribs have capitula articulating with the centra, as well as
tubercula articulating with the transverse processes; in the posterior
ones, however, only the tubercula remain. Seven pairs of well-ossified
sternal ribs generally meet the sternum. In the Physeteridae most of
the ribs are connected to the vertebrae by both capitula and
tubercula.

In the UNGULATA the ribs are generally broad and flattened, and this
is especially the case in the genera _Bos_ and _Bubalus_ (fig. 101,
6). The anterior ribs are short and nearly straight, and sternal ribs
are well developed. The Artiodactyla have twelve to fifteen pairs of
ribs, the Perissodactyla eighteen or nineteen, and _Procavia_ twenty
to twenty-two. The Elephant has nineteen to twenty-one pairs, seven of
which may be floating ribs.

[Illustration FIG. 101. SKELETON OF A CAPE BUFFALO (_Bubalus caffer_).
The left scapula is omitted for the sake of clearness × 1/17. (Brit.
Mus.)

    1. premaxillae.
    2. nasal.
    3. orbit.
    4. neural spine of first thoracic vertebra.
    5. scapula.
    6. rib.
    7. femur.
    8. patella.
    9. tibia.
    10. metatarsals.
    11. radius.
    12. metacarpals.]

In the RODENTIA there are generally thirteen pairs of ribs, which do
not present any marked peculiarities.

The CARNIVORA have thirteen to fifteen pairs of ribs, whose vertebral
portions are slender, nearly straight and subcylindrical, while their
sternal portions are long and imperfectly ossified (fig. 76, 5). There
is nothing that calls for special remark about the ribs, in either
INSECTIVORA or CHIROPTERA.

PRIMATES. In Man and the Orang (_Simia_) there are generally twelve
pairs of ribs; in the Gorilla and Chimpanzee (_Anthropopithecus_), and
Gibbons (_Hylobates_), there are thirteen, in the Cebidae twelve to
fifteen, and in the Lemuroidea twelve to seventeen pairs. The first
vertebral rib is shorter than the others, and the sternal ribs
generally remain cartilaginous throughout life, though in man the
first may ossify.


APPENDICULAR SKELETON.


THE PECTORAL GIRDLE.

By far the most primitive type of the pectoral or shoulder girdle is
found in the MONOTREMATA. The scapula (fig. 102, A, 1) is long and
recurved, and has only two surfaces, one corresponding to the
prescapular[1] fossa, the other to the postscapular[1] and
subscapular[169] fossae. The coracoid is a short bone attached above
to the scapula and below to the presternum; it forms a large part of
the glenoid cavity. In front of the coracoid there is a fairly large
flattened epicoracoid (fig. 102, 6); there is also a large =T=-shaped
interclavicle (fig. 102, 4), which is expanded behind and rests on the
presternum. The clavicles rest on and are firmly united to the
anterior border of the interclavicle. This shoulder girdle differs
greatly from that of any other mammals, and recalls that of some
Lacertilia.

[Illustration FIG. 102. _A_, SIDE VIEW, _B_, DORSAL VIEW OF THE
SHOULDER GIRDLE AND PART OR THE STERNUM OF THE SPINY ANTEATER
(_Echidna aculeata_) × 1. (After PARKER.)

    1. scapula.
    2. suprascapula.
    3. clavicle.
    4. interclavicle.
    5. coracoid.
    6. epicoracoid.
    7. glenoid cavity.
    8. presternum.
    9. second sternal rib.
    10. second vertebral rib.]

In MARSUPIALS, as in all mammals except the Monotremes, the shoulder
girdle is much reduced; there are no epicoracoids and interclavicle,
and the coracoid forms simply a small process on the scapula,
ossifying from a centre separate from that giving rise to the rest of
the bone. The scapula has a long acromion, and a clavicle is always
present except in _Perameles_. Unossified remains of the precoracoids
are found at either end of the clavicle. The scapula of _Notoryctes_
has a very high overhanging spine, and there is a second strong ridge
running along the proximal part of the glenoid border.

The shoulder girdle of the EDENTATA shows some very curious
variations. In _Orycteropus_ the scapula is of very normal form and
the clavicle is well developed. In the Pangolins and Anteaters the
scapula is very broad and rounded; there is no clavicle in the
Pangolins, and generally only a vestigial one in Anteaters. In
Armadillos, Sloths, and Megatheriidae, the acromion is very long and
the clavicle is well developed. In the Sloths, _Megatherium_, and
_Myrmecophaga_, a connection is formed between the coracoid, which is
unusually large, and the coracoid border of the scapula, converting
the coraco-scapula notch into a foramen. In _Bradypus_ the clavicle is
very small, and is attached to the coracoid, which sometimes forms a
distinct bone[170].

In the SIRENIA the scapula is somewhat narrow and curved backwards:
the spine, acromion, and coracoid process are moderately developed,
and there is no clavicle.

CETACEA. In nearly all the Odontoceti the scapula is broad and
somewhat fan-shaped; the prescapular fossa is much reduced, and the
acromion and coracoid process form flattened processes, extending
forwards nearly parallel to one another. Some of the Mystacoceti, such
as _Balaenoptera_, have a broad, fan-shaped scapula, with a long
acromion and coracoid process, extending parallel to one another.
Others, such as _Balaena_, have a higher and narrower scapula, with a
smaller coracoid process.

In UNGULATA the scapula is always high and rather narrow, and neither
acromion nor coracoid process is ever much developed. In no adult
Ungulate except _Typotherium_ is there any trace of a clavicle, but a
vestigial clavicle has been described in early embryos of sheep[171].

[Illustration FIG. 103. SKELETON OF A LLAMA (_Auchenia glama_) × 1/18.
(Brit. Mus.)

    1. hyoid.
    2. atlas vertebra.
    3. seventh cervical vertebra.
    4. scapula.
    5. imperfectly ossified suprascapula.
    6. olecranon process of ulna.
    7. metacarpals.
    8. ilium.
    9. patella.
    10. calcaneum.]

UNGULATA VERA. In the Ruminantia the suprascapular region (fig. 103,
5) is very imperfectly ossified, and when this is removed the upper
border of the scapula is very straight (fig. 101, 5). The spine is
prominent, and generally has a fairly well-marked acromion. In
_Hippopotamus_ the acromion is fairly prominent, but in the other
Suina, though the spine is prominent, the acromion is not developed.
The Perissodactyla have no acromion, but while the Equidae and
_Hyracotherium_ have the scapula long and slender, with the spine very
slightly developed, the other living Perissodactyla have the spine
prominent and strongly bent back at about the middle of its length.

SUBUNGULATA. _Typotherium_ (Toxodontia) differs from all other known
Ungulates in having well-developed clavicles; its scapula has a strong
backwardly-projecting process, much like that in _Rhinoceros_.

_Phenacodus_ (Condylarthra), has a curiously rounded scapula, with the
coracoid and suprascapular borders passing imperceptibly into one
another. The scapula resembles that of a carnivore more than does that
of any existing Ungulate.

_Procavia_ has a triangular scapula with a prominent spine and no
acromion; there is a large unossified suprascapular region.

The scapula in the Proboscidea has a large rounded suprascapular
border and a narrow, slightly concave glenoid border. The spine is
large, and has a prominent process projecting backwards from about its
middle. The spine lies towards the front end of the scapula, so that
the postscapular fossa is much larger than the prescapular fossa.

In RODENTIA the shoulder girdle is of a rather primitive type. The
scapula is generally high and narrow, somewhat as in Ruminantia; it
differs, however, from the Ruminant scapula in having a high acromion,
which is often, as in the Hares and Rabbits, terminated by a long
metacromion. The development of the clavicle varies, and sometimes it
is altogether absent. It is frequently connected by cartilaginous
bands or ligaments (fig. 104, 7 and 9), on the one hand with the
scapula, and on the other with the sternum. These unossified bands are
remains of the precoracoid. Epicoracoidal vestiges of the sternal ends
of the coracoids (fig. 104, 11) are also often present.

In the CARNIVORA VERA the scapula is large, and generally has rather
rounded borders. The spine and acromion are well developed, and the
prescapular and postscapular fossae are nearly equal in size. The
coracoid is very small, and the clavicle is never completely
developed, being often absent, as in the Bears and most of their
allies. In the Seals (Phocidae) the scapula is elongated and curved
backwards, and has a very concave glenoid border. In the Eared Seals
(Otariidae) the scapula is proportionally much larger and wider, the
prescapular fossa being specially large, and being traversed by a
ridge, which converges to meet the spine.

[Illustration FIG. 104. DORSAL VIEW OF THE STERNUM AND RIGHT HALF OF
THE SHOULDER-GIRDLE OF _Mus sylvaticus_ × 4. (After PARKER.)

    1. postscapular fossa.
    2. prescapular fossa.
    3. spine.
    4. suprascapular border unossified.
    5. coracoid process.
    6. acromion.
    7. cartilaginous vestige of precoracoid at scapular end of clavicle.
    8. clavicle.
    9. cartilaginous vestige of precoracoid at sternal end of clavicle.
    10. omosternum.
    11. epicoracoid.
    12. presternum.
    13. first segment of mesosternum.
    14. xiphisternum.
    15. cartilaginous termination of xiphisternum.
    16. 2nd sternal rib.
    17. 1st vertebral rib.]

In the INSECTIVORA the shoulder girdle is well developed and, as in
Rodents, remains are met with of various parts not generally seen in
mammals. In the Shrews the scapula is long and narrow, and has a
well-marked spine, whose end bifurcates, forming the acromion and
metacromion. The clavicle is long and slender, and is connected with
the sternum and acromion by vestiges of the precoracoid. Considerable
remains of the sternal end of the coracoid are also found. In
_Potamogale_, however, there are no clavicles. In the Mole the
shoulder girdle is greatly developed, and of very remarkable form. The
scapula is high and very narrow, with the spine and acromion very
little developed. The other shoulder girdle element is an irregular
bone, which articulates with the humerus and presternum, and is
connected by ligaments with the scapula. This bone appears to
represent both the coracoid and the clavicle, being formed partly of
cartilage bone, partly of membrane bone.

In the CHIROPTERA the scapula is large and oval, and has a moderately
high spine and a large acromion. The coracoid process is well
developed and is often forked. The clavicles are also well developed,
and vestiges of the precoracoid and of the sternal end of the coracoid
are often found.

In PRIMATES the clavicle and coracoid process are always well
developed. In Man and the Gorilla the scapula has a long straight
suprascapular border, a well-developed coracoid process and spine, and
a large curved acromion. Vestiges of the precoracoid occur at each end
of the clavicle. The shape of the scapula varies much in the lower
Primates.


THE UPPER ARM AND FORE-ARM.

In the MONOTREMATA the humerus is short, very broad at each end and
contracted in the middle. The radius and ulna are stout and of nearly
equal size, while the ulna has a greatly expanded olecranon.

In the MARSUPIALIA the humerus is generally a strong bone, broad at
the distal end and having well marked deltoid and supinator ridges,
which are specially large in _Notoryctes_. An ent-epicondylar or
supracondylar foramen (fig. 105, 5) is almost always present except in
_Notoryctes_. The radius and ulna are always distinct and well
developed, and a certain amount of rotation can take place between
them. The ulna of _Notoryctes_ has an enormous hooked olecranon which
causes the bone to be nearly twice as long as the radius.

[Illustration FIG. 105. ANTERIOR SURFACE OF THE RIGHT HUMERUS OF A
WOMBAT (_Phascolomys latifrons_). (After OWEN.)

    1. head.
    2. greater tuberosity.
    3. lesser tuberosity.
    4. deltoid ridge.
    5. ent-epicondylar (supracondylar) foramen.
    6. supinator ridge.
    7. external condyle.
    8. internal condyle.
    9. articular surface for radius.
    10. articular surface for ulna.]

EDENTATA. The Sloths have long slender arm bones; the humerus is
nearly smooth and has a very large ent-epicondylar foramen in
_Choloepus_, but not in _Bradypus_. The radius and ulna can be
rotated on one another to a considerable extent. The humerus in all
other Edentates is very strong and has the points for the attachment
of muscles much developed, especially in the Armadillos and
Megatheriidae. An ent-epicondylar foramen is found in all living
forms. The radius and ulna are well developed, but are not capable of
much rotation.

In the SIRENIA the humerus is well developed and of a normal
character. It is expanded at each end and has a prominent internal
condyle, a small olecranon fossa, and no ent-epicondylar foramen. In
the Dugong and _Rhytina_ there is a bicipital groove and the
tuberosities are distinct, but in the Manatee there is no bicipital
groove, and the tuberosities coalesce. The radius and ulna are about
equally developed and ankylosed together at both ends.

In the CETACEA the arm bones are very short and thick. The humerus has
a globular head, and a distal end terminated by two equal flattened
surfaces to which the radius and ulna are united. There is no
bicipital groove, and the tuberosities coalesce. The radius and ulna
are flat expanded bones fixed parallel to one another, but the ulna
has a definite olecranon. Scarcely any movement can take place between
them and the humerus, and in old animals the three bones are often
ankylosed together.

In the UNGULATA VERA the humerus is stout and rather short. The great
tuberosity is always large and often overhangs the bicipital groove,
it is especially large in _Titanotherium_ (_Brontops_). There is never
an ent-epicondylar foramen. The radius is always large at both ends,
but the condition of the ulna is very variable. Sometimes, as in
_Tapirus_, _Rhinoceros_, _Macrauchenia_, Suina and Tragulina, the ulna
is well developed, and quite distinct from the radius; but in most
forms, although complete, it is much reduced distally, and is fused to
the radius. Sometimes, as in the Horse and Giraffe, it is reduced to
the olecranon and to a very slender descending process which does not
nearly reach the carpus. In the Tylopoda, though the ulna is complete
and its distal end is often distinct, it has coalesced with the radius
throughout its whole length; the olecranon is generally very large.

SUBUNGULATA. In the large Condylarthra the humerus has an
ent-epicondylar foramen, and the radius and ulna are stout bones
nearly equal in size.

In _Procavia_ the humerus is rather long, and has a very prominent
greater tuberosity, and a large supra-trochlear fossa, but no
ent-epicondylar foramen.

In the Proboscidea the humerus is marked by a greatly developed
supinator ridge, and is very long, longer than the radius and ulna.
The ulna has a remarkable development, having its distal end larger
than that of the radius, it has also a larger articular surface for
the humerus than has the radius.

In RODENTIA the humerus varies much in its development according to
the animal's mode of life. In the Hares it is long and straight, with
a small distal end, and a slight deltoid ridge. In the Beaver on the
other hand the deltoid and supinator ridges are considerably
developed. There is generally a large supra-trochlear fossa, but no
ent-epicondylar foramen.

CARNIVORA. In the Carnivora vera the humerus has large tuberosities, a
prominent deltoid ridge and a deep olecranon fossa. The shaft is
generally curved, and an ent-epicondylar foramen is often found,
though not in the Canidae, Hyaenidae, and Ursidae. The radius and ulna
are never united. The radius (fig. 77, B) has a very similar
development throughout its whole length, while the ulna has a large
olecranon (fig. 77, C, 11) and a shaft tapering somewhat towards the
distal end.

In the Pinnipedia the arm bones are very strongly developed. The
humerus has a very prominent deltoid ridge, and the proximal end of
the ulna and distal end of the radius are much expanded.

In the INSECTIVORA the arm bones are well developed, and the radius
and ulna, though sometimes united, are generally distinct; as a rule
there is an ent-epicondylar foramen, but this is absent in the
Hedgehog. The Mole has an extraordinary humerus, very short and
curved, and much flattened and expanded at both ends. It articulates
both with the scapula and coraco-clavicle. The ulna has a greatly
developed olecranon.

In the CHIROPTERA both humerus and radius are exceedingly long and
slender; the ulna is reduced to little more than the proximal end and
is fused to the radius. There is no ent-epicondylar foramen.

All PRIMATES have the power of pronation and supination of the
fore-arm, by the rotation of the distal end of the radius round that
of the ulna.

In Man and the Anthropoid Apes the humerus is long and straight, and
has a globular head; neither of the tuberosities, nor the deltoid nor
supinator ridges are much developed. The olecranon fossa is deep and
there is no ent-epicondylar foramen. The radius is curved and has a
narrow proximal, and expanded distal end, the ulna is straighter than
the radius and has the distal end much smaller than the proximal; the
olecranon is not much developed.

In the lower Primates, although the radius and ulna are always quite
separate, the power of pronation and supination is not nearly so great
as in the higher forms. In most of the Cebidae and Lemurs an
ent-epicondylar foramen occurs.


THE MANUS.

The Manus is divisible into two parts, viz. the carpus or wrist, and
the hand which is composed of the metacarpals and phalanges. The
carpal bones are always modified from their primitive arrangement,
sometimes more, sometimes less. One modification however is always
found in mammals, viz. the union of carpalia, 4 and 5 to form the
_unciform_ bone. Two sesamoid bones are commonly developed, one on
each side of the carpus, the _pisiform_ or one on the ulnar side being
much the larger and more constant: it has been suggested that these
represent respectively vestiges of a prepollex and a post-minimus
digit[172].

One or more of the five digits commonly present may be lost, and
sometimes all are lost except the third. The terminal or ungual
phalanges of the digits are commonly specially modified to support
nails, claws, or hoofs. There are as a rule two small sesamoid bones
developed on the ventral or flexor side of the metacarpo-phalangeal
articulations, and sometimes similar bones occur on the dorsal or
extensor side.

MONOTREMATA. In _Echidna_ the carpus is broad, the scaphoid and lunar
are united and there is no centrale. The pisiform is large and several
other sesamoid bones occur. Each of the five digits is terminated by a
large ungual phalanx. In _Ornithorhynchus_ the manus is more slender,
but the general arrangement is the same as in _Echidna_.

MARSUPIALIA. The carpus has no centrale and the lunar is generally
small or absent. Five digits are almost always present. In _Choeropus_
however the only two functional digits are the second and third, which
have very long closely apposed metacarpals; the fourth digit is
vestigial, but has the normal number of phalanges, while the first and
fifth are absent. The manus in _Notoryctes_ is extraordinarily
modified, the scaphoid and all the distal carpalia are apparently
fused, the first, second, and fifth digits are very small, the third
and fourth, though having only one phalanx apiece, bear each an
enormous claw. Lying on and obscuring the ventral surface of the manus
is a large bone, probably a sesamoid.

Among the EDENTATA there is a great diversity in the structure of the
manus, the centrale is however always wanting, and except in _Manis_
the scaphoid and lunar are distinct. In the Sloths the manus is very
long, narrow, and curved, and terminated by two or three long hooked
claws, borne by the second and third, or the second, third and fourth
digits. The fifth digit is absent, and the fourth is represented only
by a small metacarpal. In the Anteaters the third digit is greatly
developed and bears a long hooked claw. In _Myrmecophaga_ all five
digits are fairly well though irregularly developed, in _Cycloturus_
the first, fourth, and fifth, are vestigial. In the Armadillos the
manus is broad, and has strongly developed ungual phalanges. The
digits, though almost always five in number, vary much in their
relative arrangement. In _Dasypus_ they are regular, but are
remarkably irregular in Priodon. The pollex is absent in Glyptodonts
and in _Megatherium._ In _Megatherium_ the fifth digit is clawless
while the second, third, and fourth bear enormous claws. In the
Manidae the scaphoid and lunar are united; five digits are present,
the third and fourth being very large, and all being terminated by
deeply cleft ungual phalanges. In _Orycteropus_ the pollex is absent,
while the other digits are terminated by pointed ungual phalanges.

In SIRENIA the general structure of the manus is quite of the ordinary
mammalian type. In _Manatus_ most of the bones of the carpus are
distinct, but in _Halicore_ many, especially those of the distal row,
have coalesced. The digits are always five in number and have the
normal number of flattened phalanges.

In the CETACEA, on the other hand, the manus is much modified by the
fact that the number of phalanges may be greatly increased above the
normal number of three, thirteen or fourteen sometimes occurring in
each digit. These are believed to be duplicated epiphyses. In the
Mystacoceti the manus remains largely cartilaginous, in the Odontoceti
it is better ossified, and the phalanges commonly have epiphyses at
both ends. In _Physeter_ the carpal bones also have epiphyses. The
carpus generally consists of six bones arranged in two rows of three
each. Five digits are generally present, but sometimes as in
_Balaenoptera musculus_, there are four, the third being suppressed.
Their relative development varies much. The Sperm Whale which till
recently was placed in the entrance hall of the Natural History Museum
at South Kensington has one phalanx to the first digit, four to the
second, five to the third, four to the fourth, and three to the fifth.
Generally the manus is short and broad, but sometimes, as in
_Globicephalus_, it is much elongated owing to the great development
of the second and third digits.

UNGULATA[173]. The manus of the members of this great order is of very
great classificatory and morphological importance. All the members
agree in having the scaphoid and lunar distinct, and in almost every
case the ends of the digits are either encased in hoofs or provided
with broad flat nails. It is by means of characters derived from the
manus and pes that the group is subdivided into the Ungulata vera and
the Subungulata.

In the UNGULATA VERA the manus is never plantigrade, and there are not
more than four digits, the pollex being almost always completely
suppressed: in _Cotylops_ among extinct Artiodactyla however a
vestigial pollex is found. The centrale is absent, and the magnum
articulates freely with the scaphoid, and is separated from the
cuneiform by the unciform and lunar. All the bones of the carpus
interlock strongly, and the axis of the third digit passes through the
magnum and between the scaphoid and lunar.

There is a very strong distinction between the manus of the suborders
Artiodactyla and Perissodactyla. In the Artiodactyla the axis of the
manus passes between the third and fourth digits, which are almost
equally developed and, except in the Hippopotami and some extinct
forms such as _Anoplotherium_, have their ungual phalanges flattened
on their contiguous surfaces.

In all _ARTIODACTYLA_ the third and fourth digits are large, but a
gradual reduction in the second and fifth can be well traced. Thus in
the Suina the second and fifth digits, though smaller than the third
and fourth, are well developed and all four metacarpals are distinct.
In the Tragulina too all four metacarpals are developed, and in
_Dorcatherium_ the third and fourth commonly remain distinct as in the
Suina. In the other Artiodactyla however the third and fourth
metacarpals are almost always united, though indications of their
separate origin remain. In some Ruminantia, such as many Deer, the
second and fifth digits are reduced to minute splint bones attached to
the proximal end of the fused third and fourth metacarpals, and to
small hoof-bearing phalanges, sometimes attached to splint-like distal
vestiges of the metacarpals, sometimes altogether unconnected with any
other skeletal structures. In some other Ruminants, such as the Sheep
and Oxen, the only remnants of the second and fifth digits are nodules
of bone supporting the hoofs, and in others, such as the Giraffe,
_Anoplotherium commune_, some Antelopes and the Tylopoda, all traces
of these digits have disappeared. The Camels differ from all living
Ungulata vera in not having the distal phalanges completely encased in
hoofs, and from all except the Hippopotami in placing a considerable
amount of the manus on the ground in walking.

[Illustration FIG. 106. MANUS OF PERISSODACTYLES.

_A._ LEFT MANUS OF _Tapirus_. (After VON ZITTEL.)

_B._ RIGHT MANUS OF _Titanotherium_. (After MARSH.)

_C._ LEFT MANUS OF _Chalicotherium gigantium_. (After GERVAIS.)

    1. scaphoid.
    2. lunar.
    3. cuneiform.
    4. trapezoid.
    5. magnum.
    6. unciform.
    7. trapezium.
    II, III, IV, V. second, third, fourth and fifth digits.]

While the manus of the Artiodactyla is symmetrical about a line drawn
between the third and fourth digits, that of the _PERISSODACTYLA_ is
symmetrical about a line drawn through the middle of the third digit,
which is larger than the others and has its ungual phalanx evenly
rounded and symmetrical in itself. The most reduced manus in the whole
of the mammalia is found in the Horse and its allies, in which the
third digit, terminated by a very wide ungual phalanx, is the only one
functional. Small splint bones representing the second and fourth
metacarpals are attached to the upper part of the third metacarpal. In
_Hipparion_[174] and other early horse-like animals the second and
fourth digits, though very small and functionless, are complete and
are terminated by small hoofs. In _Rhinoceros_ the second and fourth
digits are equally developed and nearly as large as the third, and
reach the ground in walking, a vestige of the fifth is also present.
In the Tapir (fig. 106, A) and _Hyracotherium_ the fifth digit is
fully developed but is scarcely functional. In _Titanotherium_
(_Brontops_) (fig. 106, B) it is nearly as well developed as any of
the others, and there is little or no difference between the relative
development of the third and fourth digits.

The Chalicotheriidae[175], though distinctly Perissodactyles in
various respects such as their cervical vertebrae and teeth, differ
not only from all other Perissodactyles, but from almost all other
Ungulates, in the very abnormal character of their manus. For while
the carpus and metacarpus are like those of ordinary Perissodactyles,
the phalanges resemble those of Edentates, each second phalanx having
a strongly developed trochlea, and each distal one being curved,
pointed and deeply cleft at its termination (fig. 106, C).

The Macraucheniidae, while agreeing with Perissodactyles in having
only three digits, with the limb symmetrical about a line drawn
through the middle of the third, have a carpus which approaches
closely to the subungulate condition, the magnum articulating
regularly with the lunar, and only to a slight extent with the
scaphoid.

In the SUBUNGULATA the manus sometimes has five functional digits, and
a considerable part of it rests on the ground in walking. The bones of
the carpus retain their primitive relation to one another, the magnum
articulating with the lunar, but not with the scaphoid. This character
does not however hold in the Toxodontia, for in most of the animals
belonging to this group the magnum does articulate with the scaphoid.
The corner of the scaphoid just reaches the magnum also in Amblypoda.

As far as is known the _TOXODONTIA_ generally have three, sometimes
five digits to the manus, and the third is symmetrical in itself--a
Perissodactyloid feature.

In _Phenacodus_ (fig. 107, B) (_CONDYLARTHRA_) all five digits are
well developed, the pollex being the smallest. The carpal bones retain
their primitive arrangement, the magnum articulating with the lunar
and not with the scaphoid. There is no separate centrale.

[Illustration FIG. 107. LEFT MANUS OF

_A. Coryphodon hamatus._ (After MARSH.) × 1/5.

_B. Phenacodus primaevus._ (After COPE.) × 1/3.

_C. Procavia (Dendrohyrax) arboreus._ (After VON ZITTEL.) × 6/7.

    1. scaphoid.
    2. lunar.
    3. cuneiform.
    4. trapezium.
    5. trapezoid.
    6. magnum.
    7. unciform.
    8. centrale.
    9. pisiform.
    I, II, III, IV, V. first, second, third, fourth and fifth digits
     respectively.]

In the _HYRACOIDEA_ (fig. 107, C) the manus is very similar to that in
_Phenacodus_, but a centrale is present and the pollex is much
reduced.

The manus of the _AMBLYPODA_, such as _Coryphodon_ (fig. 107, A) and
_Uintatherium_, is short and broad, with five well developed digits
and large carpal bones. The carpals however interlock to a slight
extent, and the corner of the magnum reaches the scaphoid.

In the _PROBOSCIDEA_ the manus is very short and broad, with large
somewhat cubical carpals which articulate by very flat surfaces and do
not interlock at all. All five digits are present, and none of them
are much reduced in size. The manus in Proboscidea and in _Coryphodon_
is subplantigrade.

In the Tillodontia the manus is plantigrade and has pointed ungual
phalanges, in this respect approaching the Carnivora. It differs
however from that of all living Carnivora in having the scaphoid and
lunar distinct.

In RODENTIA the manus nearly always has five digits with the normal
number of phalanges: the pollex may however be very small as in the
Rabbit, or absent as sometimes in the Capybara. The scaphoid and lunar
are generally united, and a centrale may be present or absent. In
_Pedetes caffer_ the radial sesamoid is double and the distal bone
bears a nail-like horny covering. In _Bathyergus_ the pisiform is
double. It is upon these facts that the contention for the former
existence of prehallux and post-minimus digits has partly been based.

In living CARNIVORA the scaphoid, lunar and centrale are always
united, forming a single bone. All five digits are present, but as a
rule in Carnivora vera the pollex is small, and in _Hyaena_ is
represented only by a small metacarpal. Sometimes, as in Cats and
Dogs, the manus is digitigrade, sometimes, as in Bears, plantigrade.
The ungual phalanges are large and pointed, and in forms like the
Cats, whose claws are retractile, they can be folded back into a deep
hollow on the ulnar side of the middle phalanx; a small radial
sesamoid is often present.

In Pinnipedia the manus is large and flat and the digits are
terminated by ungual phalanges which are blunt (sea lions and walrus),
or slightly curved and pointed (seals). The pollex is nearly or quite
as long as the second digit, and as a rule the digits then
successively diminish in size.

The Creodonta differ from living Carnivora in the fact that the
scaphoid and lunar are usually separate.

In INSECTIVORA the scaphoid and lunar are sometimes united, sometimes
separate, and a separate centrale is usually present. There are
generally five digits, but sometimes the pollex is absent. In the Mole
the manus is greatly developed and considerably modified. It is very
wide, its breadth being increased by the great development of the
radial sesamoid which is very large and sickle-shaped. The ungual
phalanges are also large and are cleft at their extremities.

In the CHIROPTERA the manus is greatly modified for the purpose of
flight. The pollex is short and is armed with a rather large curved
claw, the other digits are enormously elongated, the elongation in the
case of the Insectivorous bats being mainly due to the metacarpals,
and in the Frugivorous bats to the phalanges. In the Frugivorous bats
the second digit is clawed as well as the pollex, in other bats this
claw is always absent, and so is often the ungual phalanx, the middle
phalanx then tapering gradually to its termination.

In PRIMATES as a rule the manus is moderately short and wide. The
carpus has the scaphoid and lunar distinct, and generally also the
centrale; sometimes however, as in Man, the Gorilla, Chimpanzee, and
some Lemurs, the centrale has apparently fused with the scaphoid.
There are almost always five well-developed digits, but in the genera
_Colobus_ and _Ateles_ the pollex is vestigial.

The magnum in man is the largest bone of the carpus. The pisiform also
is well developed, but there is no radial sesamoid. In Man, the
Gorilla, Chimpanzee, and Orang, the carpus articulates only with the
radius, in most Primates it articulates also with the ulna. The third
digit of the Aye-Aye (_Chiromys_) is remarkable for its extreme
slenderness.


THE PELVIC GIRDLE.

The pelvic girdle in all mammals except the Sirenia and Cetacea
consists of two halves, usually united with one another at the
symphysis in the mid-ventral line, and connected near their upper
ends, with the sacral vertebrae. Each half forms one of the
_innominate_ bones, and includes at least three separate elements, a
dorsal bone, the ilium, and two ventral bones, the ischium and pubis.
Very often a fourth pelvic element, the acetabular or cotyloid bone,
occurs.

In the MONOTREMATA the pelvis is short and broad, and the pubes and
ischia meet in a long symphysis. The acetabulum is perforated in
_Echidna_ as in birds, but not in _Ornithorhynchus_. A pair of
elongated slender bones project forwards from the edge of the pubes
near the symphysis; these are sesamoid bones formed by ossifications
in the tendons of the external oblique abdominal muscles, and are
generally called _marsupial bones_.

In the MARSUPIALIA the ilia are generally very simple, straight, and
narrow, while the pubes and ischia are well developed and meet in a
long symphysis. Marsupial bones are nearly always prominent, but are
not developed in _Thylacinus_ or _Notoryctes_. The ischium often has a
well-marked tuberosity and in Kangaroos the pubis bears a prominent
pectineal process on its anterior border close to the acetabulum. The
pelvis in _Notoryctes_ differs much from that in all other Marsupials,
the ilium and ischium being ankylosed with six vertebrae in a manner
comparable to that of many Edentates.

In the EDENTATA the pelvis is generally well developed, but the
symphysis is very short. In the Sloths the pelvis is rather weak and
slender, the obturator foramina are very large and the ischia do not
meet in a symphysis. In the Megatheriidae the pelvis is exceedingly
wide and massive, and is firmly ankylosed with a number of vertebrae.
In the Armadillos, Glyptodonts, Anteaters, and Pangolins it is much
developed and firmly united to the vertebral column by both the ilia
and the ischia. In _Orycteropus_ however the ischium does not become
united to the vertebral column, and the pubis generally has a strongly
developed pectineal process.

In the SIRENIA the pelvis is quite vestigial. In the Dugong it
consists on each side of two slender bones, one of which represents
the ilium and the other the ischium and pubis; the two bones are
placed end to end and are commonly fused together. The ilium is
attached by ligament to the transverse process of one of the
vertebrae. In the Manatee each half of the pelvis is represented by a
triangular bone connected by ligaments with its fellow and with the
vertebral column. In neither Manatee nor Dugong is there any trace of
an acetabulum but one can be made out in _Halitherium_.

In the CETACEA the pelvis is even more vestigial than in the Sirenia,
consisting simply of a pair of small straight bones which probably
represent the ischia, and lie parallel to and below the vertebral
column at the point where the development of chevron bones commences.

In UNGULATA VERA the pelvis is generally rather long and narrow. The
ilium is flattened and expanded in front (fig. 103, 8), but becomes
much narrower and more cylindrical before reaching the acetabulum.
Both pubis and ischium contribute to the symphysis which is often very
long. The ischia are large and have prominent tuberosities, especially
in Artiodactyles. In most Ruminantia there is a deep depression, the
supra-acetabular fossa above the acetabulum, but this is not found in
the Suina or Tylopoda.

SUBUNGULATA. In _Procavia_ the pelvis is long and narrow, and bears
resemblance to that in Artiodactyles.

The Proboscidea have a very large pelvis set nearly at right angles to
the vertebral column; the ilium is very wide, having expanded iliac[1]
and gluteal[1] surfaces, and a narrow sacral[176] surface. The pubes
and ischia are rather small, but both meet their fellows in the
symphysis. _Uintatherium_ (suborder Amblypoda) also has a large and
vertically placed pelvis (fig. 108) with a much expanded ilium. The
pelvis however differs from that of the Proboscidea in the fact that
the ischia do not meet in a ventral symphysis.

In many RODENTIA the ilia have their gluteal, iliac, and sacral
surfaces of nearly equal extent; in the Hares, however, the gluteal
and iliac surfaces are confluent. The pubes and ischia are always well
developed and sometimes, as in the Hares, the acetabular bone also. In
these animals the pubis does not take part in the formation of the
acetabulum, and the ischium bears on its outer side a well-marked
ischial tuberosity.

In the CARNIVORA the pelvis is long and narrow. The iliac surfaces
(fig. 78, A, 5) are very small and the sacral large; the crest or
supra-iliac border is formed by the union of the sacral and gluteal
surfaces. The symphysis is long and includes part of both pubis and
ischium. The ischial tuberosity (fig. 78, A, 10) is often well marked,
and sometimes as in _Viverra_ the acetabular bone is distinct. In the
Pinnipedia the pelvic symphysis is little developed, or sometimes not
developed at all, and the obturator foramina are remarkably large.

In some INSECTIVORA such as _Galeopithecus_, there is a long pelvic
symphysis, in others such as _Erinaceus_ and _Centetes_, it is very
short, in others again such as _Talpa_ and _Sorex_, there is no pelvic
symphysis. The acetabular bone is exceptionally large in _Talpa_ and
_Sorex_.

In CHIROPTERA the pelvis is small and narrow, and in the great
majority of cases the two halves do not meet in a ventral symphysis.
The pubis has a strongly developed pectineal process, which
occasionally unites with a process from the ilium enclosing a large
pre-acetabular foramen.

PRIMATES. In Man and the Anthropoid Apes the pelvis is very large and
wide, and the ilium has much expanded iliac and gluteal surfaces. The
symphysis is rather short and formed by the pubis alone. The
acetabulum is deep and the obturator foramen large, and there is
frequently a well-marked ischial tuberosity. In the lower Anthropoidea
the ilium is long and narrow and has a small iliac surface. The
ischial tuberosities are large in the old world monkeys.

[Illustration FIG. 108. LEFT ANTERIOR AND POSTERIOR LIMB AND LIMB
GIRDLE OF _Uintatherium mirabile_. The anterior limb is to the left,
the posterior to the right × 1/10. (From casts, Brit. Mus.)

    1. ilium.
    2. head of femur.
    3. great trochanter.
    4. patella.
    5. fibula.
    6. tibia.
    7. second digit of pes.
    8. ungual phalanx of fifth digit of pes.
    9. calcaneum.
    10. postscapular fossa.
    11. prescapular fossa.
    12. coracoid process.
    13. humerus.
    14. radius.
    15. ulna.
    17. unciform.
    18. cuneiform.
    20. lunar.
    21. first metacarpal.
    22. fifth metacarpal.]


THE THIGH AND SHIN.

In the MONOTREMATA the femur is short, rather narrow in the middle,
and expanded at each end. The great and lesser trochanters are large
and about equally developed, but there is no third trochanter. The
fibula is very large and is expanded at its proximal end, forming a
flattened plate much resembling an olecranon. The patella is well
developed.

In the MARSUPIALIA there is no third trochanter to the femur, the
fibula is well developed but not the patella as a general rule.
_Notoryctes_ has a femur with a prominent ridge extending some little
way down the shaft from the great trochanter; the tibia has a
remarkably developed crest, and the fibula has its proximal end much
expanded and perforated; there is an irregularly shaped patella
closely connected with the proximal end of the tibia.

EDENTATA. In the Sloths the leg bones are all long and slender. The
femur has no third trochanter, and the fibula is complete and nearly
equal in size to the tibia. In the Megatheriidae the leg bones are
extraordinarily massive, the circumference of the shaft of the femur
in _Megatherium_ equalling or exceeding the length of the bone. There
is no third trochanter in _Megatherium_. In most of the remaining
Edentata the leg bones are strongly developed. The femur in the
Armadillos and Aard Varks has a strong third trochanter, and the
tibia and fibula are both large and are commonly ankylosed together at
either end. The limb bones are very massive also in the Glyptodonts.

SIRENIA. In no living Sirenian is there any trace of a hind limb, but
in _Halitherium_ a vestigial femur is found, which articulates with
the pelvis by a definite acetabulum.

[Illustration FIG. 109. LEFT FEMUR OF AN OX (_Bos taurus_) (to the
left) AND OF A SUMATRAN RHINOCEROS (_R. sumatrensis_) (to the right).
× 1/6. (Camb. Mus.)

    1. head.
    2. great trochanter.
    3. lesser trochanter.
    4. third trochanter.
    5. shaft.
    6. condyles.]

In the Mystacoceti among the CETACEA small nodules of bone or
cartilage occur connected with the vestigial pelvis, and may represent
the femur and tibia. No trace of the skeleton of the hind limb is
known in the Odontoceti.

In the UNGULATA VERA the femur is noticeable for the size of the great
trochanter (fig. 109, 2); there is no definitely constricted neck
separating the head from the rest of the bone, and the lesser
trochanter (fig. 109, 3) is not very prominent. All Perissodactyles
except the Chalicotheriidae show a strongly marked third trochanter,
but this is absent in all known Artiodactyles. The development of the
fibula in general corresponds to that of the ulna. In _Rhinoceros_,
_Macrauchenia_, _Tapirus_ and the Suina it is distinct and fairly well
developed; in the Tragulina on the other hand it is vestigial, being
reduced to the proximal end only. In the Ruminantia and Tylopoda also,
it is much reduced forming merely a small bone attached to the distal
end of the tibia, sometimes, as in the Red deer a slender vestige of
the proximal end also is preserved quite detached from the distal
portion; in the Horse this proximal portion is all that there is found
of the fibula. The progressive diminution of the fibula can be well
seen in the series of forms that are regarded as the ancestors of the
Horse. The patella of the Ungulata vera is well ossified, but
fabellae[177] are not usually found.

SUBUNGULATA. Of the Toxodontia, _Toxodon_ has no third trochanter
while _Typotherium_ and _Astrapotherium_ have one. In the Condylarthra
the femur has well-marked lesser and third trochanters, and the fibula
and patella are well developed. In the Hyracoidea there is a slight
ridge on the femur in the place of the third trochanter, the fibula is
complete, but is generally fused to the tibia at its proximal end.

Of the Amblypoda, _Coryphodon_ has a third trochanter, but
_Uintatherium_ has none; in this respect, in the vertical position and
general appearance (fig. 108) of the limb, and in the articulation of
the fibula with the calcaneum, the leg of _Uintatherium_ closely
approaches that of the Proboscidea.

In the Proboscidea the femur is very long and straight, the
development of trochanters is slight, and the fibula though slender is
complete and articulates with the calcaneum.

A third trochanter is found in the Tillodontia.

In RODENTIA the femur is variable, the great trochanter is generally
large and so sometimes is the third as in the Hares. In most Rodents
as in the Beaver the fibula is distinct, sometimes as in the Hares it
is united distally with the tibia. The patella is well developed, and
so too are the fabellae as a general rule.

CARNIVORA. In the Carnivora vera the femur (fig. 79, A) is generally
rather straight and slender, and has a very distinct head. The fibula
(fig. 79, C) is always distinct and there is generally a considerable
interval between it and the tibia. Fabellae (fig. 79, 7) are commonly
present.

In the Pinnipedia the femur is short, broad and flattened, having a
prominent great trochanter. The fibula is nearly as large as the
tibia, and the two bones are generally ankylosed together at their
proximal ends.

The Creodonta differ from all living Carnivores in having a femur with
a third trochanter.

In the INSECTIVORA a third trochanter is sometimes developed. The
fibula is sometimes distinct, sometimes fused distally with the tibia,
thus differing from that of a Carnivore.

In CHIROPTERA the femur is straight, slender and rather short, with a
small but well-developed head. The fibula may be well developed or
quite vestigial or absent. Owing to the connection of the hind limb
with the wing membrane the knee joint is directed backwards.

In PRIMATES the femur is rather long and slender, having a nearly
spherical head and large great trochanter. The tibia and fibula are
always distinct and well developed. Fabellae are not found in the
highest forms but are generally present in the others.


THE PES.

The skeleton of the pes is in most respects a counterpart of that of
the manus. Just as in the manus if one digit is absent it is the
pollex, so in the pes it is the hallux. But while in the manus the
third digit is always well developed, however much the limb may be
modified, in the pes any of the digits may be lost. In all mammals the
tibiale and intermedium fuse to form the _astragalus_, and the fourth
and fifth tarsalia to form the _cuboid_. Sesamoid bones are
considerably developed. In almost every case the phalanges and first
metatarsal have epiphyses only on their proximal ends, while the
remaining four metatarsals have epiphyses only on their distal ends.

In the MONOTREMATA all the usual tarsal bones are distinct, and the
five digits have the normal number of phalanges. Several sesamoid
bones are developed, the most important one, found only in the male,
being articulated to the tibia and bearing the curious horny spur. The
ungual phalanges of the pes like those of the manus, are deeply cleft
at their extremities. In the Echidnidae the pes is turned outwards and
backwards in walking.

In the MARSUPIALIA the pes is subject to great modifications, but in
every case the seven usual tarsal bones are distinct. In the
Didelphyidae the foot is broad, all five digits are well developed,
and the hallux is opposable to the others. In the Dasyuridae the foot
is narrow, and the hallux may be very small, or as in _Thylacinus_
completely absent. In _Notoryctes_ the pes is much less abnormal than
the manus, and all five digits have the usual number of phalanges. The
fifth metatarsal has a curious projecting process, and there is a
large sesamoid above the hallux. In the Wombats (Phascolomyidae) the
foot is short and broad, the digits are all distinct, and the hallux
is divaricated from the others.

In the remaining marsupials the second and third metacarpals and
digits are very slender, and are enclosed within a common integument.
This condition is known as _syndactylism_, and its effect is to
produce the appearance of one toe with two claws. In the Kangaroos
(Macropodidae) the pes is very long and narrow, owing to the
elongation of the metacarpals. The fourth digit is greatly developed,
the fifth moderately so, while the hallux is absent, and the second
and third digits are very small. The Peramelidae have the foot
constructed on the same plan as in the Kangaroos, and in one genus
_Choeropus_ the same type of foot is carried to a greater extreme than
even in the Kangaroos. Thus the fourth digit is enormously developed,
the second and third are small, and the fifth smaller still, while the
hallux is absent. In the Phalangers and Koalas though the second and
third toes are very slender, the hallux is well developed and
opposable.

EDENTATA. In the Sloths the pes much resembles the manus, being long
and narrow, but in both genera the second, third and fourth digits are
well developed. Most of the other Edentates have a but little modified
pes with the normal number of tarsal bones and the complete series of
digits. In _Cycloturus_ however the hallux is vestigial and it is
absent in Glyptodonts. _Megatherium_ has a greatly modified pes, the
hallux is absent, and the second digit vestigial, while the third is
very large, having an enormous ungual phalanx. The calcaneum too is
abnormally large.

No trace of the pes occurs in either SIRENIA or CETACEA.

In the UNGULATA the pes like the manus is subject to much variation
and is of great morphological importance.

In the UNGULATA VERA the pes is never plantigrade and never has more
than four digits, the hallux being absent. The cuboid always
articulates with the astragalus, and the tarsal bones strongly
interlock. As was the case also with the manus, the pes is formed on
two well-marked types characteristic respectively of the Artiodactyla
and Perissodactyla.

_ARTIODACTYLA._ Just as in the manus, the third and fourth digits are
well and subequally developed; their ungual phalanges have the
contiguous sides flat, and the axis of the limb passes between them,
and between the cuboid and navicular. The astragalus has both the
proximal and distal surfaces pulley-like, and articulates with the
navicular and cuboid by two facets of nearly equal size. The calcaneum
articulates with the lower end of the fibula if that bone is fully
developed.

In the Suina four toes are developed, and though in the Peccaries the
third and fourth metatarsals are united, they are all distinct in most
members of the group, as are all the tarsal bones. In the Hippopotami
the four digits are of approximately equal size, and the middle ones
do not have the contiguous faces of their ungual phalanges flattened.

In the Tragulina the cuboid, navicular, and two outer cuneiforms are
united forming a single bone; all four metatarsals are complete and
the two middle ones are united. In the Tylopoda and _Anoplotherium
commune_ only the third and fourth digits are developed, their
metatarsals are free distally, but are elsewhere united. In the
Ruminantia the cuboid and navicular are always united and so are the
second and third cuneiforms, while in _Cervulus_ all four bones are
united together. The third and fourth metatarsals in Ruminants are
always united in the same way as are the third and fourth metacarpals,
while the second and fifth are always wanting. In Deer the second and
fifth digits are usually each represented by three small phalanges,
but in the Giraffe and most Bovidae the bones of these digits are
wanting.

[Illustration

    FIG. 110. _A._ LEFT PES OF A TAPIR (_Tapirus americanus_). × 1/6.
    _B._ RIGHT PES OF A RHINOCEROS (_R. sumatrensis_). × 1/8.
    _C._ (CAST OF) RIGHT PES OF _Hipparion gracile_. × 1/7.
    _D._ RIGHT PES OF A HORSE (_Equus caballus_). × 1/10. (All Camb. Mus.)

    1. calcaneum.
    2. astragalus.
    3. navicular.
    4. cuboid.
    5. external cuneiform.
    6. middle cuneiform.
    7. internal cuneiform.]

In the _PERISSODACTYLA_ the pes like the manus is symmetrical about a
line drawn through the third digit; this line when continued passes
through the external cuneiform, navicular and astragalus. The
astragalus has its distal portion abruptly truncated, and the facet by
which it articulates with the cuboid is much smaller than that by
which it articulates with the navicular. The calcaneum does not
articulate with the fibula. The tarsus in _Macrauchenia_ like the
carpus differs from that of other Perissodactyles and resembles that
of Subungulates in having the bones arranged in lines with little or
no interlocking. The calcaneum resembles that of Artiodactyles in
having a small facet for articulation with the fibula. _Tapirus_ (fig.
110, A), _Rhinoceros_ (fig. 110, B) and _Titanotherium_ have a short
and broad foot with the usual tarsal bones and three well-developed
digits,--a number never exceeded by any Perissodactyle. From this
tridactylate limb a series of stages is exhibited by various extinct
forms leading gradually to the condition met with in the Horse (fig.
110, D) in which the third toe is greatly developed, while the second
and fourth are reduced to slender metatarsals attached to the proximal
half of the third metatarsal.

In _Chalicotherium_ and _Agriochoerus_ the pes has the same abnormal
characters as the manus, the digits being clawed and the ungual
phalanges in _Chalicotherium_ deeply cleft.

In the SUBUNGULATA the pes is sometimes plantigrade and
pentedactylate, the cuboid sometimes does not articulate with the
astragalus, and the tarsal bones sometimes do not interlock.

In _Typotherium_ (_TOXODONTIA_) the hallux is absent and the other
four digits are well developed; in _Toxodon_ and _Nesodon_ the pes is
tridactylate. The tarsal bones have the regular Subungulate
arrangement, the cuboid not articulating with the astragalus. The
calcaneum articulates with the fibula as in Artiodactyles. The
astragalus in most forms, but not in _Astrapotherium_, resembles that
of the Ungulata vera in having a grooved proximal surface.

In _Phenacodus_ (_CONDYLARTHRA_) the tarsus is very little modified,
five digits are present, the first and fifth being small and not
reaching the ground.

In _Procavia_ only the three middle digits are present with a vestige
of the fifth metacarpal.

In the _AMBLYPODA_ the pes (fig. 108) is very short and broad, all
five digits are functional, and at any rate in _Coryphodon_
plantigrade, the hallux being the smallest. The astragalus is very
flat, and the tarsals interlock to a slight extent, the cuboid
articulating with both calcaneum and astragalus.

The pes in the _PROBOSCIDEA_ much resembles that in the Amblypoda, but
differs in that the astragalus does not articulate with the cuboid,
the tarsals not interlocking at all.

In the RODENTIA the structure of the foot is very variable. In Beavers
the foot is very large, all five digits being well developed; the
fifth metatarsal articulates with the outer side of the fourth
metatarsal, and not with the cuboid, and there is a large sesamoid
bone on the tibial side of the tarsus. In the Rats, Porcupines and
Squirrels, there are five digits, in the Hares only four, and in the
Capybara and some of its allies only three. In the Jerboa (_Dipus_) a
curious condition of the pes is met with, as it consists of three very
long metatarsals fused together and bearing three short toes, each
formed of three phalanges. _Lophiomys_ differs from all other Rodents
in having the hallux opposable.

CARNIVORA. In the Carnivora vera the pes is regular and shows little
deviation from the normal condition. All the usual tarsal bones are
present, but sometimes as in the Dogs, Cats, and Hyaenas, the hallux
is vestigial. Sometimes as in the Bears the pes is plantigrade,
sometimes as in the Cats and Dogs it is digitigrade. In this respect
and in the character of the ungual phalanges, the pes closely
corresponds with the manus. In the Sea Otter (_Latax_) the foot is
large and flattened and approaches in character that of the
Pinnipedia.

In the Pinnipedia the pes differs much from that in the Carnivora
vera. In the Seals in which the foot cannot be used for walking, and
is habitually directed backwards, the first and fifth digits are much
longer and stouter than any of the others. In the Sea Lions which can
use the pes for walking, the digits are all of nearly the same length,
and in the Walrus the fifth is somewhat the longest.

In the INSECTIVORA the pes is almost always normal, and provided with
five digits.

In the CHIROPTERA the pes is pentedactylate, and the digits are
terminated by long curved ungual phalanges. In some genera the toes
have only two phalanges. The calcaneum is sometimes produced into a
long slender process which helps to support the membrane between the
leg and the tail.

Among the PRIMATES Man has the simplest form of pes. In Man all five
digits are well developed, the hallux being considerably the largest.
Sesamoid bones occur only under the metatarso-phalangeal joint of the
hallux.

In the other Primates the internal cuneiform has a saddle-shaped
articulating surface for the hallux, which is obliquely directed to
the side of the foot and opposable to the other digits. Two sesamoid
bones are usually developed below each metatarso-phalangeal joint, and
one below the cuboid. The second digit in Lemurs, and all except the
hallux in _Chiromys_ have pointed ungual phalanges; in all other cases
the ungual phalanges are flat. In some of the Lemuroidea, especially
_Tarsius_, the tarsus is curiously modified by the elongation of the
calcaneum and navicular.


FOOTNOTES:

[165] The figure was drawn from a photograph and the size of the jaws
relatively to the cranium is exaggerated.

[166] See W.K. Parker, "On the Structure and Development of the Skull
in the Pig." _Phil. Trans._ pp. 289-336, 1874.

[167] See W.H. Flower, "On the value of the characters of the base of
the cranium in the classification of the order Carnivora." _P.Z.S._
1869, p. 5.

[168] See W.K. Parker, _Monograph of the shoulder-girdle and sternum
of the Vertebrata_, _Ray Soc._ 1868.

[169] See p. 405.

[170] See E. Lydekker, _P.Z.S._ 1895, p. 172.

[171] See H. Wincza, _Morph. Jahr._ XVI. p. 647, 1890.

[172] See K. Bardeleben, _P.Z.S._, 1889, p. 259.

[173] See E. Cope, "The origin of the foot structures of Ungulata,"
_Journ. of Philad. Acad._ 1874. H.F. Osborn, "The evolution of the
Ungulate foot," _T. Amer. Phil. Soc._ 1889.

[174] See O.C. Marsh, various papers including "Fossil horses in
America," _Amer. Natural._ 1874; "Polydactyl horses," _Amer. J. Sci._
1879 and 1892. M. Pavlow, "Le développement des Equidés," _Bul. Soc.
Moscou_, 1887, and subsequent papers in the same. Osborn and Wortman,
"On the Perissodactyls of the White River beds," _Bull. Amer. Mus._
Dec. 23rd, 1895.

[175] See H.F. Osborn, _Chalicotherium and Macrotherium, Amer.
Natural._ 1889--91--92.

[176] See p. 409.

[177] See p. 412.




LIST OF AUTHORS REFERRED TO.


    Abbott, E.C., 112

    Ameghino, F., 351, 424

    Andrews, C.W., 299


    Balfour, F.M., 16

    Ballowitz, E., 424

    Bardeleben, K., 504

    Bateson, W., 50, 344

    Baum, H., 374

    Baur, G., 27, 189, 190, 344, 346

    Beneden, P.J. van, 353

    Benham, W.B., 51

    Bettany, G.T., 16, 87, 154

    Boulenger, G.A., 169

    Brandt, J.F., 352

    Bridge, T.W., 123

    Brühl, C.B., 210

    Burmeister, H., 351, 424


    Cope, E.D., 135, 199, 204, 351, 359, 361, 363, 368

    Credner, H., 135


    Dean, B., 63, 104

    Dobson, G.E., 369, 370


    Earle, C., 432

    Ecker, A., 151

    Ellenberger, W., 374


    Flower, W.H., 28, 42, 351, 420, 422, 434

    Fritsch, A., 135

    Fürbringer, M., 295


    Gadow, H., 40, 112, 190, 295, 343, 350

    Gegenbaur, C., 127

    Gervais, P., 353

    Günther, A.C.L.G., 70, 104


    Haslam, G., 151

    Hasse, C., 112, 113

    Haswell, W.A., 127

    Hertwig, O., 169

    Hoffmann, C.K., 190, 202, 210

    Howes, G.B., 164, 451

    Hubrecht, A.A.W., 104

    Hulke, J.W., 192, 204

    Hurst, C.H., 71, 297

    Hutton, F.W., 299

    Huxley, T.H., 11, 13, 133, 135, 191, 210, 295, 297, 334, 343, 351,
     374, 437


    Kirkaldy, J.W., 51

    Klein, E., 11

    Kölliker, A., 9

    Kükenthal, W., 349, 422


    Lankester, E. Ray, 51

    Leche, W., 344, 423

    Lindsay, B., 336

    Lydekker, R., 36, 42, 190, 195, 495


    Macbride, E.W., 50

    Marsh, O.C., 204, 209, 299, 348, 361, 364, 365, 508

    Marshall, A.M., 71, 151

    Masterman, A.T., 51

    Meyer, H. v., 135

    Miall, L.C., 135, 243

    Mivart, St G., 369

    Morgan, C. Lloyd, 11


    Newton, E.T., 283


    Osborn, H.F., 348, 420, 429, 508

    Owen, R., 191, 204, 210, 297, 348, 351, 420


    Parker, T.J., 83, 96, 299

    Parker, W.K., 16, 24, 53, 87, 154, 173, 200, 243, 465, 489

    Pavlow, M., 358, 508

    Pollard, H.B., 119

    Poulton, E.B., 422

    Pycraft, W.P., 297


    Ridewood, W.G., 106, 164

    Röse, C., 422


    Sagemehl, M., 104

    Schäfer, E., 11

    Scott, W.B., 368

    Seeley, H.G., 191, 212

    Selenka, E., 40, 295

    Shufeldt, R., 123

    Smith, E. Noble, 11

    Stirling, E.C., 423

    Swirski, G., 103


    Taeker, J., 427

    Thomas, O., 349, 362, 370, 422, 424, 425

    Tomes, C.S., 420

    Traquair, R.H., 55, 58


    Vogt, C., 297


    Wiedersheim, R., 25, 134, 136

    Wincza, H., 358, 495

    Woodward, A. Smith, 34, 54, 58, 62, 127, 210

    Wortman, J.L., 508

    Wray, R.S., 303


    Zittel, K.A. v., 36, 205, 212




INDEX.


     Every reference is to the page: words in italics are names of
     genera or species; figures in italics indicate that the
     reference relates to systematic position; figures in thick type
     refer to an illustration; _f._ = and in following page or
     pages; _n._ = note.


    Aard Vark, _44_, _352_;
      femur, 517;
      sacrum, 452;
      teeth, 425;
      see _Orycteropus_

    Aard wolf, _48_;
      see _Proteles_

    Abdominal ribs, crocodile, 260;
      reptiles, 286

    Abdominal shield, turtle, 215

    _Acanthias_, _32_;
      calcification of vertebrae, 114;
      pectoral fins, 130

    _Acanthodes_, _32_, _64_

    Acanthodii, _32_;
      general characters, 64;
      spines, 106

    _Acanthomys_, _47_;
      spines, 417

    Acanthopterygii, _34_

    Accipitres, _41_

    Acetabular bone, 25, 513;
      dog, 409 f.;
      frog, 165

    Acetabulum, 25;
      crocodile, 266;
      dog, 409;
      duck, 324;
      frog, 165;
      newt, 149;
      turtle, 235

    _Acipenser_, _32_, 117;
      exoskeleton, 67;
      distribution, 66;
      pectoral fins, 131;
      plates, 104;
      skull, 121, =122=;
      spinal column, 112

    Acipenseridae, _32_

    Acrodont, defined, 199;
      teeth of reptiles, 273

    _Acrodus_, _32_;
      teeth, 109

    Acromion, dog, 405

    _Actinotrocha_, _30_;
      organ regarded as double notochord, 51

    Ad-digital quill, duck, 303

    Adjutant, _41_;
      clavicles, 338

    Ægithognathous, 335

    Æluroidea, _48_, _369_;
      teeth, 437

    Æpyornis, _40_;
      tibio-tarsus, 341

    Æpyornithes, _40_, _299_

    Aftershaft, 328

    _Agama_, _38_;
      teeth, 273

    Agamidae, _38_;
      premaxillae, 284

    Aglossa, _36_

    Agouti, _48_;
      see _Dasyprocta_

    Agriochoeridae, _45_

    _Agriochoerus_, _45_;
      pes, 525

    Ala spuria, duck, 304

    Alcidae, _42_;
      see Auks

    Alisphenoid, 19;
      crocodile, 247;
      duck, 317;
      dog, 386

    _Alligator_, _39_, _210_, _212_;
      hyoid, =285=;
      limbs, =264=;
      pectoral girdle, =262=;
      pelvis and sacrum, =267=;
      scutes, 271;
      skull, =245=, =248=, =253=

    Alligatoridae, 39

    _Alytes_, _36_;
      fronto-parietal fontanelle, 179;
      vertebrae, 172

    Amblypoda, _47_;
      general characters, 363;
      manus, 510;
      pes, 525;
      skull, 473;
      teeth, 433;
      thigh and shin, 519

    _Amblystoma_, _35_;
      skull, 175

    American monkeys, _373_;
      see _Cebidae_

    American vultures, _41_;
      vomers, 335

    _Amia_, _33_;
      distribution, 66;
      exoskeleton, 67;
      pectoral fin, 131;
      scales, 105;
      skull, 123;
      tail, 115, 117;
      vertebrae, 114

    Amiidae, _33_

    _Ammocoetes_, _31_, _55_

    Amphibia, _35_;
      anterior limb, 185;
      exoskeleton, 168;
      general characters, 133;
      hyoid apparatus, 180;
      pectoral girdle, 184;
      pelvic girdle, 187;
      posterior limb, 188;
      ribs, 182;
      skull, 173;
      sternum, 182;
      teeth, 169;
      vertebral column, 170

    Amphicoelous, defined, 14

    _Amphioxus_, _30_;
      skeleton, 51 f.;
      spinal column, 112

    _Amphisbaena_, _38_, 272;
      loss of limbs, 289

    Amphisbaenidae, _38_, _200_;
      pectoral girdle, 288;
      skull, 277;
      vertebral column, 275

    Amphitheriidae, _43_

    _Amphiuma_, _35_, _135_;
      manus, 187;
      pes, 188;
      skull, 174

    Amphiumidae, _35_

    Anacanthini, _33_

    Anal shield, turtle, 215

    _Anas_, _41_;
      _A. boschas_, see Duck

    Ankylosis, defined, 12

    Angel fish, _32_

    Angler, attachment of teeth, 107

    Anguidae, _38_

    _Anguilla_, _33_;
      see Eel

    _Anguis_, _38_;
      loss of limbs, 289;
      scutes, 271

    Angular, 22;
      cod, 100;
      crocodile, 258;
      duck, 319;
      salmon, 94;
      turtle, 231

    Angulo-splenial, frog, 161

    Ankle joint, duck, 327;
      reptiles, 294

    Anomodontia, _36_

    Anoplotheriidae, _45_

    _Anoplotherium_, _45_;
      manus, 506;
      pes, 523;
      tail, 454;
      teeth, 428

    _Anser_, _41_

    Anseres, _41_;
      aftershaft, 329;
      claws, 330

    Anseriformes, _41_

    Anteaters, _352_;
      absence of teeth, 424;
      manus, 505;
      pectoral girdle, 495;
      pelvis, 513;
      skull, 458;
      thoraco-lumbar vertebrae, 447;
      Spiny --, _43_;
      Great and Two-toed --, _44_

    Antelope, _359_;
      manus, 507;
      Four-horned A., _46_

    Anterior limb, 26;
      Amphibia, 185;
      birds, 338;
      crocodile, 263;
      dog, 405;
      duck, 322;
      frog, 164;
      newt, 147;
      reptiles, 290;
      turtle, 232

    Anthropoid apes, _373_;
      arm-bones, 503;
      pelvis, 515

    Anthropoidea, 49;
      general characters, 372;
      sacrum, 452;
      skull, 482;
      teeth, 441

    _Anthropopithecus_, _49_;
      ribs, 493

    Antiarcha, _31_;
      general characters, 55

    Antibrachium, see fore-arm

    _Antilocapra_, _46_;
      horns, 417

    Antilocapridae, _46_

    Antitrochanter, duck, 325

    Antlers, 8, 358;
      Cervidae, 469

    Antorbital process, 18

    Anura, _36_;
      general characters, 136;
      hyoid apparatus, 180;
      pelvis, 187;
      posterior limb, 188;
      skull, 179;
      sternum, 182;
      vertebrae, 172

    _Apatornis_, _40_;
      vertebrae, 332

    Apteria, 328

    Apteryges, _40_

    _Apteryx_, _40_, _299_;
      aftershaft, 329;
      anterior nares, 333;
      claws, 330;
      foot, 342;
      manus, 338;
      pectineal process, 341;
      pectoral girdle, 338;
      pneumaticity of skeleton, 331;
      _A. oweni_, pelvic girdle and sacrum, =340=

    Aqueductus vestibuli, dogfish, 74

    Arcade:
      infratemporal--, crocodile, 255;
      _Sphenodon_, 283;
      inner --, duck, 318;
      outer --, duck, 318;
      supratemporal --, crocodile, 257;
      reptiles, 281

    Archaeoceti, _44_;
      general characters, 356;
      skull, 461;
      teeth, 426

    _Archaeopteryx_, _40_, _297_;
      claws, 330;
      fibula, 341;
      mandible, 335;
      metatarsals, 342;
      pelvis, 341;
      ribs, 336;
      sacrum, 333;
      skull, 333;
      tail, 333;
      teeth, 330;
      wing, 338

    Archaeornithes, _40_;
      characters, 297

    _Archegosaurus_, _35_, _136_;
      palatines, 177

    Archipterygium, _Ceratodus_, 127;
      Ichthyotomi, 62

    Arcifera, _36_, 185

    Arctoidea, _48_, 369;
      teeth, 438

    _Ardea_, _41_;
      see Heron

    Ardeae, _41_

    Arm, see fore-arm and upper arm

    Armadillo, _44_, _352_;
      cervical vertebrae, 443;
      femur, 517;
      humerus, 501;
      lumbar vertebrae, 447;
      manus, 505;
      pectoral girdle, 495;
      pelvis, 513;
      ribs, 491;
      sacrum, 452;
      scales, 417;
      scutes, 419;
      skull, 459;
      teeth, 424

    Armour plates, 8

    Arthrodira, _34_;
      characters, 70

    Articular, 22;
      cod, 100;
      crocodile, 258;
      duck, 319;
      newt, 144;
      salmon, 94;
      turtle, 231

    Artiodactyla, _45_;
      characters, 358;
      manus, 506;
      odontoid process, 445;
      pes, 522;
      ribs, 491;
      skull, 465;
      teeth, 427;
      thoraco-lumbar vertebrae, 448

    Asses, _360_

    _Asterolepis_, _31_, _55_

    Asterospondyli, 114

    Astragalus, 27;
      crocodile, 268;
      dog, 414;
      mammals, 521

    Astrapotheriidae, _46_

    _Astrapotherium_, _46_, 361;
      dental formula, 432;
      femur, 519;
      pes, 525

    _Ateles_, _49_;
      pollex, 512;
      tail, 454

    Atlantosauridae, _38_

    Atlas, 15;
      crocodile, 240;
      dog, =379=, 380;
      duck, 309;
      ox, =445=;
      turtle, 219

    Attachment of teeth, 4;
      in fish, 107

    _Auchenia_, _45_;
      see Llama

    Auditory aperture or meatus:
      external --, crocodile, 250;
      dog, 402;
      turtle, 228;
      internal --, crocodile, 246, 251;
      dog, 392;
      turtle, 228

    Auditory capsule, 20;
      cod, 96;
      crocodile, 250;
      dog, 390;
      dogfish, 74;
      frog, 156;
      newt, 143;
      turtle, 227

    Auditory ossicles, crocodile, 251;
      dog, 393;
      duck, 320;
      mammals, 485 f.;
      turtle, 228

    Auks, _42_;
      thoracic vertebrae, 332

    Autostylic, 61, 119

    Aves, _40_;
      characters, 295;
      see Birds

    Axial skeletal rods, 50

    Axial skeleton, crocodile, 239;
      cod, 83;
      dog, 377;
      dogfish, 72;
      duck, 307;
      frog, 152;
      newt, 138;
      turtle, 218

    Axis vertebra, crocodile, 241;
      dog, 380;
      duck, 309;
      turtle, 220

    Axolotl, _35_;
      see _Siredon_

    Aye Aye, _49_;
      see _Chiromys_


    _Babirussa_, _45_;
      dental formula, 428

    Baboon, _49_;
      see _Cynocephalus_

    _Balanoglossus_, _30_, 50

    _Balaena_, _44_, _357_;
      scapula, 495;
      _B. mysticetus_ baleen, 419

    Balaenidae, _44_

    Balaenoidea, _44_;
      general characters, 356

    _Balaenoptera_, _44_, 357;
      manus, 506;
      thoracic vertebrae, 448;
      scapula, 495;
      _B. musculus_, cervical vertebrae, =444=

    Baleen, 3, 418

    _Balistes_, _33_;
      teeth, 111

    Balistidae, _33_

    Ball and socket joints, 13

    Bandicoot, _43_

    Barb, 302

    Barbule, 303

    Barramunda, _34_;
      see _Ceratodus_

    Basalia, dogfish, 79

    Basibranchial, dogfish, 78;
      cod, 101;
      duck, 320;
      newt, 145;
      salmon, 95

    Basi-branchiostegal, cod, 101;
      salmon, 95

    Basicranial axis, 19;
      dog, 384

    Basidorsalia, dogfish, 72

    Basi-hyal, dogfish, 78;
      dog, 399;
      duck, 320

    Basilar plate, 17

    Basilingual plate, Anura, 180;
      crocodile, 259;
      frog, 161;
      turtle, 231

    Basi-occipital, 19;
      crocodile, 246;
      cod, 97;
      dog, 386;
      duck, 315;
      salmon, 89;
      turtle, 224

    Basipterygium, cod, 103;
      dogfish, 82

    Basisphenoid, 19;
      crocodile, 247;
      dog, 386;
      salmon, 91;
      turtle, 225

    Bastard wing, duck, 304

    Bathyerginae, palate, 366

    _Bathyergus_, _47_;
      auditory ossicles, 488;
      manus, 511

    Batoidei, _32_, 64

    _Batrachoseps_, _35_;
      teeth, 169

    Bats, claws, 418;
      Horseshoe bats, _49_;
      see Chiroptera

    _Bdellostoma_, _31_, _55_;
      teeth, 57

    Beak, 3;
      birds, 329;
      duck, 302;
      _Siren_, 168;
      tadpoles of Anura, 168;
      turtle, 215

    Bears, _48_, _369_;
      manus, 511;
      pes, 526;
      sacral vertebrae, 452;
      skull, 479;
      Isabelline -- mandible, =438=

    Beaver, _47_;
      fibula, 520;
      humerus, 502;
      pes, 526;
      sacrum, 452;
      tail, 454

    _Belodon_, _39_, 211;
      frontals, 277;
      palate, 281;
      vertebrae, 275

    Bichir, _33_;
      see _Polypterus_

    Bicipital groove, dog, 405

    Bilophodont, defined, 345;
      teeth of Tapiridae, 429

    _Bipes_, _38_;
      limbs, 289

    Birds, anterior limb, 338;
      endoskeleton, 331 f.;
      exoskeleton, 328 f.;
      general characters, 295;
      hyoid, 336;
      pectoral girdle, 336;
      pelvic girdle, 339;
      posterior limb, 341;
      ribs, 336;
      skull, 333;
      sternum, 336;
      teeth, 330;
      vertebral, 332

    _Bison_, _46_;
      occipital crest, 468

    Blind snake, _38_;
      see _Typhlops_

    Blind worm, _38_;
      see _Anguis_

    Boidae, _38_

    _Bombinator_, _36_;
      vertebrae, 172

    Bone, development of, 10 f.

    Bone cells, 10

    Bony Ganoids, fins, 105;
      pelvic fin, 132;
      ribs, 126;
      skull, 123;
      vertebral column, 114;
      see _Holostei_

    Border:
      alveolar --, of dog's jaw, 398;
      coracoid, glenoid, and suprascapular -- of dog's scapula, 405

    _Bos_, _46_;
      occipital crest, 468;
      ribs, 491;
      see Ox

    Bottlenose, _44_;
      see _Hyperoödon_

    Bovidae, _46_;
      pes, 523;
      skull, 468

    Bow-fin, _33_;
      see _Amia_

    Brachial ossicles, cod, 103

    Brachium; see upper arm

    Brachydont, defined, 345;
      teeth of Ungulates, 429 f.

    _Brachycephalus_, _36_;
      bony plates of, 168

    Brain case, crocodile, 245;
      dog, 384;
      duck, 314;
      frog, 154;
      newt, 140;
      turtle, 224

    Bradypodidae, _43_;
      see Sloths

    _Bradypus_, _43_;
      cervical vertebrae, 443;
      pectoral girdle, 495;
      skull, 457;
      thoraco-lumbar vertebrae, 447

    Branchial arches, Amphibia, 180 f.;
      cod, 101;
      dogfish, 78;
      fish, 120 f.;
      newt, 145;
      salmon, 95;
      -- basket, Marsipobranchii, 38;
      -- skeleton, _Amphioxus_, 52;
      Balanoglossus, 50

    _Branchiosaurus_, _35_;
      branchial arches, 180

    Branchiostegal rays, cod, 100

    _Brontops_, _46_;
      see _Titanotherium_

    _Brontosaurus_, _38_, _207_;
      sternum, 288

    _Bubalus_, _46_;
      ribs, 491;
      see Buffalo

    Buccal skeleton, _Amphioxus_, 52

    _Buceros_, _42_;
      see Hornbill

    Buckler, of Labyrinthodonts, 168, 184

    Buffalo, _46_;
      Cape --, skeleton of, =492=

    _Bufo_, _36_;
      hyoid, 182;
      jaws, 169;
      _B. viridis_, carpus, 186

    Bufonidae, _36_

    Bunodont, defined, 345;
      teeth of Ungulata, 427 f.

    Buno-selenodont, defined, 432


    Caeciliidae, _35_

    _Caiman_, _39_;
      _C. latirostris_ hyoid, =285=,
        limbs, =264=,
        lateral view of skull, =248=,
        palatal view of cranium and mandible, =245=,
        longitudinal section of skull, =253=,
        pectoral girdle, =262=,
        pelvic girdle and sacrum, =267=;
      _C. sclerops_, scutes, 271

    Ca'ing whale, _45_;
      see _Globicephalus_

    _Calamoichthys_, _33_;
      distribution, 66

    Calamus, 302

    Calcaneum, 27;
      crocodile, 268;
      dog, 414

    Calcar, of frog, 167

    _Callorhynchus_, _32_, _66_;
      teeth, 110

    Camel, _45_, _359_;
      manus, 507;
      teeth, 428

    Camelidae, _45_

    _Camelus_, _45_;
      see Camel

    Camptosauridae, _39_

    Canal:
      alisphenoid --, dog, 402;
      carotid --, duck, 315;
      Eustachian --, crocodile, 247;
      dog, 402;
      duck, 316;
      interorbital --, dogfish, 76

    Canaliculi, 10

    Canidae, _48_;
      humerus, 502;
      skull, 479;
      see Dog

    Canine, 344;
      dog, 376 f.

    _Canis_, _48_;
      thoraco-lumbar vertebrae, 450;
      see Dog

    _Capitosaurus_, _35_;
      skull, =176=

    Capybara, _48_;
      manus, 511;
      pes, 526;
      skull, 476;
      tail, 454

    Carapace, Chelonia, 271;
      _Dermochelys_, 272;
      Glyptodonts, 419;
      Green turtle, 215;
      Loggerhead turtle, =216=

    Carcharidae, _32_

    Carina sterni, duck, 321

    Carinatae, _40_;
      general characters, 300;
      quadrate, 334

    Carnassial teeth, 368;
      carnivora, =436=;
      dog, 376 f.

    Carnivora, _48_;
      arm bones, 502;
      auditory ossicles, 488;
      cervical vertebrae, 446;
      general characters, 367;
      manus, 511;
      pelvis, 515;
      pes, 526;
      ribs, 493;
      sacral vertebrae, 452;
      skull, 478;
      sternum, 490;
      tail, 454;
      teeth, 437;
      thigh and shin, 520;
      thoraco-lumbar vertebrae, 450

    Carnivora vera, _48_;
      general characters, 368;
      scapula, 497

    Carp, _33_;
      pharyngeal teeth, 111

    Carpo-metacarpus, duck, 324

    Carpus, 26;
      crocodile, 265;
      dog, 408;
      duck, 323;
      frog, 164;
      newt, 147;
      turtle, 233

    Cartilage, structure of, 10

    Cartilaginous ganoids, cranium, 121;
      pelvic fin, 132;
      spinal column, 112;
      see Chondrostei

    Cassowary, _40_, _299_;
      aftershaft, 328;
      bony crest, 334;
      claws, 330;
      pelvic girdle and sacrum, =340=;
      secondaries, 329

    _Castor_, _47_;
      see Beaver

    Castoridae, _47_

    _Casuarius_, _40_;
      see Cassowary

    Cataphracti, _34_

    Cat, _48_, _369_;
      hallux, 526;
      manus, 511;
      skull, 479

    Cat-fish, _33_

    Cathartae, _41_

    _Cathartes_, _41_;
      see American vulture

    Caudal fin, Cetacea, 453;
      fish, 116;
      -- vertebrae, crocodile, 243;
      cod, 85;
      dog, 383;
      duck, 312;
      general characters, 16;
      newt, 140;
      turtle, 222

    _Cavia_, _48_;
      tail, 454

    Caviidae, _48_

    Cebidae, _49_, _373_;
      ribs, 493;
      skull, 484;
      teeth, 441

    _Coenolestes_, _43_, 424

    Cement, 5

    _Centetes_, _49_;
      caudal vertebrae, 454;
      pelvic symphysis, 515;
      spines, 417;
      teeth, 440;
      thoraco-lumbar vertebrae, 450

    Centetidae, _49_;
      auditory ossicles, 488;
      skull, 480

    Centrale, 27;
      see Carpus and Tarsus

    Centre of motion, 448

    Centrum, 14

    _Cephalaspis_, _31_, _55_

    Cephalic shield, armadillos, 419

    _Cephalochordata_, _30_, 51

    _Cephalodiscus_, _30_, 50

    _Ceratodus_, _34_, _70_;
      branchial arches, 124;
      cranium, =125=;
      skeleton, =128=;
      skull, 117, 124;
      spinal column, 113;
      teeth, 111

    Cerato-branchial, cod, 101;
      dogfish, 78;
      duck, 320;
      salmon, 95

    Cerato-hyal, 23;
      cod, 100;
      dog, 399;
      dogfish, 78;
      salmon, 95

    _Ceratophrys_, _36_;
      bony plates of, 168;
      teeth, 170

    _Ceratops_, _39_;
      see _Polyonax_

    Ceratopsia, _39_;
      characters, 209;
      premaxillae, 284

    Ceratopsidae, _39_

    _Ceratosaurus_, _38_, 208;
      supratemporal fossae, 283;
      _C. nasicornis_, skeleton, =206=

    Cercopithecidae, _49_, _373_

    Cervical ribs, crocodile, 260;
      reptiles, 285

    Cervical vertebrae, crocodile, 239;
      dog, 380;
      duck, 307;
      general characters, 15;
      mammals, 442;
      turtle, 219

    Cervidae, _46_;
      skull, 469

    _Cervulus_, _46_;
      pes, 523

    _Cervus_, 46;
      _C. megaceros_, antlers, 469

    _Cestracion 32_;
      calcification of vertebrae 114;
      external branchial arches 121;
      pectoral fin 130;
      skull =118=;
      suspensorium 119;
      teeth 109;
      vertebral column 114

    Cestraciontidae _32_

    Cetacea _44_, 522;
      arm bones 501;
      auditory ossicles 487;
      caudal vertebrae 453;
      cervical vertebrae 444;
      characters 353;
      exoskeleton 416 f.;
      hind limb 518;
      manus 505;
      pectoral girdle 495;
      pelvis 514;
      position of limbs 28;
      ribs 491;
      skull 461 f.;
      sternum 489;
      teeth 426;
      thoraco-lumbar vertebrae 448

    Cetiosauridae _38_

    _Chalcides_ _38_;
      limbs 289

    Chalicotheriidae _46_;
      femur 519;
      manus 509

    _Chalicotherium_ _46_;
      femur 360;
      pes =508=, 525;
      teeth 432

    _Chamaeleon_ _38_, 199 f.;
      epipubis 293;
      ilia 291;
      manus 291;
      skull 278

    Chamaeleonidae _38_

    Charadriidae _42_

    Charadriiformes _42_

    _Chauna_ _41_;
      interorbital septum 333;
      ribs 336;
      _C. derbiana_, spurs 330

    _Chelone_ _37_, _194_;
      plastron 271, =218=;
      see Turtle

    Chelonia _37_;
      beaks 271;
      carapace 271;
      general characters 193;
      humerus 290;
      limbs 290;
      palate 281;
      pectoral girdle 288;
      pelvic girdle 291;
      skull 277 f.;
      tarsus 293;
      vertebrae 275 f.

    Chelonidae _37_

    Chelydae _37_

    _Chelydra_ _37_;
      carpus 26, 291

    Chelydridae _37_

    _Chelys_ _37_, _195_

    Chersidae _37_

    Chevron bones 16;
      crocodile 243;
      mammals 453 f.;
      reptiles 276

    Chevrotain _45_, _359_;
      teeth 429

    _Chimaera_ _32,_ _66_;
      attachment of fins 130;
      pelvic girdle 127;
      skull =65=;
      teeth 110

    Chimaeridae _32_

    Chimaeroidei, general characters 65

    Chimpanzee _49_;
      carpus 512;
      ribs 493;
      thoraco-lumbar vertebrae 450

    _Chinchilla_ _47_;
      auditory ossicles 488

    Chinchillidae _47_

    Chiromyidae _49_

    _Chiromys_ _49,_ _372_;
      manus 512;
      pes 527;
      teeth 441

    Chiroptera _49_;
      auditory ossicles 488;
      arm bones 503;
      cervical vertebrae 446;
      general characters 370;
      manus 512;
      pelvis 515;
      pes 527;
      sacrum 452;
      shoulder girdle 499;
      skull 481;
      sternum 490;
      tail 454;
      teeth 440;
      thigh and shin 520;
      thoraco-lumbar vertebrae 450

    _Chirotes_ _38_;
      limbs 289

    _Chlamydophorus_ _44,_ _272_;
      scutes 419;
      skull 459

    _Chlamydoselache_ _31_;
      branchial arches 121

    _Choeropus_ _43_;
      manus 504;
      pes 522

    _Choloepus_ _43_;
      ribs 491;
      shifting of pelvis 451;
      skull =458=;
      sternum 489;
      thoraco-lumbar vertebrae 447;
      _C. hoffmanni_ cervical vertebrae 443

    Chondrocranium, salmon 87

    Chondroid tissue, _Balanoglossus_ 50

    Chondrostei _32_;
      fins 105;
      general characters 67;
      teeth 110;
      see Cartilaginous ganoids

    Chordal sheath, _Amphioxus_ 52

    Chrysochloridae _49_

    _Chrysochloris_ _49_;
      auditory ossicles 488;
      claws 418;
      teeth 440

    _Ciconia_ _41_;
      see Stork

    Ciconiiformes _41_

    Cingulum 376

    Civet _48,_ _369_;
      teeth 437

    _Cladoselache_ _31,_ _63_; fin 129

    Clasper 132;
      dogfish 82

    Clavicle 25;
      birds 338;
      cod 102;
      duck 322;
      dog 405;
      fish 126;
      frog 163;
      mammals 494 f.;
      reptiles 289

    Claws 3;
      birds 330;
      crocodile 237;
      dog 374;
      duck 302;
      mammals 417;
      turtle 215

    Clupeidae, _33_

    _Clupeus_, _33_

    Cnemial crest, dog, 412;
      duck, 326

    _Coccosteus_, _34_, _70_

    Coccyx, man, 454

    Cochliodontidae, _31_

    _Cochliodus_, _31_;
      dental plates, 109

    Cod, _33_;
      appendicular skeleton, 101 f.;
      cranium, 96;
      mandibular and hyoid arches, =99=;
      median fins, 86;
      pectoral girdle and fin, =102=;
      ribs, 86;
      skull, 96 f.;
      vertebral column, 83 f.

    _Coelogenys_, _48_;
      zygomatic arch, 477

    _Coenolestes_, _43_, 424

    Coffer-fish, _33_;
      see, _Ostracion_

    _Colobus_, _49_;
      pollex, 512

    Colubridae, _38_

    Columbae, _42_

    Columbidae, _42_

    Columella, crocodile, 251;
      duck, 320;
      frog, 157;
      turtle, 228

    Columella cranii, 200 n;
      see epipterygoid

    Colymbi, _40_

    Colymbiformes, _40_

    Compsognathidae, _38_

    _Compsognathus_, _38_, 208

    Condylar ridge, duck, 326

    Condyle of humerus, dog, 406;
      of mandible, dog, 398

    Condylarthra, _47_;
      femur, 519;
      general characters, 361;
      manus, 509;
      skull, 472;
      teeth, 432

    Contour feather, duck, 303

    Copula, 23

    _Coracias_, _42_;
      see Roller

    Coraciae, _42_

    Coraciiformes, _42_

    Coracoid, 25;
      cod, 103;
      crocodile, 263;
      duck, 322;
      frog, 163;
      Monotremata, 493;
      newt, 147;
      reptiles, 288;
      turtle, 232

    Coracoid groove, duck, 321

    Cormorant, _41_;
      foot, 342;
      skull, 335

    Cornu, see hyoid

    Cornua trabeculae, 18

    _Coryphodon_, _47_;
      femur, 519;
      manus, 510;
      pes, 525;
      skull, 473;
      teeth, 433;
      _C. hamatus_, manus, =510=

    Coryphodontidae, _47_

    Costal plate, turtle, 215;
      -- process, duck, 321;
      -- shield, turtle, 214

    Cotyloid bone, 25, 513;
      see Acetabular bone

    Cotylopidae, _45_

    _Cotylops_, _45_;
      pollex, 506;
      skull, 468

    Coverts, 306, 328

    Cranium, 18;
      cod, 96 f.;
      crocodile, 244 f.;
      development of, 16 f.;
      dog, 384 f.;
      dogfish, 73 f.;
      duck, 314;
      frog, 154 f.;
      newt, 140 f.;
      turtle, 222 f.

    Cranio-facial axis, dog, 384

    Creodonta, _48_;
      carpus, 512;
      femur, 520;
      general characters, 368;
      skull, 479;
      teeth, 439

    Cribriform plate, dog, 388, 400

    Crocodile, _210_, _212_;
      anterior limb, 263;
      exoskeleton, 237;
      pectoral girdle, 262;
      pelvic girdle, 266;
      posterior limb, 268;
      ribs and sternum, 259;
      skeleton, 237 f.;
      skull, 243 f.;
      tarsus, 293;
      teeth, 238;
      vacuities in surface of cranium, 256;
      vertebral column, 239

    Crocodilia, _39_;
      general characters of, 210;
      palate, 281;
      skull, 277 f.;
      succession of teeth, 274;
      teeth, 273

    Crocodilidae, _39_

    _Crocodilus_, _39_;
      _C. palustris_, sternum and associated bones, =261=;
      late thoracic and first sacral vertebrae, =242=;
      _C. vulgaris_, cervical vertebrae, =239=

    Crossopterygii, _33_;
      general characters, 68

    Crotalidae, _38_

    _Crotalus_, _38_;
      jaws 280;
      see Rattlesnake

    Crows, _42_

    Crura of stapes, dog, 393

    Cruro-tarsal, ankle joint, 345

    Crus, 26;
      crocodile, 268;
      dog, 412;
      duck, 326;
      frog, 166;
      newt, 149;
      turtle, 235

    Crusta petrosa, 5

    _Cryptobranchus_, _35_, 135;
      skull, 175;
      _C. lateralis_, sacral vertebrae, 171

    Cryptodira, _37_;
      characters, 194

    Ctenoid scales, 8, 60, 105

    Cubitals, 303 f.

    Cuboid, 27;
      dog, 415

    Cuckoo, foot, 342

    Cuculi, _42_

    Cuculiformes, _42_

    Cuneiform bones, 27;
      dog, 414 f.

    _Cyclodus_, _38_;
      see _Tiliqua_

    Cycloid scales, 8, 60, 105;
      cod, 83

    _Cyclopidius_, _45_;
      skull, 468

    Cyclospondyli, 114

    Cyclostomata, _31_;
      general characters, 53

    _Cycloturus_, _44_;
      hallux, 522;
      manus, 505

    _Cygnus_, _41_;
      see Swan

    _Cynocephalus_, _49_;
      cervical vertebrae, 446;
      skull, 482

    Cynoidea, _48_, _369_;
      dental formula, 437

    _Cynognathus_, _36_;
      occipital condyle, 277;
      teeth, 273

    Cyprinidae, _33_

    _Cyprinus_, _33_;
      see Carp

    Cypseli, _42_

    Cypselidae, _42_;
      see Swifts

    Cystignathidae, _36_


    _Dactylopterus_, _34_;
      pectoral fins, 131

    Dasypodidae, _44_

    _Dasyprocta_, _48_;
      auditory ossicles, 488;
      thoraco-lumbar vertebrae, 450

    Dasyproctidae, _48_

    _Dasypus_, _44_;
      manus, 505;
      skull, 459;
      stapes, 487;
      teeth, 424

    Dasyuridae, _43_, _350_;
      dentition, 423;
      pes, 521;
      skull, 456

    Deer, _359_;
      manus, 507;
      pes, 523;
      Chinese water --, _46_,
        see _Hydropotes_;
      Musk --, _46_,
        see _Moschus_;
      Red -- fibula, 519

    Delphinidae, _45_

    _Delphinus_, _45_, 357;
      lumbar vertebrae, 448;
      skull, 462 f.

    Deltoid ridge, crocodile, 263;
      dog, 406;
      frog, 164

    _Dendrohyrax_, 363

    Dental formula, regular, 344, 422;
      Anthropoidea, 441;
      _Astrapotherium_, 432;
      _Babirussa_, 428;
      Camel, 428;
      _Chiromys_, 441;
      Chiroptera (many), 440;
      Cynoidea, 437;
      _Dinotherium_, 434;
      Dog, 376;
      Duplicidentata, 435;
      _Elephas_, 434;
      _Erinaceus_, 440;
      _Felis_, 437;
      _Galeopithecus_, 440;
      _Hippopotamus_, 427;
      Horse, 430;
      _Hydromys_, 436;
      Hyracoidea, 362;
      Macropodidae, 423;
      _Manatus_, 425;
      _Notoryctes_, 423;
      _Otaria_, 439;
      _Procavia_, 432;
      _Pteropus_, 441;
      Rodentia (most), 435;
      Ruminantia, 429;
      _Squalodon_, 427;
      _Sus_, 428;
      Tapiridae, 429;
      _Thylacinus_, 423;
      _Uintatherium_, 433;
      _Ursus_, 439;
      _Zeuglodon_, 426

    Dentary, 22;
      crocodile, 258;
      cod, 100;
      duck, 320;
      frog, 161;
      newt, 144;
      salmon, 94;
      turtle, 230

    Dentine, 5

    Derbian Screamer, spurs, 330

    Dermal exoskeleton, crocodile, 237;
      fish, 105;
      mammals, 419;
      reptiles, 271;
      turtle, 215

    Dermo-supra-occipital, Labyrinthodontia, 177;
      _Polypterus_, 122

    Dermochelydidae, _37_

    _Dermochelys_, _37_, _194_, 214, 270;
      carapace and plastron, 272

    Dermoptera, _48_;
      general characters, 370

    Derotremata, _35_

    _Desmodus_, _49_;
      teeth, 441

    Desmognathous, 319, 335

    Development of bone, 10;
      of cranium, 16;
      of teeth, 7

    _Dicynodon_, _36_, _192_;
      beak, 271;
      supratemporal fossa, 283;
      teeth, 273

    Didelphia, _43_;
      general characters, 349

    Didelphyidae, _43_, _350_;
      auditory ossicles, 486;
      pes, 521;
      teeth, 423

    _Didelphys_, _43_;
      atlas, 443;
      teeth, 422

    _Didus_, _42_;
      see Dodo

    Digitigrade, defined, 358 n.

    Digits, _26_;
      see Manus and Pes

    _Dimetrodon_, _36_;
      thoracic vertebrae, 276

    _Dinichthys_, _34_, _70_

    Dinocerata, 364;
      see Uintatheriidae

    Dinornithes, _40_, _299_;
      see Moas

    Dinosauria, _38_;
      general characters, 204;
      humerus, 290;
      ischium, 291;
      pectoral girdle, 288;
      pes, 293;
      pre-orbital vacuity, 284;
      ribs, 285;
      vertebrae, 275 f.

    Dinotheriidae, _47_

    _Dinotherium_, _47_, 365;
      dental formula, 434;
      teeth, 345

    _Diodon_, _33_;
      beaks, 111;
      _D. hystrix_, scales, 105

    Diphycercal tail, 60, 116

    Diphyodont, defined, 7, 344

    _Diplacanthus_, _32_, _64_

    Dipneumona, _34_

    Dipnoi, _34_;
      general characters, 69;
      pelvic fins, 131;
      skull, 124;
      spinal column, 113;
      tail, 116;
      teeth, 111

    Dipodidae, _47_

    Diprotodont, 423

    Diprotodontia, _43_;
      characters, 350

    Dipteridae, _34_, _70_;
      cranium, 124;
      tail, 117;
      teeth, 111

    _Dipus_, _47_;
      cervical vertebrae, 446;
      pes, 526

    Discoglossidae, _36_

    _Discoglossus_, _36_;
      ribs, 182;
      vertebrae, 172

    Distal, defined, 23 n.

    Divers, _40_;
      thoracic vertebrae, 332

    _Docidophryne_, _36_;
      shoulder girdle and sternum, =183=

    Dodo, _42_;
      wing, 338

    Dog, _48_;
      arm bones, =407=;
      anterior limb, 405;
      atlas and axis, =379=;
      cranium, 384, =389=, =396=;
      dentition, =375=;
      innominate bone, =410=;
      leg bones, =411=;
      manus, 408, =413=, 511;
      pectoral girdle, 404;
      pelvic girdle, 409;
      pes, =413=, 414;
      posterior limb, 412;
      ribs, 402;
      second lumbar vertebra, =382=;
      second thoracic vertebra, =382=;
      skull, 383, =387=;
      sternum, =403=, 404;
      vertebral column, 378

    Dogfish, _64_;
      cranium, 73;
      exoskeleton, 71;
      median fins, 79;
      pectoral girdle and fin, 79;
      pelvic girdle, 81;
      pelvic fin, 81;
      ribs, 73;
      skull, 73, =75=;
      vertebral column, 72;
      visceral skeleton, 77;
      Spotted and Spiny --, _32_

    Dolphin, _45_, _357_;
      lumbar vertebrae, 448;
      Gangetic --, _45_,
        see _Platanista_

    Donkey, skull, =431=

    _Dorcatherium_, _45_;
      manus, 507

    Dorsal vertebra, 16

    Dorsal shield, crocodile, 238

    Down feathers, 306

    _Draco_, _38_;
      ribs, 286

    Dromaeognathous, 335

    _Dromaeus_, _40_, _299_;
      see Emeu

    Duck, _41_, 334;
      beak, 329;
      claws, 330;
      cranium, =313=;
      exoskeleton, 302;
      pectoral girdle, 321;
      pelvic girdle, 324, =311=, =325=;
      pes, 327;
      posterior limb, 326;
      ribs, 320;
      skull, 312, =312=, =313=;
      sternum, 321;
      vertebral column, 307;
      wing, 322, =304=, =305=

    Duckbill, _43_;
      see _Ornithorhynchus_

    Dugong, _44_;
      humerus, 501;
      pelvis, 514;
      thoraco-lumbar vertebrae, 448;
      see _Halicore_

    Duplicidentata, _48_, 366;
      dental formula, 435;
      skull, 478


    Eagles, 335

    Eared Seals, _369_;
      scapula, 498;
      see Otariidae

    _Echidna_, _43_;
      caudal vertebrae, 453;
      manus, 504;
      pelvis, 513;
      sacral vertebrae, 451;
      shoulder-girdle and sternum, =494=;
      skull, 455;
      spines, 417;
      spur, 418;
      thoraco-lumbar vertebrae, 447

    Echidnidae, _43_;
      pes, 521

    Ectethmoid, 21 n.

    Ectocondylar ridge, dog, 406

    Edentata, _43_;
      auditory ossicles, 487;
      arm bones, 500;
      caudal vertebrae, 453;
      cervical vertebrae, 443;
      manus, 504;
      pectoral girdle, 495;
      pes, 522;
      pelvis, 513;
      ribs, 491;
      sacrum, 452;
      skull, 457;
      sternum, 489;
      teeth, 424;
      thigh and shin, 517;
      thoraco-lumbar vertebrae, 447

    Eel, _33_;
      scales, 105

    Elasmobranchii, _31_;
      cranium, 118 f.;
      clasper, 132;
      general characters, 61;
      pelvic fins, 131;
      ribs, 125;
      teeth, 109;
      vertebral column, 113 f.;
      visceral arches, 120

    _Elasmotherium_, _46_;
      mesethmoid, 470

    Elephant, _47_;
      auditory ossicles, 487;
      caudal vertebrae, 453;
      ribs, 491;
      skull, 473 f., =474= and =475=;
      tusks, 420;
      see also Proboscidea

    Elephantidae, _47_

    _Elephas_, _47_, _364_;
      dental formula, 434;
      _E. planifrons_, 435;
      see Elephant

    _Elginia_, _36_;
      skull, 191, 283

    Embolomerous, 172

    Emeu, _40_, _299_;
      aftershaft, 328;
      claws, 330

    Enamel, 4;
      -- cap, 7;
      -- organ, 7

    Endochondral ossification, 11

    Endoskeleton, Amphibia, 170;
      birds, 331 f.;
      cod, 83 f.;
      crocodile 239, f.;
      dog, 377 f.;
      dogfish, 71 f.;
      duck, 306 f.;
      fish, 112 f.;
      frog, 151 f.;
      mammals, 442 f.;
      newt, 138 f.;
      reptiles, 275 f.;
      turtle, 218 f.

    Engystomatidae, _36_

    Entoplastron, turtle, 217

    Epanorthidae, _43_, _350_

    Epi-branchial, cod, 101;
      dogfish, 78;
      salmon, 94

    Epicoracoid, 25;
      frog, 163;
      turtle, 232;
      Monotremes, 493;
      vestiges of in Rodentia, 497

    _Epicrium_, _35_;
      orbit, 179

    Epidermal exoskeleton, birds, 328;
      crocodile, 237;
      dog, 374;
      duck, 302;
      mammals, 416;
      reptiles, 270;
      turtle, 214

    Epi-hyal, cod, 100;
      dog, 399;
      salmon, 94

    Epi-otic, 20;
      cod, 96;
      crocodile, 250;
      Labyrinthodontia, 177;
      reptiles, 278;
      salmon, 89;
      turtle, 227

    Epiphysis, 11

    Epiplastron, turtle, 217

    Epiprecoracoid, Amphibia, 184;
      turtle, 232

    Epipterygoid, Lacertilia, 200;
      reptiles, 278

    Epipubis, crocodile, 267;
      newt, 149;
      turtle, 235

    Episternum, 217;
      frog, 163

    Equidae, _46_;
      mane, 416;
      scapula, 496;
      skull, 471

    _Equus_, _46_;
      see Horse

    Erinaceidae, _49_

    _Erinaceus_, _49_;
      dental formula, 440;
      pelvic symphysis, 515;
      presternum, 490;
      see Hedgehog

    Esocidae, _33_

    Esox, _33_;
      attachment of teeth, 107

    Ethmoid, 394;
      see median ethmoid

    Ethmoidal plane, 390;
      -- region, 21

    Ethmo-palatine ligament, dogfish, 77

    Ethmo-turbinal, dog, 395

    _Euchirosaurus_, _35_;
      vertebrae, 171

    Eustachian canal, see Canal

    Eusuchia, _39_;
      general characters, 212

    Eutheria, _43_;
      general characters, 351

    _Exocaetus_, _33_;
      pectoral fins, 131

    Exoccipital, 19;
      cod, 97;
      crocodile, 246;
      dog, 386;
      duck, 314;
      frog, 154;
      newt, 141;
      salmon, 89;
      turtle, 224

    Exoskeleton, 2;
      Amphibia, 168;
      birds, 328;
      crocodile, 237;
      dog, 374;
      dogfish, 71;
      duck, 302;
      fish, 104;
      ganoids, 66;
      mammals, 442;
      reptiles, 270;
      turtle, 214

    Extensor side, defined, 29

    Extra-branchial, dogfish, 79

    Extra-columella, crocodile, 251;
      turtle, 228


    Fabella, dog, 412

    _Falco_, _41_

    Falcon _41_, 335

    Falconiformes, _41_

    Feathers, 3, 328;
      duck, 302

    Felidae, _48_;
      claws, 418

    _Felis_, _48_;
      dental formula, 437;
      thoraco-lumbar vertebrae, 450

    Femoral shield, turtle, 215

    Femur, 26;
      crocodile, 268;
      dog, 412;
      duck, 326;
      frog, 166;
      mammals, 517 f.;
      newt, 149;
      ox and rhinoceros, =518=;
      turtle, 235

    Fenestra ovalis, crocodile, 250 f.;
      dog, 392;
      duck, 316;
      frog, 157;
      turtle, 228;
      -- rotunda, dog, 392;
      duck, 316

    Fenestral recess, duck, 316

    Fibula, 26;
      crocodile, 268;
      dog, 412;
      duck, 327;
      frog, 166;
      newt, 149;
      turtle, 235

    Fibulare, 27;
      see Tarsus

    File-fish, _33_;
      see _Balistes_

    Filoplume, 306

    Finches, _42_

    Fins, fish, 115;
      caudal --, Cetacea, 453;
      cod, 87;
      fish, 116;
      Ichthyosauria, 195;
      median --, cod, 86;
      dogfish, 79;
      pectoral --, cod, 103;
      dogfish, 79;
      pelvic --, cod, 103;
      dogfish, 81

    Fin-rays, 105, 115;
      cod, 83, 103;
      dogfish, 79

    Firmisternia, _36_, 185

    Fish, appendicular skeleton, 126;
      endoskeleton, 112 f.;
      exoskeleton, 104;
      general characters, 60 f.;
      paired fins, 127 f.;
      ribs, 125 f.;
      skull, 117 f.;
      spinal column, 112 f.;
      teeth, 106 f.

    Fissipedia, _48_;
      general characters, 368

    Flamingo, 335

    Flexor side, defined, 29

    Floating ribs, dog, 402;
      mammals, 490

    Flower, Sir W.H., on succession of teeth in elephants, 434

    Flying-fish, _33_;
      -- fox, _49_, _371_,
        skull, 481,
        see _Pteropus_;
      -- gurnard, _34_,
        see _Dactylopterus_;
      -- lemur, _48_,
        see _Galeopithecus_;
      -- lizard, _38_,
        see _Draco_

    Fontanelle, salmon, 89;
      anterior --, dogfish, 74;
      frog, 154;
      posterior --, frog, 154

    Foot, crocodile, 269;
      dog, 414;
      frog, 167;
      newt, 149 f.;
      turtle, 236

    Foramen:
      anterior palatine --, dog, 401;
      condylar --, dog, 401;
      -- cordiforme, reptiles, 292;
      ect-epicondylar --, _Sphenodon_, 290;
      ent-epicondylar --, 191 n.;
      Carnivora vera, 502;
      Cebidae, 503;
      _Choloepus_, 500;
      Condylarthra, 362, 502;
      Creodonta, 368;
      Insectivora, 503;
      Lemurs, 503;
      Marsupials, 500;
      reptiles, 290;
      external mandibular --, crocodile, 258;
      inferior dental --, dog, 399;
      infra-orbital --, dog, 401;
      Rodents, 477;
      ilio-sciatic --, duck, 325;
      internal mandibular --, crocodile, 258;
      internal orbital --, dog, 401;
      interparietal --, Labyrinthodontia, 173, 177;
      reptiles, 277;
      -- lacerum anterius, dog, 388, 400;
      -- lacerum medium, dog, 402;
      -- lacerum posterius, dog, 392, 401;
      lachrymal --, dog, 394, 401;
      -- magnum, cod, 97;
      crocodile, 257;
      dog, 386, 402;
      dogfish, 76;
      duck, 314;
      frog, 154;
      newt, 141;
      salmon, 89;
      turtle, 224;
      mental --, dog, 399;
      obturator --, duck, 326;
      dog, 410;
      ophthalmic --, dogfish, 74;
      optic --, dog, 400;
      dogfish, 74;
      orbitonasal --, dogfish, 74;
      -- ovale, crocodile, 249;
      dog, 400;
      pneumatic --, duck, 323;
      pneumogastric --, dogfish, 76;
      posterior palatine --, dog, 401;
      postglenoid --, dog, 402;
      pre-acetabular --, Chiroptera, 515;
      -- rotundum, dog, 400;
      stylomastoid --, dog, 392 f., 400;
      thyroid --, dog, 410;
      trigeminal --, duck, 316;
      -- triosseum, duck, 322

    Fore-arm, 26;
      crocodile, 265;
      dog, 406;
      duck, 323;
      frog, 164;
      newt, 147;
      turtle, 233

    Fossa:
      cerebellar --, dog, 392;
      cerebral --, dog, 392;
      digital --, dog, 412;
      floccular --, dog, 392;
      infratemporal --, see lateral temporal --;
      lachrymal --, Ruminants, 469;
      lateral temporal --, crocodile, 257;
      _Sphenodon_, 283;
      olecranon --, dog, 406;
      prescapular --, dog, 405;
      postscapular --, dog, 405;
      post-temporal --, _Sphenodon_, 283;
      pterygoid --, crocodile, 257;
      suborbital --, Ruminants, 469;
      supra-acetabular --, Ruminants, 514;
      supratemporal --, crocodile, 249, 256;
      reptiles, 283;
      supra-trochlear --, dog, 406;
      temporal --, dog, 398;
      trochanteric --, dog, 412

    Fowl, _41_, 335;
      claws, 330;
      skeleton, =301=

    Fox, _48_

    Frigate bird, _41_;
      clavicles, 338

    Frog, anterior limb, 164;
      cranium, =155=, =157=;
      hyoid apparatus, 161;
      pelvic girdle, 165;
      posterior limb, 166;
      shoulder-girdle and sternum, =183=;
      skull, 154 f., =155=, =159=;
      teeth, 151;
      vertebral column, 152;
      Common --, Edible --, Fire-bellied --, Green-tree --,
       Horned --, Midwife --, Painted -- and Toad --, _36_;

    Frontal, 19;
      cod, 96;
      crocodile, 249;
      dog, 388;
      duck, 314;
      newt, 141;
      salmon, 91;
      turtle, 225;
      -- segment, crocodile, 249;
      dog, 388;
      turtle, 225

    Fronto-parietal, frog, 156

    Frugivorous bats, manus 512;
      see Pteropidae

    Fulcra 67;
      _Polypterus_ 106

    Furcula 296; duck 322


    Gadidae _33_

    _Gadus_ _33_;
      see Cod

    Galeopithecidae _48_

    _Galeopithecus_ _48_, _370_;
      dental formula 440;
      inter centra 450;
      pelvic symphysis 515;
      skull 480

    _Galesaurus_ _36_, _192_; teeth 273

    _Galeus_, _32_;
      occipital joint 118

    Galli _41_

    Galliformes _41_

    _Gallus_ _41_;
      _G. bankiva_ skeleton =301=

    Gannet, _41_; wing =339=

    Ganoid scales 8, 60, 104

    Ganoidei _32_;
      general characters and distribution 66;
      pectoral girdle 126;
      pelvic fins 132;
      teeth 110;
      skull 121 f.;
      spinal column 112 and 114

    Garialidae _39_

    _Garialis_ _39_, _212_

    Garial 210

    Gar pike 33;
      see _Lepidosteus_

    _Gavialis_ _39_

    _Gazella_ _46_

    Gazelle _46_;
      skull 468

    Geckonidae _37_;
      see Gecko

    Gecko _37_;
      epipubis 293;
      parietals 277;
      supratemporal fossa 283;
      vertebrae 275

    Gibbon _49_;
      ribs 493;
      skull 482

    Gill-rays, dogfish 78;
      salmon 95

    _Giraffa_ _46_

    Giraffe _46_, _359_;
      cervical vertebrae 445;
      manus 507;
      pes 523;
      ulna 501

    Giraffidae _46_; skull 469

    Girdle bone, frog 156

    Glenoid cavity 25;
      crocodile 263;
      dog 405;
      duck 322;
      frog 162;
      newt 146;
      turtle 232;
      --fossa, dog 394

    Globe-fish _33_

    _Globicephalus_ _45_;
      cervical vertebrae 354;
      manus 506;
      skull 463

    Gluteal surface of ilium, dog 410

    _Glyptodon_ _44_;
      carapace 419;
      cervical vertebrae 443;
      caudal vertebrae 453;
      manus 505;
      pelvis 513;
      pes 522;
      teeth 425;
      thoraco-lumbar vertebrae 447

    Glyptodontidae _44_, _352_;
      skull 459;
      see also _Glyptodon_

    Gnathostomata _31_, _59_

    Golden mole _49_;
      see Chrysochloris

    Goniopholidae _39_

    _Goniopholis_ _39_;
      vertebrae 275

    Goose 334;
      beak 329;
      Spur-winged--41

    _Gorilla_ _49_;
      carpus 512;
      ribs 493;
      scapula 499;
      skull =483=;
      thoraco-lumbar vertebrae 450

    Gruidae _41_;
      see Cranes

    Gruiformes _41_

    Guinea-pig _48_;
      tail 454

    Gular shield, turtle 215

    Gulls _42_, 335;
      aftershaft 328

    Gymnodontidae _33_;
      beaks 111

    Gymnophiona _35_;
      branchial arches 180;
      general characters 136;
      ribs 182;
      scales 168;
      skull 177;
      teeth 169;
      vertebrae 172

    _Gymnura_ _49_;
      teeth 440;
      zygomatic arch 481

    _Gypogeranus_ _41_;
      claws 330

    _Gyrinophilus_ _35_;
      vertebral column 171


    Haddock _33_

    Hadrosauridae _39_

    _Hadrosaurus_ _39_;
      skull 284

    Hag or hag-fish _31_, 54 f.

    Hair 3;
      dog 374;
      mammals 416

    _Halicore_ _44_, _352_;
      manus 505;
      skull 460;
      teeth 425;
      see Dugong

    Halicoridae _44_

    Halitheriidae _44_

    _Halitherium_ _44_, 352;
      femur 518;
      pelvis 514;
      teeth 425

    Hallux 26;
      dog 415;
      duck 327;
      frog 167

    Hamular process, dog 397

    Hand, crocodile 266;
      dog 408 f.;
      duck 324;
      frog 165;
      newt 147;
      turtle 233

    _Hapale_ _49_

    Hapalidae _49_, 372 f.;
      teeth 441

    Hare _48_, 366;
      acetabular bone 515;
      dental formula 435;
      femur 520;
      humerus 502;
      pelvis 515;
      scapula 497;
      skull 476;
      tail 454;
      thoraco-lumbar vertebrae 449;
      Cape jumping --, _47_;
      see _Pedetes_

    _Harpagus_, _41_;
      serrated beak, 334

    _Harriotta_, _32_, _66_

    _Hatteria_, _37_, _197_;
      see _Sphenodon_

    Haversian canals, 10;
      -- system, 10

    Hawks, beak of, 330

    Hedgehog, _49_, _370_;
      auditory ossicles, 488;
      humerus, 503;
      presternum, 490;
      skull, 480;
      spines, 417;
      see _Erinaceus_

    Hemichordata, _30_, _50_

    _Heptanchus_, _31_;
      branchial arches, 63, =120=;
      vertebrae, 114

    Herbivorous dentition, 427, 430

    Heron, _41_, 335;
      interorbital septum, 333;
      powder down feathers, 329

    Herring, _33_

    _Hesperornis_, _40_, _299_;
      caudal vertebrae, 333;
      clavicles, 338;
      teeth, 330;
      wing, 338

    Heterocercal tail, 60, 116

    Heterodont, 7

    Heterostraci, _31_;
      general character, 54

    _Hexanchus_, _31_;
      branchial arches, 63, 121

    Hinge joint, 13

    _Hipparion_, _46_;
      manus, 508;
      pes, =524=

    Hippopotamidae, _45_

    _Hippopotamus_, _45_, _359_;
      dental formula, 427;
      hair, 416;
      mandible, 467;
      manus, 506;
      pes, 523;
      scapula, 496;
      skull, 467;
      teeth, 345

    Hoatzin, _41_;
      see _Opisthocomus_

    Holocephali, _32_, 65, 104;
      clasper, 132;
      spinal column, 113;
      tail, 116;
      teeth, 109

    Holoptychiidae, _33_

    _Holoptychius_, _33_;
      scales, 105

    Holostei, _33_;
      general characters, 68;
      teeth, 110;
      see Bony Ganoids

    Hominidae, _49_, _373_

    _Homo_, _49_;
      see Man

    Homocercal tail, 60, 69, 117;
      codfish, 87

    Homodont, defined, 7

    Hoofs, 3, 418

    Hoopoe, _42_, 335

    _Hoplopterus_, spur, 330

    Hornbill, _42_, 331;
      bony crest, 334;
      interorbital septum, 333

    Horns, 3, 417

    Horny plates on palate, 418;
      -- teeth, Lampreys, 4;
      Myxinoids, 57;
      _Ornithorhynchus_, 4

    Horse, _46_, _360_;
      fibula, 519;
      malleus, 487;
      manus, 507;
      pes, =524=;
      skull, 471;
      teeth, 345, 430;
      ulna, 501

    Howling monkey, _49_;
      see _Mycetes_

    Humerals, duck, 303 f.

    Humeral shield, turtle, 215

    Humerus, 26;
      crocodile, 263;
      dog, 405;
      duck, 323;
      frog, 164;
      newt, 147;
      turtle, 232;
      wombat, =500=

    Humming-birds, _42_, 335

    Humpbacked whale, _44_, _357_

    _Hyaena_, _48_, _369_;
      hallux, 526;
      pollex, 511;
      sacral vertebrae, 452;
      teeth, 437

    Hyaenidae, _48_;
      humerus, 502

    _Hyaenodon_, _48_, _368_

    Hyaenodontidae, _48_

    Hyaline cartilage, 10

    _Hydrochaerus_, _48_;
      teeth, 437;
      see Capybara

    _Hydromys_, _47_;
      dental formula, 436

    Hydrophidae, _38_;
      scales, 270

    _Hydropotes_, _46_;
      canines, 429

    _Hyla_, _36_;
      fronto-parietal fontanelle, 179;
      sternum, 184

    Hylidae, _36_

    _Hylobates_, _49_;
      ribs, 493;
      skull, 482

    Hyoid, 21;
      alligator, =285=;
      Amphibia, 180;
      birds, 336;
      cod, 100;
      crocodile, 259;
      dogfish, 77;
      dog, 399;
      duck, 320;
      frog, 161;
      newt, 144;
      reptiles, 284;
      salmon, 94;
      turtle, 231, =285=

    Hyomandibular, 23;
      cod, 100;
      dogfish, 78;
      salmon, 94

    _Hyomoschus_, _45_

    Hyoplastron, turtle, 217

    Hyostylic, 61, 119

    _Hyotherium_, _45_;
      teeth, 427

    _Hyperodapedon_, _37_, _198_;
      premaxillae, 284

    _Hyperoödon_, _44_;
      skull, 464;
      sternum, 489;
      thoracic vertebrae, 448

    Hypo-branchial, cod, 101;
      dogfish, 78

    Hypo-hyal, cod, 100;
      salmon, 95

    Hypo-ischium, Lacertilia, 292

    Hypoplastron, turtle, 217

    Hyporachis, 328

    _Hypsilophodon_, _39_;
      predentary bone, 284

    Hypsodont, defined, 345, 429

    Hypural bone, cod, 85

    Hyracidae, _47_

    Hyracoidea, _47_;
      femur, 519;
      general characters, 362;
      manus, 510;
      nails, 418;
      skull, 472;
      teeth, 432

    _Hyracotherium_, _46_;
      manus, 508;
      scapula, 496

    _Hyrax_, _47_, _363_;
      see _Procavia_

    Hystricidae, _47_

    Hystricomorpha, _47_;
      auditory ossicles, 488

    _Hystrix_, _47_;
      auditory ossicles, 488;
      see Porcupine


    Ichthyodorulites, 106

    Ichthyoidea, _35_;
      general characters, 134

    Ichthyopsida, _31_;
      general characters, 59

    Ichthyopterygium, 130

    _Ichthyornis_, _40_;
      mandible, 335;
      pelvis, 341;
      teeth, 330;
      vertebrae, 332

    Ichthyornithiformes, _40_, 300

    Ichthyosauria, _37_;
      general characters, 195;
      ribs, 285

    Ichthyosauridae, _37_

    _Ichthyosaurus_, _37_, _197_;
      limbs, 290;
      palatines, 281;
      pectoral girdle, 288;
      position of limbs, 28;
      skull, =196=;
      teeth, 273;
      vertebral column, 275

    Ichthyotomi, _31_;
      general characters, 62;
      fins, 130 f.;
      tail, 116

    _Iguana_, _38_;
      teeth, 273

    Iguanidae, _38_;
      zygosphenes, 200, 276

    _Iguanodon_, _39_, 208 f.;
      jaws, 292;
      predentary, 284;
      sternum, 288;
      teeth, 272 f.;
      vertebrae, 275

    Iguanodontidae, _39_

    Iliac surface of ilium, dog, 410

    Ilium, 25;
      crocodile, 266;
      dog, 409;
      duck, 325;
      frog, 165;
      mammals, 513 f.;
      newt, 149;
      reptiles, 291;
      turtle, 235

    Incisors, dog, 376 f.;
      mammals, 344

    Incus, dog, 393;
      man, dog and rabbit, =485=

    Infra-marginal shield, turtle, 215

    Infra-pharyngeal bone, cod, 101

    _Inia_, _45_;
      cervical vertebrae, 444;
      lumbar vertebrae, 448

    Innominate bone, dog, 409;
      mammals, 513

    Insectivora, _48_;
      arm bones, 503;
      auditory ossicles, 488;
      cervical vertebrae, 446;
      general characters, 369 f.;
      manus, 512;
      pelvis, 515;
      pes, 527;
      sacrum, 452;
      shoulder-girdle, 499;
      skull, 480;
      sternum, 490;
      tail, 454;
      teeth, 440;
      thigh and shin, 520;
      thoraco-lumbar vertebrae, 450

    Insectivora vera, _49_;
      general characters, 370

    inter centra, 15;
      _Galeopithecus_, 370;
      Ichthyosauria, 195;
      Labyrinthodontia, 172;
      _Sphenodon_, 198;
      _Talpa_, 450

    Interclavicle, 25;
      crocodile, 263;
      Monotremata, 494;
      reptiles, 289

    Intercondylar notch, dog, 412

    Intergular shield, turtle, 215

    Interhyal, cod, 100

    Intermedium, 27;
      see Carpus and Tarsus

    Intermuscular bones, cod, 86

    Internasal septum, dogfish, 76

    Interorbital septum, birds, 333;
      crocodile, 247;
      duck, 317;
      reptiles, 277

    Interspinous bones, cod, 86

    Intertarsal ankle joint, 190

    Intervertebral discs, 15, 378

    Ischial tuberosity, dog, 411

    Ischium, 25;
      crocodile, 266;
      dog, 410;
      duck, 325;
      frog, 165;
      newt, 149;
      turtle, 235

    _Ischyodus_, _32_, _66_

    Ivory, 5


    Jacana, _42_;
      see _Parra_

    _Jacare_, _39_;
      scutes, 271

    Jaws, 21;
      cod, 98 f.;
      crocodile, 252 f.;
      dog, 395 f.;
      dogfish, 77;
      duck, 317 f.;
      frog, 158 f.;
      newt, 143 f.;
      salmon, 93 f.;
      turtle, 229 f.

    Jerboa, _47_;
      cervical vertebrae, 446;
      pes, 526

    Joints, kinds of, 13

    Jugal, 22;
      crocodile, 255;
      dog, 398;
      duck, 318;
      turtle, 229

    Jugulares, 132


    Kangaroo, _43_;
      dental formula, 423;
      lumbar vertebrae, 447;
      pectineal process, 513;
      pes, 522;
      tail, 453;
      teeth, 345

    Kestrel, claws, 330

    Killer, _45_;
      see _Orca_

    Kiwi, _40_;
      see _Apteryx_

    Knee-cap, see patella

    Koala, _43_;
      lumbar vertebrae, 447;
      pes, 522;
      tail, 453

    Kükenthal, W., on teeth of Cetacea, 426;
      on teeth of Marsupials, 422


    Labial cartilage, dogfish, 77;
      _Squatina_, 119

    Labridae, _33_

    _Labrus_, _33_;
      see Wrasse

    Labyrinthodontia, _35_;
      buckler, 168;
      general characters, 135;
      interparietal foramen, 173;
      pelvis, 187;
      ribs, 182;
      skull, 176;
      teeth, 169

    Lacertilia, _37_;
      general characters, 199;
      pectoral girdle, 288;
      skull, 277;
      vertebrae, 275

    Lachrymal, 20;
      cod, 97;
      crocodile, 251;
      dog, 394;
      duck, 317;
      salmon, 93

    Lacunae, 10

    _Lagenorhynchus_, _45_;
      skull, 462

    _Lagostomus_, _47_;
      maxillae, 477

    Lambdoidal crest, duck, 315

    Lamella of malleus, dog, 393

    Lamnidae, _32_

    Lamprey, _31_, 55 f.

    Lancelet, _30_;
      see _Amphioxus_

    Laridae, _42_;
      see Gulls

    Larks, _42_

    Larvacea, _30_;
      notochord, 51

    _Latax_, _48_;
      pes, 526

    Lateral ethmoid, 21;
      cod, 97;
      salmon, 89 f.

    Leathery turtle, _37_;
      see _Dermochelys_

    Lemuroidea, _49_;
      caudal vertebrae, 454;
      general characters, 372;
      nails, 418;
      ribs, 493;
      sacrum, 452;
      skull, 482;
      thoraco-lumbar vertebrae, 450;
      see Lemurs

    Lemurs, carpus, 512;
      pes, 527;
      teeth, 441;
      see Lemuroidea

    Lenticular, 485;
      dog, 393

    _Lepidosiren_, _34_, _70_;
      branchial arches, 125;
      fins, 130

    Lepidosteidae, _33_

    _Lepidosteus_, _33_;
      attachment of teeth, 108;
      distribution, 66;
      pectoral fins, 131;
      scales, 67, 104;
      skull, 123;
      tail, 117;
      vertebrae, 68

    _Lepidotus_, _33_;
      teeth, 110

    Leporidae, _48_

    Lepospondyli, _35_

    _Lepus_, _48_;
      see Hare

    _Leptoptilus_, _41_;
      see Adjutant

    _Lialis_, _37_, 289

    Limbs, general account, 26;
      modifications in position of, 28;
      reptiles, 289

    Llama, _45_, 359;
      cervical vertebrae, 445;
      skeleton, =496=;
      teeth, 428

    Limicolae, _42_

    Lingual apparatus, lampreys, 58;
      myxinoids, 57

    Lion, _48_

    _Loemanctus longipes_, shoulder girdle and sternum, =287=

    Loggerhead turtle, carapace, =216=

    _Lophiodon_, _46_;
      teeth, 345, 429

    Lophiodontidae, _46_

    Lophiomyidae, _47_

    _Lophiomys_, _47_;
      pes, 526;
      skull, 476

    _Lophius_, attachment of teeth, 107

    Lower jaw, see Mandible

    Lumbar vertebrae, 16;
      crocodile, 242;
      dog, 378 f.;
      duck, 311

    Lunar, 27;
      dog, 408


    _Macacus_, _49_;
      cervical vertebrae, 446

    _Machaerodus_, _48_;
      upper canines, 437

    _Macrauchenia_, _46_, 358;
      calcaneum, 360;
      cervical vertebrae, 445;
      fibula, 519;
      tarsus, 523;
      ulna, 501

    Macraucheniidae, _46_, 509

    Macropodidae, _43_, _350_;
      dental formula, 423;
      pes, 522

    _Macropus_, _43_;
      see Kangaroo

    Macroscelidae, _49_

    _Macroscelides_, _49_;
      skull, 480

    Magnum, 27;
      see Carpus

    Malar, 22;
      see jugal

    Malleus, dog, 393;
      man, dog and rabbit, =485=

    Mammalia, _42_;
      auditory ossicles, 485;
      cervical vertebrae, 442;
      exoskeleton, 416;
      general characters, 343;
      manus, 503;
      Mesozoic --, 348;
      pectoral girdle, 493;
      pelvic girdle, 512;
      pes, 521;
      ribs, 490;
      sacral and caudal vertebrae, 451;
      skull, 455;
      sternum, 489;
      thigh and shin, 517;
      thoraco-lumbar vertebrae, 447

    Man, _49_;
      arm bones, 503;
      auditory ossicles, 488;
      caudal vertebrae, 454;
      cervical vertebrae 446;
      pelvis, 515;
      pes, 527;
      ribs, 493;
      scapula, 499;
      skull, 483;
      sternum, 490;
      teeth, 441

    Manatee, _44_;
      see _Manatus_

    Manatidae, _44_

    _Manatus_, _44_;
      cervical vertebrae, 444;
      dental formula, 425;
      humerus, 501;
      manus, 505;
      pelvis, 514;
      skull, 460;
      sternum, 489;
      teeth, 345;
      thoraco-lumbar vertebrae, 448

    Mandible, birds, 335;
      cod, 100;
      crocodile, 258;
      dog, 398;
      duck, 319;
      frog, 160;
      Hippopotamus, =467=;
      Isabelline bear, =438=;
      newt, 144;
      salmon, 94;
      turtle, 230

    Manidae, _44_;
      see _Manis_

    _Manis_, _44_;
      auditory ossicles, 487;
      manus, 504;
      scales, 3, 417;
      skull, 459;
      _M. macrura_ xiphisternum, 489;
      see Pangolin

    Manubrium of malleus 486;
      dog, 393;
      -- sterni, dog 404

    Manus, 26;
      crocodile, 265;
      dog, 408, =413=;
      duck, 323;
      frog, 164;
      mammalia, 503;
      newt, 147;
      Perissodactyles, =508=;
      turtle, 233

    Marginal plate, turtle 216;
      -- ray, 131;
      -- shield, turtle, 214

    Marmoset, _49_, 372 f.

    Marmot, frontals, 476

    Marsipobranchii, _31_, _53_;
      spinal column, 56

    Marsupial bones, 513

    Marsupial mole, 43;
      see _Notoryctes_

    Marsupialia, _43_;
      arm bones, 499;
      auditory ossicles, 486;
      cervical vertebrae, 443;
      caudal vertebrae, 453;
      general characters, 349;
      manus, 504;
      pectoral girdle, 494;
      pelvis, 513;
      pes, 521;
      ribs, 491;
      sacral vertebrae, 451;
      skull, 456;
      teeth, 422;
      thigh and shin, 517;
      thoraco-lumbar vertebrae, 447

    _Mastodon_, _47_, _365_;
      teeth, 434

    _Mastodonsaurus_, _35_, _136_;
      pelvis, 187

    Mastoid portion of periotic, dog, 391

    Maxilla, 22;
      cod, 98;
      crocodile, 254;
      dog, 397;
      duck, 318;
      frog, 159;
      newt, 144;
      turtle, 229

    Maxillo-mandibular arch, 21

    Maxillo-palatine, duck, 318

    Maxillo-turbinal, dog, 395

    Meatus, external auditory --, crocodile, 250;
      dog, 393;
      turtle, 228;
      internal auditory --, crocodile, 251;
      dog, 392, 400;
      turtle, 228

    Meckel's cartilage, 22;
      cod, 100;
      dogfish, 77;
      salmon, 94

    Median ethmoid, 21;
      cod, 98;
      Gymnophiona, 179;
      salmon, 91;
      -- fin, Amphibia, 52;
      cod, 86;
      dogfish, 79

    Megachiroptera, _49_;
      general characters, 371

    _Megalobatrachus_, _35_, _135_;
      carpus, 186;
      skull, 175

    Megalosauridae, _38_

    _Megalosaurus_, _38_, _208_

    _Megapodius_, spur, 330

    _Megaptera_, _44_, _357_

    Megatheriidae, _44_, 352;
      humerus, 501;
      leg bones, 517;
      pelvis, 513;
      sacrum, 452;
      teeth, 424;
      thoraco-lumbar vertebrae, 447

    _Megatherium_, _44_;
      femur, 517;
      manus, 505;
      pectoral girdle, 495;
      pes, 522;
      skull, 458

    Megistanes, _40_, _299_

    Membranous cranium, 17

    Menobranchidae, _35_

    _Menobranchus_, _35_, _135_;
      carpus, 185;
      pes, 188;
      skull, 174;
      teeth, 169

    _Menopoma_, _35_;
      see _Cryptobranchus_

    Mento-meckelian, 22;
      frog, 161;
      reptiles, 284

    Merganser, _41_;
      beak, 329

    _Mergus_, _41_

    Merrythought, duck, 322

    Mesethmoid, 20;
      dog, 390;
      duck, 317

    _Mesoplodon_, _44_;
      teeth, 427

    Mesopterygium, 79

    Mesosauridae, _37_

    _Mesosaurus_, _37_

    Mesosternum, dog, 404

    Metacarpal quill, duck, 303

    Metacarpo-digital, duck, 303

    Metacarpus, 26;
      see Manus

    Metacromion, hares and rabbits, 497

    Meta-pterygium, 79

    Metatarsus 26;
      see Pes

    Metatheria, _43_;
      general characters, 349

    _Metriorhynchus_, _39_, 278

    Microchiroptera, _49_;
      general characters, 371

    _Microgale_, _49_;
      caudal vertebrae, 454

    Mid-digital quill, duck, 303

    Milk-teeth, 344;
      dog, 377;
      horse, 430

    Moa, _40_, _299_;
      aftershaft, 328;
      pectoral girdle, 336;
      wing, 338

    Molar teeth, 344;
      dog, 376 f.

    Mole, _49_, _370_;
      auditory ossicles, 488;
      cervical vertebrae, 446;
      humerus, 503;
      manus, 512;
      presternum, 490;
      shoulder girdle, 499;
      skull, 481;
      teeth, 440;
      Golden --, _49_;
      see _Chrysochloris_;
      Marsupial --, _43_;
      see _Notoryctes_

    _Molge_, _35_, _135_;
      see Newt

    Monitor, _38_;
      see _Varanus_

    Monkey, _49_, _373_;
      see under Primates

    Monodelphia, _43_;
      characters of, 351

    _Monodon_, _45_, _357_;
      see Narwhal

    Monophyodont, defined, 7, 344

    Monopneumona, _34_

    Monotremata, _42_;
      arm bones, 499;
      auditory ossicles, 486;
      caudal vertebrae, 453;
      cervical vertebrae, 443;
      general characters, 346;
      manus, 504;
      pectoral girdle, 493;
      pelvis, 513;
      pes, 521;
      ribs, 490;
      sacral vertebrae, 451;
      skull, 455;
      sternum, 489;
      teeth, 422;
      thigh and shin, 517;
      thoraco-lumbar vertebrae, 447

    _Morosaurus_, _38_, _207_;
      pes, 294

    _Mosasaurus_, _38_, _204_

    _Moschus_, _46_;
      canines, 429

    Mouse, _47_;
      teeth, 437;
      see _Mus_

    Mud-fish, _34_

    Multituberculata, _43_, _348_

    Muntjac, _46_;
      see _Cervulus_

    Muraenidae, _33_

    Muridae _47_

    _Mus_, _47_;
      _M. musculus_, teeth, 437;
      _M. sylvaticus_, sternum and shoulder girdle, 498

    Musk deer, _46_;
      canines, 429

    Mustelidae, _48_;
      teeth 439

    _Mycetes_, 49;
      hyoid, 485;
      mandible, 484;
      skull, 482

    Myliobatidae, _32_;
      teeth, 109

    Myomorpha, _47_

    _Myrmecobius_, teeth, 423

    _Myrmecophaga_, _44_;
      manus 505;
      pectoral girdle, 495;
      skull, 458

    Myrmecophagidae, _44_, 424;
      see Anteaters

    Mystacoceti, _44_;
      general characters, 356;
      hind limb, 518;
      manus, 505;
      pectoral girdle, 495;
      skull, 461;
      teeth, 426

    _Myxine_, _31_, 55;
      fins, 115;
      notochordal sheath, 9

    Myxinoidei, _31_, 55


    Nails, 3;
      Amphibia, 168;
      mammals, 417

    Nares:
      anterior --, crocodile, 252, 257;
      dog, 401;
      duck, 317;
      newt, 143;
      turtle, 225, 229;
      posterior --, crocodile, 257;
      dog, 402;
      duck, 318;
      frog, 158;
      newt, 143;
      turtle, 230

    Narial cavity, salmon, 89;
      -- passage, crocodile, 254;
      dog, 395;
      -- septum, dog, 401

    Narwhal, _45_, _357_;
      teeth, 427

    Nasal 21;
      crocodile, 252;
      dog, 394;
      duck, 317;
      frog, 158;
      newt, 143;
      turtle, 228;
      -- capsule, 20;
      cod, 97;
      crocodile, 252;
      dog, 394;
      dogfish, 74;
      frog, 158;
      newt, 143;
      turtle, 228;
      -- cavity, dog, 388;
      -- fossae, salmon, 89;
      -- horns, rhinoceros, 3

    Navicular, 27;
      dog, 414

    Neornithes, _40_;
      general characters, 298

    _Nesodon_, _46_, _361_;
      pes, 525;
      teeth, 432

    Nesodontidae, _46_

    Neural arch, 14;
      -- plate, turtle, 215;
      -- spine, 14

    Neuromere, defined, 112

    Newt, _35_;
      anterior limb, 147;
      hyoid apparatus or visceral arches, 144, =181=;
      pelvic girdle, 149;
      ribs, 145;
      shoulder girdle, =146=;
      skull, 140;
      sternum, 145;
      vertebral column, 138

    Notidanidae, _31_;
      calcification of vertebrae, 114;
      pectoral fins, 130;
      vertebral column, 113;
      visceral arches, 63, 119 f.

    Notochord, _Amphioxus_ 52;
      Balanoglossus, 50;
      dogfish, 72;
      Tunicates, 51

    _Nothosaurus_, _37_, _193_;
      supratemporal fossae, 283

    Nothosauridae, _37_

    _Notoryctes_, _43_;
      arm bones, 500;
      caudal vertebrae, 453;
      cervical vertebrae, 443;
      claws, 418;
      dental formula, 423;
      manus, 504;
      pelvis, 513;
      pectoral girdle, 494;
      pes, 521;
      ribs, 491;
      sacrum, 452;
      skull, 457;
      sternum, 489;
      thigh and shin, 517

    Notoryctidae, _43_, _350_

    Nuchal plate, turtle, 215;
      -- shield, crocodile, 238;
      turtle, 214

    _Nyrania_, _35_;
      palatines, 177


    Occipital condyle, crocodile, 246;
      dog, 386;
      duck, 315;
      frog, 154;
      turtle, 224;
      -- crest, dog, 386;
      -- segment, crocodile, 246;
      dog, 384;
      turtle, 224

    _Odontaspis_, _32_;
      succession of teeth, =107=

    Odontoblast, 7

    Odontoceti, _44_;
      general characters, 357;
      manus, 505;
      pectoral girdle, 495;
      skull, 462;
      sternum, 489;
      teeth, 426

    Odontolcae, _40_;
      general characters, 299

    _Odontopteryx_, _40_;
      jaws, 334

    _Ogmorhinus_, _48_;
      mandibular ramus, =439=

    Olecranon process, dog, 406;
      duck, 323;
      frog, 164

    Olfactory capsule, see nasal capsule;
      -- cavity, dog, 388;
      -- chamber, dog, 395;
      -- fossa, dog, 390

    Olm, _35_

    _Omosaurus_, exoskeleton, 272

    Omosternum, frog, 163

    _Onychodactylus_, _35_;
      nails, 168

    Operculum, cod, 101;
      salmon, 95

    Ophidia, _38_;
      general characters, 202;
      jaw bones, 280;
      scales, 270;
      skull, 277 f.;
      vertebral column, 275

    _Ophisaurus_, _38_;
      limbs, 289;
      pectoral girdle, 289

    Opisthocoelous, defined, 14

    _Opisthocomus_, 41;
      skull, 334

    Opisthotic, 20;
      cod, 96;
      crocodile, 250;
      salmon, 89 f.;
      turtle, 227

    Opossum, _43_;
      caudal vertebrae, 453;
      teeth, 423

    Optic capsule, 20;
      crocodile, 251;
      dog, 394;
      turtle, 228

    Orang, _49_;
      carpus, 512;
      ribs, 493;
      thoraco-lumbar vertebrae, 450

    Orbit, crocodile, 257;
      dogfish, 74;
      duck, 317

    Orbital ring, cod, 97;
      salmon, 93

    Orbitosphenoid, 19;
      dog, 388;
      duck, 317;
      newt, 141

    _Orca_, _45_;
      teeth, 427

    _Oreodon_, _45_;
      see _Cotylops_

    Ornithodelphia, _42_;
      general characters, 346

    Ornithosauria, _212_

    Ornithorhynchidae, _43_

    _Ornithorhynchus_, _43_;
      beak, 3, 418;
      caudal vertebrae, 453;
      manus, 504;
      pelvis, 513;
      sacral vertebrae, 451;
      shoulder girdle, =347=;
      skull, 455;
      spur, 418;
      tarsus, 27 n.;
      teeth, 4, 346, 422;
      thoraco-lumbar vertebrae, 447

    Ornithopoda, _39_;
      general characters, 209

    Orthopoda, _39_;
      general characters, 208;
      pubes, 292

    Orycteropodidae, _44_;
      teeth 425

    _Orycteropus_, _44_;
      hair, 416;
      manus, 505;
      pectoral girdle, 495;
      pelvis, 513;
      skull, 459;
      see Aard Vark

    Osborn, H.F., on Mesozoic Mammals, 348

    Os entoglossum, duck, 320

    Osteoblast, 11

    Osteoclast, 11

    Osteodentine, 108

    Osteostraci, _31_;
      general characters, 54

    _Ostracion_, _33_, 69;
      plates, 105

    Ostracionidae, _33_

    Ostracodermi, _31_;
      general characters, 54

    Ostrich, _40_, _299_;
      aftershaft, 329;
      cervical vertebrae, =331=;
      claws, 330;
      foot, 342;
      manus, 338;
      pelvic girdle and sacrum, =340=;
      pubis, 341;
      tibio-tarsus, 341;
      wing, =339=

    _Otaria_, _48_;
      dentition, 439;
      tympanic bulla, 480

    Otariidae, _48_, _369_;
      auditory ossicles, 488;
      scapula, 498;
      skull, 480

    Owen's apteryx, pelvic girdle and sacrum, =340=

    Owen's chameleon, epidermal horns, 271

    Owls, _42_, 335;
      aftershaft, 329;
      foot, 342

    Owl-parrot, _42_;
      see _Stringops_

    Ox, _46_, 359;
      atlas and axis, =445=;
      three cervical vertebrae, =15=;
      femur, =518=;
      manus, 507;
      teeth, 345;
      two thoracic vertebrae, =449=


    Paca, _48_

    Paired fins, 127

    Palaeoniscidae, _32_

    _Palaeoniscus_, _32_;
      scales, 67

    Palaeospondylidae, _31_

    _Palaeospondylus_, _31_, 58

    _Palaeosyops_, _46_;
      teeth, 432

    Palaeotheriidae, _46_

    _Palaeotherium_, 46;
      skull, 471;
      teeth, 430

    _Palamedea_, _41_;
      spur, 330, 338

    Palamedeae, _41_

    Palate, reptiles, 280 f.

    Palatine, cod, 98;
      crocodile, 254;
      dog, 397;
      duck, 318;
      frog, 160;
      salmon, 93;
      turtle, 230

    Palato-pterygo-quadrate bar, 22;
      dogfish, 77;
      fish, 120 f.;
      salmon, 93

    Palm civet, _48_

    Pangolin, _44_;
      pectoral girdle, 495;
      pelvis, 513;
      caudal vertebrae, 453;
      see _Manis_

    Parachordals, 17

    _Paradoxurus_, _48_;
      tail, 454

    Parasphenoid, 21;
      cod, 97;
      frog, 156;
      newt, 141;
      reptiles, 278;
      salmon, 93

    Parasuchia, _39_;
      general characters, 211

    Parethmoid, 21 n.

    Pariasauria, _36_

    _Pariasaurus_, _36_, _192_;
      pectoral girdle, 289;
      pelvis, 292;
      supratemporal fossa, 283;
      teeth, 273

    Parietal, 19;
      cod, 96;
      crocodile, 247;
      dog, 386;
      duck, 314;
      newt, 141;
      salmon, 91;
      turtle, 225;
      -- segment, crocodile, 247;
      dog, 386;
      turtle, 225

    Paroccipital process, dog, 386

    _Parra_, _42_;
      spur, 330

    Parrots, 335;
      aftershaft, 328;
      beak, 330;
      epiphyses of centra, 332;
      foot, 342;
      powder-down feathers, 329;
      skull, 334

    Parrot fish, _33_;
      see _Scarus_

    Passeres, aftershaft, 328

    Passeriformes, _42_

    Patella, dog, 412;
      duck, 327

    _Pavo_, _41_;
      _P. cristatus_, shoulder girdle and sternum, =337=

    Peacock, _41_;
      see _Pavo_

    Peccary, pes, 523

    Pecora, _46_, _359_;
      teeth, 429

    Pectinated incisors, _Galeopithecus_, 370, 440;
      _Procavia_, 362

    Pectineal process, duck, 326

    Pectoral fins, cod, 103;
      dogfish, 79;
      -- girdle, 24;
      Amphibia, 184;
      birds, 336;
      cod, 101;
      crocodile, 262;
      dog, 404;
      dogfish, 79;
      duck, 321;
      fish, 126;
      frog, 162;
      mammalia, 493;
      newt, 145;
      reptiles, 288;
      turtle, 231;
      -- shield, turtle, 215

    _Pedetes_, _47_;
      manus, 511;
      tail, 454

    Pelican, _41_, 335;
      clavicles, 338

    _Pelicanus_, _41_;
      _P. conspicillatus_ shoulder girdle and sternum, =337=

    _Pelobates_, _36_;
      vertebrae, 172;
      _P. cultripes_ teeth, 169

    Pelobatidae, _36_

    Pelvic fins, cod, 103;
      dogfish, 82;
      fish, 131;
      -- girdle, 25;
      Amphibia, 187;
      birds, 339;
      crocodile, 266;
      dog, 409;
      dogfish, 81;
      duck, 324;
      fish, 127;
      frog, 165;
      mammals, 512;
      newt, 149;
      Ratitae, =340=;
      Reptilia, 291;
      turtle, 235

    Penguin, _40_;
      distribution of feathers, 328;
      fibula, 341;
      foot, 342;
      manus, 338;
      metatarsus, 342;
      pneumaticity of skeleton, 331;
      skull, 333;
      sternum, 336;
      thoracic vertebrae, 332;
      wing, 329, =339=

    Penna, duck, 303

    Pentedactylate, defined, 26

    _Perameles_, _43_;
      atlas, 443;
      pectoral girdle, 494

    Peramelidae, _43_, _350_;
      auditory ossicles, 486;
      pes, 522

    _Perca_, _34_

    Perch, _34_;
      pelvic fin, 132;
      urostyle, 117

    Percidae, _34_

    Perennibranchiata, _35_;
      characters, 135

    Perichondrium, 10

    Perichordal sheath, 16

    Periosteal ossification, 10

    Periosteum, 10

    Periotic, dog, 390;
      -- capsule, see Auditory capsule

    Perissodactyla, _46_;
      cervical vertebrae, 445;
      general characters, 359;
      manus, 507;
      pes, 523;
      ribs, 491;
      scapula, 496;
      skull, 470;
      teeth, 429;
      thoraco-lumbar vertebrae, 448

    Persistent pulps, 5

    Pes, 26;
      crocodile, 268;
      dog, =413=, 414;
      duck, 327;
      frog, 166;
      mammals, 521;
      reptiles, 293;
      turtle, 236;
      of Tapir, Rhinoceros, _Hipparion_ and Horse, =524=

    _Petromyzon_, _31_, 55 f.;
      notochordal sheath, 9

    Petromyzontidae, _31_, 55

    Petrous portion of periotic, dog, 391

    _Pezophaps_, _42_;
      see Solitaire

    _Phacochaerus_, _45_;
      teeth, 428

    _Phaëthon_, _41_;
      metatarsals, 342

    _Phalacrocorax_, _41_

    Phalangeridae, _43_, 350

    Phalanges, 26;
      see Manus and Pes

    Phaneroglossa, _36_

    Pharyngo-branchial, cod, 101;
      dogfish, 78;
      salmon, 95

    Pharyngognathi, _33_

    _Phascolarctus_, _43_;
      see Koala

    Phascolomyidae, _43_, _350_

    _Phascolomys_, _43_, 349;
      see Wombat

    _Phascolotherium_, _43_, 348

    Phenacodontidae, _47_

    _Phenacodus_, _47_, _362_;
      caudal vertebrae, 454;
      manus, =510=;
      pes, 525;
      scapula, 497;
      skull, 472;
      thoraco-lumbar vertebrae, 449

    _Phocaena_, _45_, _357_;
      skull, 462;
      thoraco-lumbar vertebrae, 448;
      _P. phocaenoides_, ossicles, 420

    Phocidae, _48_, _369_;
      scapula, 497;
      tympanic bulla, 480

    _Phoronis_, _30_, 50 f.

    _Phororhacos_, _41_;
      anterior nares, 333;
      ischia, 341

    _Physeter_, _44_;
      cervical vertebrae, 444;
      manus, 505;
      skull, 464;
      teeth, 426

    Physeteridae, _44_;
      ribs, 491;
      thoraco-lumbar vertebrae, 448

    _Physodon_, _44_;
      teeth, 426

    Physodontidae, _44_

    Physostomi, _33_

    Phytosauridae, _39_

    _Phytosaurus_, _39_;
      see _Belodon_

    Pici, _42_

    _Picus_, _42_;
      see Woodpecker

    Pig, _45_, _359_;
      skull, 465 f., =466=;
      teeth, 345, 427

    Pigeons, _42_, 334 f.;
      aftershaft, 329;
      pneumaticity of skeleton, 331

    Pike, _33_;
      pelvic fin, 132;
      teeth, 107, 110

    Pinnipedia, _48_;
      arm bones, 502;
      auditory ossicles, 488;
      general characters, 369;
      manus, 511;
      pelvis, 515;
      pes, 526;
      skull, 480;
      teeth, 439;
      thigh and shin, 520;
      thoraco-lumbar vertebrae, 450

    _Pipa_, _36_;
      hyoid apparatus, 182;
      jaws, 169;
      skull, 180;
      sternum, 184;
      vertebrae, 172

    Pipidae, _36_

    Pisces, _31_;
      general characters, 60

    Piscivorous dentition, 426, 440

    Pisiform, 345, 504;
      crocodile, 265;
      dog, 408;
      turtle, 233

    Pituitary fossa, crocodile, 247;
      -- space, 17

    Placodontia, _36_

    _Placodus_, _36_, _192_;
      teeth 273

    Placoid scale, 4, 60, 104

    Plantigrade, defined, 358 n.

    Plastron, _Dermochelys_, 272;
      _Chelone midas_, 217, =218=, 271

    Platanistidae, _45_

    _Platanista_, _45_;
      cervical vertebrae, 444;
      skull, 464

    Plectognathi, _33_;
      vertebrae, 115

    _Plectropterus_, _41_;
      _P. gambensis_, spur, 330

    Plesiosauridae, _37_;
      limbs, 193;
      parasphenoid, 192;
      skull, 278

    _Plesiosaurus_, _37_, _193_;
      position of limbs, 28

    Pleuracanthidae, _63_;
      fins, 115

    Pleurodira, _37_;
      general characters, 195

    Pleurodont, 159, 199, 273

    Pleuronectidae, _33_

    Pleuropterygii, _31_, 63

    _Pliosaurus_, _37_, _193_

    Plovers, _42_, 334;
      thoracic vertebrae, 332

    Pneumaticity of bird's skeleton, 331

    _Polacanthus_, _39_;
      exoskeleton, 272

    Pollex, 26;
      see Manus

    _Polyodon_, _32_, 104;
      distribution, 66;
      pectoral fins, 131;
      skull, 122;
      spinal column, 112;
      teeth, 110

    Polyodontidae, _32_

    _Polyonax_, _39_, _209_;
      beak, 271;
      frontals, 277;
      jaw, 274;
      predentary, 284

    Polyprotodont, 423

    Polyprotodontia, _43_;
      general characters, 350

    Polypteridae, _33_

    _Polypterus_, _33_, _68_;
      distribution, 66;
      exoskeleton, 67;
      pectoral fins, 131;
      pelvic fins, 132;
      pelvis, 127;
      scales, 104;
      skull, 122;
      tail, 116

    _Pontoporia_, _45_;
      cervical vertebrae, 444;
      teeth, 426

    Porcupine, _47_;
      pes, 526;
      skull, 476, =477=;
      spines, 417

    Porpoise, _45_, _357_;
      thoraco-lumbar vertebrae, 448

    Postaxial, 28

    Posterior cornu, duck, 320;
      turtle, 231;
      -- limb, 26;
      Amphibia, 188;
      birds, 341;
      dog, 412;
      duck, 326;
      frog, 166;
      newt, 149, =148=;
      reptiles, 293;
      turtle, 235, =234=

    Postfrontal, 21;
      crocodile, 250;
      turtle, 225

    Postorbital bar, crocodile, 250, 255 f.;
      _Hatteria_, 283;
      -- groove, dogfish, 76

    Post-temporal, cod, 102;
      reptiles, 283;
      -- bar, crocodile, 256;
      _Hatteria_, 283

    _Potamogale_, _49_, 367, 370;
      shoulder girdle, 499;
      teeth, 440

    Potamogalidae, _49_

    Powder-down feathers, 329

    Pre-axial, 28

    Precoracoid, 25;
      frog, 163;
      newt, 147;
      reptiles, 288;
      turtle, 232

    Predentary, reptiles, 284

    Predigital quill, duck, 303

    Prefrontal, 21;
      crocodile, 249;
      reptiles, 278;
      turtle, 225

    Prefronto-lachrymal, newt, 141

    Prehallux, frog, 167 f.

    Premaxilla, 22;
      cod, 98;
      crocodile, 252;
      dog, 398;
      duck, 314, 318;
      frog, 158;
      newt, 143;
      salmon, 94;
      turtle, 230

    Premolar, dog, 370, 377;
      mammals, 344

    Prenasal process, frog, 158

    Pre-orbital vacuity, reptiles, 283

    Presphenoid, 19;
      dog, 388

    Prespiracular ligament, dogfish, 77

    Presternum, dog, 404

    Primaries, duck, 303

    Primates, _49_;
      arm bones, 503;
      auditory ossicles, 488;
      cervical vertebrae, 446;
      general characters, 372;
      manus, 512;
      pelvis, 515;
      pes, 527;
      ribs, 493;
      sacrum, 452;
      shoulder girdle, 499;
      skull, 482 f.;
      sternum, 490;
      tail, 454;
      teeth, 441;
      thigh and shin, 520;
      thoraco-lumbar vertebrae, 450

    _Priodon_, _44_;
      caudal vertebrae, 453;
      manus, 505;
      stapes, 487;
      sternum, 489;
      teeth, 424

    Pristidae, _32_

    _Pristis_, _32_;
      snout or rostrum, 109, 119

    Proboscidea, _47_;
      arm bones, 502;
      cervical vertebrae, 445;
      general characters, 364;
      femur, 519;
      manus, 511;
      pelvis, 514;
      pes, 526;
      scapula, 497;
      skull, 473;
      teeth, 433;
      thoraco-lumbar vertebrae, 449

    _Procavia_, _47_, _363_;
      auditory ossicles, 487;
      caudal vertebrae, 453;
      dental formula, 432;
      humerus, 502;
      manus, 510;
      pelvis, 514;
      pes, 525;
      ribs, 491;
      scapula, 497;
      skull, 433, 472;
      tarsus, 27;
      thoraco-lumbar vertebrae, 449

    Process, alinasal --, frog, 158;
      basi-pterygoid --, birds, 334;
      coracoid --, dog, 405;
      coronoid -- (of mandible), dog, 398;
      duck, 319;
      coronoid -- (of ulna), dog, 408;
      pectineal --, duck, 326;
      postfrontal --, duck, 316;
      postglenoid --, dog, 394;
      postorbital -- (of frontal), dog, 388;
      postorbital -- (of jugal), dog, 398;
      posterior articular --, duck, 319;
      zygomatic --, dog, 394

    Processus brevis, 486;
      -- gracilis, 486;
      -- longus, 486;
      dog, 393

    Procoelous, defined, 14

    _Prodelphinus_, _45_;
      skull, 462

    Proganosauria, _37_

    Prone position, 29

    Prongbuck, _46_;
      horns, 417

    Pro-otic, 20;
      frog, 157;
      turtle, 227

    Pro-pterygium, dogfish, 79

    Proteidae, _35_

    _Proteles_, _48_;
      teeth, 437

    Protelidae, _48_

    Proterosauridae, _37_

    _Proterosaurus_, _37_;
      teeth, 198, 274;
      vertebrae, 197

    _Proteus_, _35_, 135, 182;
      branchial arches, 180;
      digits, 187;
      pes, 188;
      skull, 174

    _Protopterus_, _34_, _70_, 117;
      branchial arches, 121, 124;
      fins, 130;
      skull, 124;
      vestigial gill on pectoral girdle, 129

    Prototheria, _42_;
      general characters, 346

    Proximal, defined, 23 n.

    _Psephurus_, distribution, 66

    _Pseudopus_, _38_;
      limbs, 289

    Psittaci, _42_;
      see Parrots

    _Pteranodon_, _39_, _274_;
      pectoral girdle, 289

    Pteranodontidae, _39_

    _Pteraspis_, _31_, _54_

    _Pterichthys_, _31_, _55_

    _Pterocles_, _42_;
      see Sandgrouse

    Pteroclidae, _42_

    Pterodactylidae, _39_

    _Pterodactylus_, _39_, 213

    Pteropidae, _49_;
      skull, 481

    _Pteropus_, _49_;
      dental formula, 441;
      tail, 454

    Pterosauria, _39_;
      general characters, 212;
      ischia, 292;
      limbs, 291;
      pre-orbital vacuity, 284;
      ribs, 285;
      sternum, 287;
      vertebrae, 275 f.

    Pterotic, 20;
      cod, 96;
      salmon, 90 f.

    Pterygoid, cod, 98;
      crocodile, 255;
      dog, 397;
      duck, 318;
      frog, 160;
      newt, 144;
      salmon, 93;
      turtle, 230;
      -- fossa, crocodile, 255;
      -- plate, dog, 388

    Pterylae, 328

    Pubis, 25;
      crocodile, 266 f.;
      duck, 325;
      dog, 411;
      frog, 165;
      newt, 149;
      reptiles, 292;
      turtle, 235

    Pygal plate, turtle, 217;
      -- shield, turtle, 214

    Pygopodidae, _37_

    Pygostyle, duck, 307, 312

    _Python_, _38_;
      ischio-pubis, 292;
      jaws, 280;
      vestiges of limbs, 289, 293

    Pythonomorpha, _38_;
      general characters, 204;
      limbs, 290;
      teeth, 273


    Quadrate, 22;
      cod, 98;
      crocodile, 255;
      duck, 319;
      frog, 160;
      newt, 144;
      salmon, 93;
      turtle, 229

    Quadratojugal, 22;
      crocodile, 255;
      duck, 318;
      frog, 160;
      turtle, 229

    Quill, duck, 302


    Rabbit, _48_, 366;
      pollex, 511

    Raccoon, 369

    Rachis, duck, 302

    Rachitomous, defined, 171

    Radiale, 27;
      see Carpus

    Radialia, 115;
      dogfish, 79 f.

    Radio-ulna, frog, 164

    Radius, 26;
      crocodile, 265;
      dog, 406;
      duck, 323;
      newt, 147;
      turtle, 233

    _Raia_, _32_;
      calcification of vertebrae, 114

    Raiidae, _32_

    _Rana_, _36_;
      see Frog

    Ranidae, _36_;
      shoulder girdle, 185

    Rat, pes, 526

    Ratitae, 40;
      caudal vertebrae, 333;
      clavicles, 338;
      foot, 342;
      general characters, 298;
      skull, 333;
      sternum, 336;
      vomers, 334;
      wing, 338

    Rattlesnake, _38_;
      rattle, 3, 270

    Ray, pectoral fin, 130;
      Eagle --, Electric -- and Sting --, _32_

    Rectrices, 303, 329

    Reed-fish, _33_

    Reindeer, antlers, 469

    Remicle, duck, 304

    Remiges, 303, 329

    Reptiles, anterior limb, 290;
      exoskeleton, 270;
      fossae in skull, 281;
      pectoral girdle, 288;
      pelvic girdle, 291;
      posterior limb, 293;
      ribs, 285;
      skull, 276;
      sternum, 287;
      teeth, 272;
      vertebral column, 275

    Reptilia, _36_;
      general characters, 190;
      see Reptiles

    _Rhabdopleura_, _30_, 50

    _Rhamphastos_, _42_;
      see Toucan

    Rhamphorhynchidae, _39_

    _Rhamphorhynchus_, _39_, 213, 274

    _Rhea_, _40_;
      aftershaft, 329;
      claws, 330;
      ischia, 341;
      manus, 338;
      _R. macrorhyncha_, pelvic girdle and sacrum, =340=

    Rheornithes, _40_

    _Rhina_, _32_;
      see _Squatina_

    Rhinal process, frog, 158

    _Rhinoceros_, _46_, 360, 419;
      femur, =518=;
      fibula, 519;
      malleus, 487;
      manus, 508;
      nasal horns, 3, 417;
      pes, 525;
      skull, =421=, 470;
      teeth, 430;
      ulna, 501;
      _R. antiquitatis_, 470

    Rhinocerotidae, _46_

    Rhinolophidae, _49_

    Rhiptoglossa, _38_

    Rhizodontidae, _33_

    _Rhizodus_, _33_;
      teeth, 110

    Rhynchocephalia, _37_;
      general characters, 197;
      humerus, 290;
      teeth, 273 f.;
      vertebrae, 275

    Rhynchosauridae, _37_;
      maxillae, 198

    _Rhytina_, _44_, 352, 425;
      humerus, 501;
      skull, =460=

    Rhytinidae, _44_

    Ribs, 23;
      Amphibia, 182;
      birds, 336;
      cod, 86;
      crocodile, 259;
      dog, 402;
      dogfish, 73;
      duck, 320;
      fish, 125;
      frog, 153;
      mammalia, 490;
      newt, 145;
      reptiles, 285

    Ridge, supra-orbital and suborbital, dogfish, 74

    Rodentia, _47_;
      auditory ossicles, 488;
      cervical vertebrae, 446;
      dental formula, 435;
      general characters, 365;
      pelvis, 515;
      pes, 526;
      humerus, 502;
      manus, 511;
      ribs, 493;
      sacrum, 452;
      shoulder girdle, 497;
      skull, 476;
      sternum, 489;
      tail, 454;
      teeth, 421;
      thigh and shin, 520;
      thoraco-lumbar vertebrae, 449

    Roller, _42_, 335

    Rooted teeth, defined, 5

    Rorqual, _44_, 357;
      cervical vertebrae, 444

    Rostrum, crocodile, 247;
      dogfish, 74;
      duck, 316;
      _Pristis_, 119;
      -- of sternum, duck, 321

    Röse, C., on teeth of Marsupials, 422

    Ruminantia, _46_, _359_;
      auditory ossicles, 487;
      fibula, 519;
      horny plates on palate, 418;
      hyoid, 470;
      manus, 507;
      odontoid process, 445;
      pes, 523;
      scapula, 495;
      teeth, 420, 429


    Sabre-toothed lion, _48_;
      see _Machaerodus_

    Sacral ribs, crocodile, 243;
      -- surface of ilium, dog, 409;
      -- vertebrae, 16;
      crocodile, 243;
      dog, 383;
      duck, 312;
      frog, 153;
      newt, 140;
      turtle, 222

    Sacrum, duck, 310;
      see Sacral vertebrae

    Sagittal crest, dog, 386

    _Saiga_, skull, 468

    Salamander, _35_

    _Salamandra_, _35_, _135_;
      antibrachium and manus of larva, =186=;
      manus of larva, 185;
      tarsus, 27

    Salamandrina, _35_, _135_;
      skull, 175;
      sternum, 182

    _Salmo_, _33_

    Salmon, _33_;
      branchial arches, 95;
      chondrocranium, 87;
      opercular bones, 95;
      pectoral fins, 131;
      skull, 87

    Salmonidae, _33_

    Sandgrouse, _42_, 335

    _Sarcophilus_, _43_;
      teeth, 423

    Sauropoda, _38_;
      general characters, 205;
      teeth, 273;
      vertebrae, 276

    Sauropsida, _36_;
      general characters, 189

    Sauropterygia, _37_;
      general characters, 192;
      limbs, 290;
      palate, 281;
      pectoral girdle, 288;
      vertebrae, 276

    Saw-fish, _32_;
      see _Pristis_

    Scales, cod, 83;
      crocodile, 237;
      ctenoid, 8;
      cycloid, 8;
      duck, 302;
      ganoid, 8;
      Gymnophiona, 168;
      mammals, 417

    Scale-foot, 37

    Scalpriform, 366

    _Scaphirhynchus_, _32_, _104_;
      distribution, 66;
      exoskeleton, 67;
      spinal column, 112

    Scaphoid, 27;
      mammals, 504 f.

    Scapho-lunar, dog, 408

    Scapula, 25;
      cod, 103;
      crocodile, 263;
      dog, 404;
      duck, 322;
      frog, 162;
      newt, 146;
      turtle, 232

    Scapular shield, armadillo, 419

    Scapus, duck, 302

    _Scarus_, _33_;
      beaks, 111

    Scelidosauridae, _39_

    Schizognathous, defined, 335

    Scincidae, _38_

    _Scincus_, _38_;
      scutes, 271

    Sciuromorpha, _47_

    Sclerotic, turtle, 228

    Screamer, _41_;
      spurs, 330

    Scutes, armadillos, 419;
      crocodile, 237;
      reptiles, 271

    Scylliidae, _32_

    _Scyllium_, _32_;
      calcification of vertebrae, 114;
      pectoral fins, 130;
      suspensorium, 119;
      see Dogfish

    _Scymnus_, _32_, _118_;
      calcification of vertebrae, 114;
      mandibular arch, 120;
      pectoral fins, 130

    _Scythrops_, _42_;
      interorbital septum, 333

    Sea leopard, _48_;
      see _Ogmorhinus_;
      -- lion, _48_;
      manus, 511;
      pes, 526;
      position of limbs, 29;
      -- otter, _48_;
      pes, 526

    Seal, _369_;
      manus, 511;
      pes, 526;
      sacral vertebrae, 452;
      scapula, 497

    Secondaries, duck, 303 f.

    Secretary-bird, _41_;
      claws, 330

    Selachii, _31_;
      general characters, 63;
      teeth, 108

    Selenodont, defined, 345, 428

    Sella turcica, crocodile, 247;
      dog, 386

    Semionotidae, _33_

    Semiplumae, 328

    Sense capsules, see Auditory, Nasal and Optic capsule

    _Seps_, 38;
      limbs, 289

    Shagreen, 61

    Shaft of feather, 302

    Shark, 64;
      Frill-gilled --, _31_;
      see _Chlamydoselache_;
      Port Jackson --, _32_;
      see _Cestracion_

    Sheep, _359_;
      manus, 507;
      teeth, 345

    Shields of turtle, 214

    Shin, 26;
      see Crus

    Shoulder girdle, see Pectoral girdle

    Shrew, _49_, _370_;
      auditory ossicles, 488;
      cervical vertebrae, 446;
      presternum, 490;
      skull, 481; teeth, 440

    Sigmoid notch, dog, 406

    Siluridae, _33_;
      plates, 105

    _Simia_, _49_;
      ribs, 493;
      skull, 484;
      thoraco-lumbar vertebrae, 450

    Simiidae, _49_, _373_

    Simplicidentata, _47_, 366

    _Siphonops_, _35_;
      _S. annulatus_, skull, =178=

    _Siredon_, _35_;
      skull, 175;
      teeth, 169;
      visceral arches, =181=

    _Siren_, _35_, _135_, 188;
      beaks, 168;
      branchial arches, 180;
      digits, 187;
      skull, 174;
      teeth, 169

    Sirenia, _44_, 522;
      arm bones, 501;
      caudal vertebrae, 453;
      cervical vertebrae, 443;
      general characters, 352;
      hair, 416;
      horny plates, 418;
      manus, 505;
      pectoral girdle, 495;
      pelvis, 514;
      ribs, 491;
      skull, 459;
      sternum, 489;
      teeth, 425;
      thoraco-lumbar vertebrae, 448

    Sirenidae, _35_

    Sirenoidei, _34_;
      general characters, 70

    _Sivatherium_, _46_;
      skull, 469

    Skate, _32_

    Skeletogenous layer, 14, 16;
      _Amphioxus_, 52, 112

    Skeleton, defined, 1;
      Cape Buffalo, =492=;
      _Ceratodus_, =128=;
      cod, 83 f.;
      crocodile, 237 f.;
      dog, 374 f.;
      duck, 302 f.;
      frog, 151 f.;
      llama, =496=;
      newt, 138 f.;
      turtle, 214 f.

    Skink, _38_;
      see _Tiliqua_

    Skull, 16 f.;
      Amphibia, 173 f.;
      Anura, 179 f.;
      birds, 333 f.;
      cod, 96 f.;
      crocodile, 243 f.;
      diagram of Mammalian, 385;
      Dipnoi, 124;
      dog, 383 f.;
      dogfish, 73 f.;
      donkey, =431=;
      duck, 312 f.;
      fish, 117;
      frog, 154 f., =159=;
      _Globicephalus_, =463=;
      Gymnophiona, 177;
      Indian elephant, =474=;
      Mammalia, 455;
      Marsipobranchii, 57;
      pig, =466=;
      _Procavia_, =433=;
      reptiles, 276 f.;
      Rhinoceros, =421=;
      _Rhytina_, =460=;
      sloth, =458=;
      Tasmanian wolf, =456=;
      Teleostei, 124;
      turtle, 222 f.;
      wombat, =456=

    Sloth, _43_, _352_;
      auditory ossicles, 487;
      arm bones, 500;
      claws, 418;
      leg bones, 517;
      manus, 505;
      pectoral girdle, 495;
      pelvis, 513;
      pes, 522;
      ribs, 491;
      sacrum, 452;
      skull, 457;
      sternum, 489;
      teeth, 424;
      thoraco-lumbar vertebrae, 447

    Snake, _38_;
      see Ophidia

    Sole, _33_

    _Solea_, _33_

    _Solenodon_, _49_;
      teeth, 440

    Solenodontidae, _49_

    Solitaire, _42_, _330_;
      wing, 338;
      wrist, 330

    _Sorex_, _49_;
      pelvis, 515;
      see Shrew

    Soricidae, _49_;
      skull, 481

    Spalacidae, _47_

    _Spatularia_, _32_;
      distribution, 66

    _Spelerpes_, _35_;
      branchial arches, 180;
      ribs, 182;
      _S. belli_ teeth, 169

    Sperm whale, _44_, _357_;
      see _Physeter_

    _Sphargis_, _37_;
      see _Dermochelys_

    Sphenethmoid, frog, 156

    Sphenisci, _40_;
      see Penguins

    Sphenisciformes, _40_

    _Sphenodon_, _37_, 197 f.;
      carpus, 291;
      cervical vertebrae, 275;
      fossae in skull, 281;
      humerus, 290;
      interparietal foramen, 277;
      ribs, 286;
      skull, =282=;
      tarsus, 293;
      teeth, 274

    Sphenodontidae, _37_

    Sphenoidal fissure, dog, 388

    Sphenotic, 20;
      cod, 97;
      salmon, 89

    Spider monkey, _49_;
      see _Ateles_

    Spinacidae, _32_

    Spinal column, 13;
      Dipnoi, 113;
      fish, 112;
      Holocephali, 113;
      Marsipobranchii, 56

    Spines, Elasmobranchs, 61;
      mammals, 417

    Spiny ant-eater, _43_;
      see _Echidna_;
      -- mouse, 47;
      see _Acanthomys_

    Splenial, 22;
      crocodile, 258;
      duck, 320;
      turtle, 231

    Spurs, birds, 330;
      Monotremata, 418

    Spur-winged goose, 330;
      -- plover, 330

    Squalidae _31_, _64_

    _Squalodon_, _45_, _357_;
      dental formula, 427

    Squalodontidae, _45_

    Squamata, _37_;
      general characters, 198;
      position of teeth, 272;
      skull, 278

    Squamosal, 21;
      crocodile, 256;
      dog, 394;
      duck, 316;
      frog, 160;
      newt, 144;
      turtle, 229

    _Squatina_, _32_;
      calcification of vertebrae, 114;
      labial cartilages, 119;
      tail, 63;
      vertebral column, 114

    Squatinidae, _32_

    Squirrels, frontals, 476;
      pes, 526

    Stapes, dog, 393;
      frog, 157;
      man, dog, rabbit, =485=;
      newt, 141

    Steganopodes, _41_

    Stegosauria, _39_;
      general characters, 209

    Stegosauridae, _39_

    _Stegosaurus_, _39_, _208_ f.;
      exoskeleton, 272

    Steller's sea-cow, _44_;
      see _Rhytina_

    Stereornithes, _41_

    Stereospondyli, _35_

    Sternal ribs, crocodile, 259;
      dog, 402;
      duck, 320;
      mammals, 490 f.

    Sternebra, dog, 404

    Sternum, 24;
      Amphibia, 182;
      birds, 336;
      crocodile, 260, =261=;
      dog, =403=;
      duck, 321;
      frog, 163;
      Mammalia, 489;
      newt, 145;
      reptiles, 287

    Stork, 335;
      White --, _41_

    Striges, _42_;
      see Owls

    _Stringops_, _42_;
      sternum, 336

    _Struthio_, _40_, _299_;
      see Ostrich

    Struthiornithes, _40_, _299_

    Sturgeon, _32_;
      see _Acipenser_

    Stylo-hyal, dog, 399

    Suborbital bar, duck, 318;
      -- ridge, dogfish, 76

    Subplantigrade, defined, 358 n.

    Subungulata, _46_;
      arm bones, 502;
      general characters, 360;
      manus, 509;
      pelvis, 514;
      pes, 525;
      shoulder girdle, 497;
      skull, 471;
      teeth, 432;
      thigh and shin, 519

    Suidae, _45_

    Suina, _45_, 358 f.;
      fibula, 519;
      manus, 507;
      odontoid process, 445;
      pelvis, 514;
      pes, 523;
      ulna, 501

    Sula, _41_;
      see Gannet

    Supinator ridge, dog, 406

    Supine position, defined, 29

    Supra-angular, 22;
      crocodile, 258;
      duck, 319;
      turtle, 230 f.

    Supracaudal shield, turtle, 214

    Supraclavicle, cod, 102

    Supra-occipital, 19;
      crocodile, 247;
      dog, 386;
      duck, 315;
      turtle, 224

    Supra-orbital, 20;
      crocodile, 251

    Suprapharyngeal bone, cod, 101

    Suprascapula, crocodile, 263;
      frog, 162

    Supratemporal arcade, crocodile, 256;
      reptiles, 281

    Surinam toad, _36_;
      see _Pipa_

    Sus, _45_;
      dental formula, 428;
      see Pig

    Suspensorium, Amphibia, 173;
      dogfish, 78;
      duck, 319;
      frog, 160;
      newt, 144;
      Pisces, 61

    Sutures, 12

    Swan, _41_;
      cervical and thoracic vertebrae, 332

    Swift, _42_, 335;
      foot, 342

    Symplectic, cod, 100;
      salmon, 94


    Tails, fish, 60

    Talpa, _49_;
      pelvis, 515;
      see Mole

    Talpidae, _49_

    Tapir, _46_, 360;
      malleus, 487;
      pes, =524=, 525;
      teeth, 345;
      see _Tapirus_

    Tapiridae, _46_;
      dental formula, 429

    _Tapirus_, _46_;
      fibula, 519;
      manus, =508=;
      skull, 471;
      see Tapir

    Tarsier, _49_

    Tarsiidae, _49_

    _Tarsipes_, _43_, 349;
      mandible, 457

    _Tarsius_, _49_, 372;
      pes, 527

    Tarso-metatarsus, duck, 327

    Tarsus, 26 f.;
      crocodile, 268;
      dog, 414;
      frog, 166;
      newt, 150;
      turtle, 236

    Tasmanian devil, _43_;
      see _Sarcophilus_;
      -- wolf, _43_;
      see _Thylacinus_

    _Tatusia_, _44_;
      stapes, 487;
      teeth, 424

    Tectospondyli, 114

    Tectrices, duck, 306

    Teeth, =6=;
      Amphibia, 169;
      birds, 330;
      cod, 83;
      crocodile, 238;
      development, 7;
      dog, 374 f.;
      fish, 106 f.;
      frog, 158 f.;
      horses, 5;
      mammals, 344, 420 f.;
      pharyngeal, 8;
      reptiles, 272 f.;
      structure, 4;
      succession, 7

    Teleosauridae, _39_

    _Teleosaurus_, _39_;
      palate, 281;
      scutes, 271;
      vertebrae, 275

    Teleostei, _33_;
      general characters, 69;
      ribs, 126;
      skull, 124;
      tail, 117;
      teeth, 110;
      vertebral column, 115

    Temnospondyli, _35_

    Tenrec, _49_;
      see _Centetes_

    Tentorium, dog, 392

    Terrapin, _37_

    _Testudo_, _37_, _194_

    _Tetraceros_, _46_;
      horns, 417

    _Thalassochelys_, carapace, =216=

    Thecodont, defined, 273

    Theriodontia, _36_

    Theromorpha, _36_;
      general characters, 191;
      humerus, 290;
      pectoral girdle, 288;
      ribs, 285;
      skull, 278;
      teeth, 273;
      vertebral column, 275 f.

    Theropoda, _38_;
      general characters, 207;
      teeth, 273

    Thoracic ribs, crocodile, 259;
      see Ribs;
      -- vertebrae, 16;
      crocodile, 241, =242=;
      dog, 381, =382=;
      duck, 310;
      turtle, 221

    Thoraco-lumbar vertebrae, mammals, 447 f.

    Thornback skate, 104

    _Thylacinus_, _43_;
      atlas, 443;
      dental formula, 423;
      pelvis, 513;
      pes, 521;
      skull, =456=

    _Thylacoleo_, _43_;
      skull, 457

    Thyro-hyal, dog, 399

    Tibia, 26;
      crocodile, 268;
      dog, 412;
      newt, 149;
      turtle, 235

    Tibiale 27;
      see Tarsus

    Tibio-fibula, frog, 166

    Tibio-tarsus, duck, 326

    Tichorhine Rhinoceros, 470

    Tiger, _48_

    _Tiliqua_, _38_;
      scutes, 200, 271

    Tillodontia _47_, 365;
      femur, 520;
      manus, 511;
      teeth, 435

    Tinamidae 300;
      caudal vertebrae, 333;
      vomers, 334

    Tinamiformes, _41_

    _Tinamus_, _41_;
      ischia, 341

    Titanotheriidae, _46_;
      skull, 470;
      teeth, 432

    _Titanotherium_, _46_;
      humerus, 501;
      manus, =508=;
      pes, 525

    Toad, _36_;
      shoulder girdle, 185

    Tope, _32_

    Torpedinidae, _32_

    _Torpedo_, _32_, 104

    Tortoise, _37_;
      position of limbs, 28

    _Tortrix_, ischio-pubis, 292;
      traces of posterior limb, 293

    Toucan, _42_;
      foot, 342

    _Toxodon_, _46_, 361;
      femur, 519;
      pes, 525;
      teeth, 432

    Toxodontia, _46_;
      general characters, 361;
      manus, 509;
      skull, 472;
      teeth, 432

    Toxodontidae, _46_

    Trabeculae 11;
      -- cranii, 17

    Tragulidae, _45_

    Tragulina, _45_, 359;
      fibula, 519;
      manus, 507;
      odontoid process, 445;
      pes, 523;
      skull, 468;
      teeth, 429;
      ulna, 501

    Transpalatine, crocodile, 255;
      reptiles, 278

    Trapezium, 27;
      dog, 408

    Trapezoid, 27;
      dog, 408

    Trichechidae, _48_, 369

    _Trichechus_, _48_;
      see Walrus

    Trionychia, _37_;
      general characters, 194

    Trionychidae, _37_

    _Trionyx_, _37_, 193 f.;
      exoskeleton, 214, 270;
      skull, 283;
      vestiges of teeth, 274

    _Trissolepis_, _32_;
      scales, 104

    _Tritylodon_, _43_;
      teeth, 348

    Trochanter, dog 412;
      duck, 326

    Trochilidae, _42_;
      see Humming-birds

    Trochlea, crocodile, 263;
      dog, 405 f.;
      duck, 323;
      turtle, 232

    Trogon, _42_;
      foot, 342

    Trogonidae, _42_

    _Tropidonotus_, _38_;
      jaws, 280;
      skull, =279=

    Trunk vertebrae, cod, 84;
      see thoracic and lumbar vertebrae

    _Trygon_, _32_;
      calcification of vertebrae, 114;
      caudal spine, 106

    Trygonidae, _32_

    Tuberosities of humerus, dog, 405;
      of ischium, dog, 411

    Tunicata, _30_, 51

    _Tupaia_, skull, 480;
      thoraco-lumbar vertebrae, 450

    Turbinals, dog, 395

    _Tursiops_, _45_;
      skull, 462

    Turtle, _37_;
      anterior limb, 232, =234=;
      cranium, 222 f., =226=;
      hyoid, 231, =285=;
      mandible, 230;
      pectoral girdle, 231;
      pelvic girdle, 235;
      pes, 236;
      plastron, 217, =218=;
      posterior limb, =234=, 235;
      sense capsules, 227;
      skull, 222;
      vertebral column, 219;
      Leathery --, see _Dermochelys_;
      Snapping --, see _Trionyx_

    Tylopoda, _45_, 359;
      fibula, 519;
      manus, 507;
      odontoid process, 445;
      pelvis, 514;
      pes, 523;
      skull, 468;
      teeth, 428;
      ulna, 502

    Tympanic, dog, 392;
      -- cavity, crocodile, 250;
      diagram of mammalian, =391=;
      dog, 393;
      duck, 315 f.;
      turtle, 228;
      -- recess, duck, 315

    Tympano-hyal, dog, 399

    Typhlopidae, _38_;
      scales, 270;
      skull, 278

    _Typhlops_, _38_;
      ischio-pubis, 292;
      traces of posterior limb, 293

    Typotheriidae, _46_

    _Typotherium_, _46_, 358, 361;
      clavicle, 495, 497;
      femur, 519;
      pes, 525;
      skull, 472;
      teeth, 432


    _Udenodon_, _36_, _192_;
      beak, 271

    Uintatheriidae, _47_;
      skull, 364

    _Uintatherium_, _47_;
      dental formula, 433;
      leg, 519;
      limbs and limb girdles, =516=;
      manus, 510;
      pelvis, 514;
      skull, 473

    Ulna, 26;
      crocodile, 265;
      dog, 406;
      duck, 323;
      frog, 164;
      newt, 147;
      turtle, 233

    Ulnare, 27;
      see Carpus

    Umbilicus, inferior and superior, duck, 303

    Uncinate process, 190;
      crocodile, 259;
      duck, 320

    Unciform, 27, 345, 504;
      dog, 408

    Ungulata, _45_;
      auditory ossicles, 487;
      caudal vertebrae, 453;
      cervical vertebrae, 445;
      general characters, 357;
      manus, 506;
      pectoral girdle, 495;
      pes, 522;
      ribs, 491;
      sacrum, 452;
      skull, 464 f.;
      sternum, 489;
      teeth, 427 f.;
      thoraco-lumbar vertebrae, 448

    Ungulata vera, _45_;
      arm bones, 501;
      general characters, 358;
      manus, 506;
      pelvis, 514;
      thigh and shin, 519

    Upper arm, 26;
      crocodile, 263;
      dog, 405;
      duck, 323;
      frog, 164;
      newt, 147;
      turtle, 232

    _Upupa_, _42_;
      see Hoopoe

    Urochordata, _30_, 51

    Urodela, _35_;
      general characters, 134;
      pelvis, 187;
      ribs, 182;
      skull, 174

    Urohyal, cod, 101;
      duck, 320

    Urostyle, Anura 172;
      cod, 85;
      frog, 153;
      Teleostei, 117

    Ursidae, _48_;
      humerus, 502

    _Ursus_, _48_;
      dental formula, 439;
      see Bears


    Vacuities, anterior palatine --, crocodile, 252, 258;
      -- in reptilian skull, 281;
      posterior palatine --, crocodile, 254, 257;
      pre-orbital --, reptiles, 283 f.

    Vampire, _49_;
      teeth, 441

    Vane, of feather, 303

    Varanidae, _38_

    _Varanus_, _38_;
      shoulder girdle, =202=;
      skull, =201=

    Vasodentine, 108, 272

    Vertebral column, 14;
      Amphibia, 170;
      birds, 332;
      cod, 83;
      crocodile, 239;
      dog, 378;
      duck, 307;
      Elasmobranchs, 113;
      frog, 152;
      mammals, 442;
      newt, 138;
      turtle, 219;
      -- ribs, crocodile, 259;
      dog, 402;
      duck, 320;
      -- shield, turtle, 214

    Vertebrata, general characters, 53

    Vexillum, of feather, 303

    Vibrissae, dog, 374

    Viscacha, _47_

    Visceral skeleton, 21;
      dogfish, 77;
      Elasmobranchs, 119 f.

    _Viverra_, _48_;
      acetabular bone, 515

    Viverridae, _48_

    Vomer, 21;
      cod, 98;
      crocodile, 252;
      dog, 395;
      duck, 317;
      frog, 158;
      salmon, 93;
      turtle, 229

    Vomero-palatine, newt, 143

    _Vultur_, _41_

    Vulture, _41_;
      Black --, shoulder girdle and sternum, =337=


    Waders, 335

    Walrus, _48_, 367, 369;
      canines, 420;
      manus, 511;
      pes, 526;
      position of limbs, 29;
      skull, 480;
      teeth, 440

    Warblers, _42_

    Wart hog, _45_;
      teeth, 428

    Weasel, _369_

    Whale, baleen, 3, 418;
      Ca'ing --, _45_,
        see _Globicephalus_;
      Humpbacked --, _44_, 357;
      Right --, _44_, 357;
      Sperm --, _44_, 357,
        see _Physeter_;
      True or Whalebone --, 356

    Whiting, _33_

    Wild duck, _41_;
      see Duck

    Wing, duck, 322;
      Gannet, Ostrich, and Penguin, =339=

    Wolf, _48_

    Wombat, _43_;
      atlas, 443;
      pes, 521;
      sacrum, 451;
      skull, =456=;
      tail, 453;
      teeth, 423

    Woodpecker, _42_, 335;
      foot, 342;
      hyoid, 336

    Wrasse, _33_;
      teeth, 111


    _Xenacanthus_, _31_;
      pectoral fins, 130

    Xenopidae, _36_

    _Xenopus_, _36_;
      branchial arches, 182;
      nails, 168;
      pelvis, 188;
      ribs, 182

    Xiphiplastron, turtle, 217

    Xiphisternal horn, crocodile, 260

    Xiphisternum, dog, 404;
      frog, 163

    Xiphoid process, duck, 321


    _Zeuglodon_, _44_, 353, 356;
      dental formula, 426;
      dermal plates, 420

    Zeuglodontidae, _44_

    Zygantra, defined, 199 n.;
      reptiles, 276

    Zygapophyses, cod, 84;
      crocodile, 240 f.;
      dog, 379 f.;
      duck, 308 f.;
      frog, 152 f.;
      newt, 139;
      turtle, 219 f.

    Zygosphene, defined, 199 n.;
      reptiles, 276


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     researches into the past.

     _Natural Science._ A most successful treatment of a difficult
     subject. All of importance is brought forward and impartially
     discussed.... Such a book has long been a desideratum. Mr
     Seward's style is clear and concise, and the many pitfalls into
     which beginners are apt to stumble are clearly pointed out.


=Zoology. An Elementary Text-Book=. By A.E. SHIPLEY, M.A., F.R.S., and
E.W. MACBRIDE, M.A. (Cantab.), D.Sc. (London), Professor of Zoology in
McGill University, Montreal. Demy 8vo. With numerous Illustrations.
10_s._ 6_d._ _Net_.

     _Pilot._ A very business-like and convenient manual of modern
     Zoology.

     _School World._ As a thoroughly trustworthy and instructive
     text-book for serious students, the work can be strongly
     recommended. Its value is enhanced by the large number of
     excellent illustrations, many of which are delightfully fresh.

     _Oxford Magazine._ It is readable, well arranged, well printed,
     copiously and admirably illustrated, and it covers the whole
     field of zoology.

     _Nature._ There pervades the pages of the work a freshness of
     style and unconventionality which render them pleasant reading
     and attractive; while, in the frequent allusion to the
     commonest occurrences of daily life and human affairs, the
     interest of the reader is assured.

     _Pall Mall Gazette._ Precisely the sort of book which, if it
     came into a thoughtful boy's hands, would turn him from a
     smatterer into a student.... One of the most instructive and
     attractive books that could be put into the hands of a young
     naturalist.

=Grasses=: a Handbook for use in the Field and Laboratory. By H.
MARSHALL WARD, Sc.D., F.R.S., Fellow of Sidney Sussex College,
Professor of Botany in the University of Cambridge. With 81 figures.
Crown 8vo. 6_s._

     _Pilot._ Brimful of matter.

     _Field._ The work is essentially suited to the requirements of
     those desirous of studying the grasses commonly grown in this
     country, and it can fairly be said that it furnishes an amount
     of information seldom obtained in more pretentious volumes.

     _Athenaeum._ Botanists and Agriculturists alike have reason to
     thank Prof. Ward for this very serviceable addition to the
     literature of grasses.

=Trees=: A Handbook of Forest Botany for the Woodlands and the
Laboratory. By H. MARSHALL WARD, Sc.D., F.R.S., Fellow of Sidney
Sussex College, Honorary Fellow of Christ's College and Professor of
Botany in the University of Cambridge. In six volumes. 1. Buds and
Twigs, 2. Leaves, 3. Inflorescences and Flowers, 4. Fruits and Seeds,
5. Seedlings, 6. General Characters. Vol. I. Buds and Twigs. Crown
8vo. Illustrated. 4_s._ 6_d._ _net_.

=A Treatise on the British Freshwater Algae.= By G.S. WEST, M.A.,
A.R.C.S., F.L.S., Professor of Natural History at the Royal
Agricultural College, Cirencester. Demy 8vo. 10_s._ 6_d._ _net_.

=A Manual and Dictionary of the Flowering Plants and Ferns=. By J.C.
WILLIS, M.A., Director of the Royal Botanic Gardens, Ceylon. _Second
Edition._ Complete in one volume. Crown 8vo. 10_s._ 6_d._

=Elementary Palaeontology--Invertebrate.= By HENRY WOODS, M.A., F.G.S.,
University Lecturer in Palaeozoology. Crown 8vo. _Third Edition._
Revised and enlarged, with 112 Illustrations. 6_s._

=Outlines of Vertebrate Palaeontology for students of Zoology.= By
ARTHUR SMITH WOODWARD, M.A., F.R.S., Keeper of the Department of
Geology in the British Museum. Demy 8vo. With numerous Illustrations.
14_s._

     _Athenaeum._ The author is to be congratulated on having produced
     a work of exceptional value, dealing with a difficult subject in a
     thoroughly sound manner.


_In preparation._

=Morphology and Anthropology.= By W.L.H. DUCKWORTH, M.A., Fellow and
Lecturer of Jesus College, University Lecturer in Physical Anthropology.

=The Origin and Influence of the Thorough-bred Horse.= By W. RIDGEWAY,
M.A., Disney Professor of Archaeology and Fellow of Gonville and Caius
College. With numerous Illustrations. Demy 8vo.

=The Morphology of Plants.= By J.C. WILLIS, M.A.

    =London=: C.J. CLAY AND SONS,
    CAMBRIDGE UNIVERSITY PRESS WAREHOUSE,
    AVE MARIA LANE,
    AND
    H.K. LEWIS, 136, GOWER STREET, W.C.

    =Glasgow=: 50, WELLINGTON STREET

       *       *       *       *       *


  Transcriber Notes

  Italic text is denoted by _underscores_ and bold text by =equal
  signs=. Subscripts are represented using braces, e.g. V{1}, and
  superscripts are introduced with a caret, e.g. 2^e

  Obvious punctuation and spelling errors, and inconsistent hyphenation
  have been corrected.

  The oe ligature in the text has been replaced with the characters oe.





End of Project Gutenberg's The Vertebrate Skeleton, by Sidney H. Reynolds