Produced by Thomas Cosmas. Produced from files made
available on The Internet Archive.







Transcriber's Notes

    Text emphasis is denoted as _Text_ for italic and =Text= for bold.
    Whole and fractional parts are shown as 4-2/3.
    OE and oe ligature converted to Oe and oe respectively.


       *       *       *       *       *




MEMOIRS OF

THE CONNECTICUT ACADEMY

OF ARTS AND SCIENCES

VOLUME VII             DECEMBER, 1920



The Appendages, Anatomy, and Relationships of Trilobites



BY



PERCY E. RAYMOND, Ph.D.

ASSOCIATE PROFESSOR OF PALAEONTOLOGY, AND CURATOR OF INVERTEBRATE

PALAEONTOLOGY IN THE MUSEUM OF COMPARATIVE ZOOLOGY,

HARVARD UNIVERSITY


[Illustration: (logo)]


NEW HAVEN, CONNECTICUT

PUBLISHED BY THE

CONNECTICUT ACADEMY OF ARTS AND SCIENCES

AND TO BE OBTAINED ALSO FROM THE

YALE UNIVERSITY PRESS






[Illustration (photo)]

[Illustration (signature)]





MEMOIRS OF

THE CONNECTICUT ACADEMY
OF ARTS AND SCIENCES

VOLUME VII DECEMBER, 1920



The Appendages, Anatomy, and Relationships
of Trilobites



BY



PERCY E. RAYMOND, Ph.D.

ASSOCIATE PROFESSOR OF PALAEONTOLOGY, AND CURATOR OF INVERTEBRATE

PALAEONTOLOGY IN THE MUSEUM OF COMPARATIVE ZOOLOGY,

HARVARD UNIVERSITY

[Illustration: (logo)]


NEW HAVEN, CONNECTICUT

PUBLISHED BY THE

CONNECTICUT ACADEMY OF ARTS AND SCIENCES

AND TO BE OBTAINED ALSO FROM THE

YALE UNIVERSITY PRESS





THE TUTTLE, MOREHOUSE & TAYLOR COMPANY




TO THE MEMORY OF


CHARLES EMERSON BEECHER


SKILLFUL WITH HAND, BRAIN, AND PEN; REVEALER OF THE MYSTERIES

OF TRILOBITES;

THIS MEMOIR IS DEDICATED






FOREWORD.


By CHARLES SCHUCHERT.


Trilobites are among the most interesting of invertebrate fossils and
have long attracted the attention of amateur collectors and men of
science. These "three-lobed minerals" have been mentioned or described
in books at least since 1698 and now several thousand species are
known to palæontologists. To this group of students they are the most
characteristic animals of the seas of Palæozoic time, and even though
they are usually preserved as dismembered parts, thousands upon
thousands of "whole ones" are stored in the museums of the world. By
"whole ones" perfect individuals are not meant, for before they became
fossils the wear and tear of their time and the process of
decomposition had taken away all the softer parts and even most of the
harder exterior covering. What is usually preserved and revealed to us
when the trilobites weather out of the embrace of their entombing
rocks is the test, the hard shell of the upper or dorsal side. From
time to time fragments of the under or limb-bearing side had been
discovered, first by Elkanah Billings, but before 1876 there was no
known place to which one could go to dig out of the ground trilobites
retaining the parts of the ventral side.

Students of trilobites have always wanted specimens to be delivered to
them weathered out of the rock by nature and revealing the ventral
anatomy without further work than the collecting, but the wish has
never been fulfilled. In the Utica black shales, near Rome, New York,
there was finally discovered in 1892 a layer less than ten millimeters
thick, bearing hundreds of _Triarthrus becki_ with most of the ventral
anatomy intact. The collector's first inkling that such were present
in the Utica formation came to him in a chance find in 1884, and for
eight years he sought off and on for the stratum whence this specimen
came. His long search was finally rewarded by the discovery of the
bed, and lo! here were to be had, in golden color, prostrate specimens
with the breathing and crawling legs and the long and beautifully
curved feeling organs all replaced by iron pyrites. Fool's gold in
this case helped to make a palæontologic paradise. The bed contained
not only such specimens of _Triarthrus becki_, but also, though more
rarely, of _Cryptolithus tessellatus_ and exceptionally of _Acidaspis
trentonensis_. This important discovery, which has figured so largely
in unraveling the evolution of the Crustacea and even has a bearing on
that of most of the Arthropoda, was made by Mr. W. S. Valiant, then
curator of the Museum of Rutgers College.

There were, however, great material difficulties to overcome before
the specimens revealed themselves with all of their information
exposed for study. No surgeon was needed, but a worker knowing the
great scientific value of what was hidden, and with endless patience
and marked skill in preparation of fossils. Much could be revealed
with the hammer, because specimens were fairly abundant. A chance
fracture at times showed considerable portions, often both antennæ
entire, and more rarely the limbs protruding beyond the test, but the
entire detail of any one limb or the variation between the limbs of
the head, thorax, and tail was the problem to be solved. No man ever
loved a knotty problem more than Charles E. Beecher. Any new puzzle
tempted him, and this one of _Triarthrus becki_ interested him most of
all and kept him busy for years. From the summer of 1893. when he
quarried out two tons of the pay stratum at Rome, until his death in
1904, his time was devoted in the main to its solution by preparing
these trilobites and learning their anatomical significance.

The specimens of _Triarthrus becki_ from Rome are pseudomorphs
composed of iron pyrites, as has been said, and are buried in a
gray-black carbonaceous shale. A little rubbing of the specimens soon
makes of them bronze images of the former trilobite and while under
preparation they are therefore easily seen. However, as the average
individual is under an inch in length and as all the limbs other than
the antennæ are double or biramous, one lying over the other, and the
outer one fringed with a filamentous beard, the parts to be revealed
by the preparator are so small and delicate that the final touch often
obliterates them. These inherent difficulties in the material were
finally overcome by endless trials on several thousand specimens, each
one of which revealed something of the ventral anatomy. Finally some
500 specimens worthy of detailed preparation were left, and on about
50 of these Beecher's descriptions of _Triarthrus_ and _Cryptolithus_
were based.

The black shale in which the specimens are buried is softer than the
pseudomorphous trilobites, a condition that is of the greatest value
in preparation. With chisel and mallet the trilobites are sought in
the slabs of shale and then with sharp chisels of the dental type they
are revealed in the rough. At first Beecher sought to clean them
further by chemical methods, and together with his friends, the
chemist Horace L. Wells, and the petrologist Louis V. Pirsson, several
solutions were tried, but in all cases the fossils were so much
decomposed as to make them useless in study. Therefore Beecher had to
depend wholly oh abrasives applied to the specimens with pieces of
rubber. Much of this delicate work was done on a dental lathe, but in
the final cleaning most of it was done with patient work by hand.
Rubber has the great advantage of being tough and yet much softer than
either specimen or shale. As the shale is softer than the iron
pyrites, the abrasives (carborundum, emery, or pumice) took away the
matrix more quickly than the trilobite itself. When a part was fully
developed, the rubbers were cut to smaller and smaller dimensions and
the abrading reduced to minute areas. So the work went on and on,
helped along from time to time by the dental chisels. Finally Beecher
became so expert with these fossils that after one side was developed
he would embed the specimen in Canada balsam and fix it on a glass
slide, thus enabling him to cut down from the opposite side. This was
done especially with _Cryptolithus_ because of the great scarcity of
material preserving the limbs, and two of these revealed both sides of
the individuals, though they were then hardly thicker than writing
paper.

Then came illustrations, which at first were camera-lucida drawings in
pencil smoothed out with pen and ink. "In some quarters," however, it
has been said, "his methods unknown, their results were not accepted;
they were regarded as startling, as iconoclastic, and even
unreliable." He therefore decided to rework his material and to
illustrate his publications with enlarged photographs. The specimens
were black, there was little relief between fossil and matrix, and the
ammonium chloride process of coating them white and photographing
under artificial light was unsuitable. Nevertheless, after many
trials, he finally succeeded in making fine enlarged photographs of
the trilobites immersed in liquid Canada balsam, with a contact cover
of glass through which the picture was taken, the camera standing
vertically over the horizontal specimen. Beecher had completed this
work in 1903 and in the winter of 1903-1904 was making the drawings,
nearly all of which are here reproduced. On Sunday morning, February
14, 1904, as he was working at home on a large wash drawing of
_Cryptolithus_, death came to him suddenly, leaving the trilobite
problem but partially solved.

When the writer, in the autumn of 1904, succeeded Professor Beecher in
the chair of Palæontology at Yale, he expected to find considerable
manuscript relating to the ventral anatomy of the trilobites, but
there was only one page. It was Beecher's method first to prepare and
thoroughly study the material in hand, then to make the necessary
illustrations, and between times to read what others had written.
There was no written output until everything had been investigated and
read, certain passages being marked for later reference. Then when all
was assimilated, he would write the headings of topics as they came to
him, later cutting them apart and arranging them in a logical
sequence. When the writer visited him in his home in January 1904, he
was primed for his final trilobite memoir, but the writing of it had
not been begun.

The writer has never made the trilobites his special subjects for
study as he has the brachiopods, and therefore felt that he should not
try to bring to light merely the material things that Beecher had so
well wrought out. It seemed at first an impossible task to find the
specialist and friend to do Beecher justice, but as the years have
passed, one of Beecher's students, always especially interested in
trilobites, has grown into a full appreciation of their structures and
significance, and to him has fallen the continuation of his master's
work. If in the following pages he departs here and there from the
accepted interpretation and the results of others, it is because his
scientific training, in desiring to see with his own eyes the
structures as they are, has led him to accept only those
interpretations that are based on tangible evidence as he understands
such. Furthermore, in seeking the relationship of the trilobites to
the rest of the Arthropoda, his wide study of material and literature,
checked up by the ontogeny of fossil and recent forms, has led him in
places from the beaten path of supposedly ascertained phylogenies. His
results, however, have been won through a detailed study of the
interrelations of the Arthropoda, starting from the fact that the
Trilobita are chronogenetically the oldest and most primitive. The
trilobites are held by him to be the most simple, generalized, ancient
Crustacea known, and the progenitors, directly and indirectly, of all
Arthropoda.

It is now twenty-six years since Professor Beecher began his
publications on the class Trilobita, and in commemoration of him and
his work, Professor Percy E. Raymond of Harvard University presents
this memoir, to bring to fruition the studies and teachings of his
honored guide. It has been with Professor Raymond a labor of love, and
it is for the writer of this foreword a long-desired memorial to the
man to whose position in the Museum and University he had the
privilege of succeeding.

  Yale University, New Haven, Connecticut.







PREFACE.


The primary object of this memoir is, as has been stated by Professor
Schuchert, to rescue from oblivion the results of the last few years
of Professor Beecher's investigations on the ventral anatomy of
trilobites. Since he left his data in the form of drawings and
photographs, without even rough notes, it became necessary, in order
to write a text to accompany the plates, to restudy the entire
subject. Under these circumstances, it seemed best to include all that
is known about the appendages of trilobites, thus bringing together a
summary of present information on the subject.

The growth of the memoir to its present size has been a gradual one.
As first completed in 1917, it contained an account of the appendages
only. Thoughts upon the probable use of the appendages led to the
discussion of possible habits, and that in turn to a consideration of
all that is known or could be inferred of the structure and anatomy of
the trilobite. Then followed an inquiry into the relationships to
other Arthropoda, which ultimately upset firmly established
preconceptions of the isolated position of the group, and led to a
modification of Bernard's view of its ancestry.

During the progress of the work, I have had the opportunity of
examining most of the known specimens retaining appendages. From the
Marsh collection in the Yale University Museum were selected the
forty-six specimens showing best the appendages of _Triarthrus_,
_Cryptolithus_, and _Acidaspis_. Dr. Charles D. Walcott very kindly
returned to the Museum of Comparative Zoology the slices of
_Ceraurus_, _Calymene_, and _Isotelus_ which were the basis of his
paper of 1881, and which had been loaned him for further study. He
loaned also eight of the more important specimens of _Neolenus
serratus_, and two of _Triarthrus becki_. At the United States
National Museum I saw the specimens of _Isotelus_ described by
Mickleborough and the isolated limbs of _Calymene_ from near
Cincinnati. The _Isotelus_ at Ottawa I had already studied with some
care while an officer of the Geological Survey of Canada.

This memoir consists, as shown in the table of contents, of four
parts. The appendages of _Neolenus_, _Isotelus_, _Ptychoparia_,
_Kootenia_, _Ceraurus_, _Calymene_, and _Acidaspis_ are discussed, as
fully as circumstances warrant, in the first part, and new
restorations of the ventral surfaces of _Neolenus_, _Isotelus_,
_Triarthrus_, _Ceraurus_ and _Cryptolithus_ are included It is not
supposed that these restorations will be of permanent value in all of
their detail, but they are put forward as the best approximations to
the real structure that the writer is able to present from the
materials so far discovered. I am greatly indebted to Doctor Elvira
Wood for the care and skill with which she has worked up these
restorations from my rather sketchy suggestions. She has put into them
not only a great amount of patient work, but also the results of
considerable study of the specimens.

Part II is a discussion of the internal anatomy of the trilobite and a
brief statement of some of the possible habits and methods of life of
these animals. Part III, which begins with a survey of the
relationships of the trilobites to other Arthropoda, is largely taken
up with an attempt to demonstrate the primitive characteristics of the
former, and their probable ancestral position. The form of the
ancestor of the trilobite is deduced from a study of the morphology,
ontogeny, and phylogeny of the group, and evidence adduced to indicate
that it was a depressed, flattened, free-swimming animal of few
segments.

In Part IV are included somewhat detailed descriptions of a few of the
best specimens of _Triarthrus_ and _Cryptolithus_. Professor Beecher,
while an observer of the minutest details, believed in publishing only
the broader, more general results of his investigations. This method
made his papers brief, readable, and striking, but it also resulted in
leaving in some minds a certain amount of doubt about the correctness
of the observations. In a matter so important as this, it has seemed
that palæontologists are entitled to the fullest possible knowledge of
the specimens on which the conclusions are based. The last part is,
therefore, a record of the data for the restorations of _Triarthrus_
and _Cryptolithus_.

The illustrations in the plates were nearly all made by or under the
supervision of Professor Beecher, as were also text figures 45 and 46.

In conclusion, I wish to express my thanks to Mrs. Charles E. Beecher
for the use of drawings which were the personal property of Professor
Beecher; to Doctor Charles D. Walcott for photographs of the limbs of
_Calymene_, and for his kindness in sending me the slices of
trilobites from Trenton Falls and specimens of _Neolenus_ and
_Triarthrus_; to Doctor R. V. Chamberlin for suggestions and
criticisms in regard to the relationship of trilobites to Insecta,
Arachnida, Chilopoda, and Diplopoda; to Mr. Samuel Henshaw, Director
of the Museum of Comparative Zoology, for permission to use the time
which has been devoted to this work; and to Miss Clara M. Le Vene, for
assistance in the preparation of the manuscript. My greatest debt is
to Professor Charles Schuchert, to whom the work owed its inception,
who has assisted in many ways during its prosecution, and who read the
manuscript, and arranged for its publication. To him I can only
express my warmest thanks for the favors which I have received and for
the efforts which he has put forth to make this a worthy memorial to
our friend and my teacher, Professor Charles Emerson Beecher.

  Harvard University, Cambridge, Mass.
     November, 1919.






TABLE OF CONTENTS.


  Historical review                                      17

  Part I. The appendages of trilobites                   20

    Terminology                                          20

    The appendages of _Neolenus_                         21
      Historical                                         21
      _Neolenus serratus_ (Rominger)                     21
        Cephalon                                         21
        Thorax                                           22
        Pygidium                                         23
        Epipodites and exites                            23
        Description of individual specimens              23
      Restoration of _Neolenus_                          30
      _Nathorstia transitans_ Walcott                    31

    The appendages of _Isotelus_                         32
      Historical                                         32
      _Isotelus latus_ Raymond                           34
      _Isotelus maximus_ Locke                           35
      Restoration of _Isotelus_                          37
      _Isotelus gigas_ Dekay                             37
      _Isotelus arenicola_ Raymond                       39

    The appendages of _Triarthrus_ (see also Part IV)    39
      _Triarthrus becki_ Green                           39
        Historical                                       40
      Restoration of _Triarthrus_                        42
        Relation of cephalic appendages to marking on
          dorsal surface of glabella                     43
        Anal plate                                       44

    The appendages of _Ptychoparia_                      45
      _Ptychoparia striata_ (Emmrich)                    45
      _Ptychoparia cordilleræ_ (Rominger)                45
      _Ptychoparia permulta_ Walcott                     45

    The appendages of _Kootenia_                         46
      _Kootenia dawsoni_ Walcott                         46

    The appendages of _Calymene_ and _Ceraurus_          46
      Historical                                         46
      Comparison of the appendages of _Calymene_ and
        _Ceraurus_ with those of _Triarthrus_            47
      Spiral branchiæ                                    48
      Ventral membrane                                   50
      Appendifers                                        51

    _Calymene senaria_ Conrad                            52
      Cephalic appendages                                52
      Thoracic appendages                                53
      Pygidial appendages                                54
      Relation of hypostoma to cephalon in _Calymene_    55
      Restoration of _Calymene_                          56

    _Calymene_ sp. ind.                                  56

    _Ceraurus pleurexanthemus_ Green                     57
      Cephalic appendages                                58
      Thoracic appendages                                59
      Pygidial appendages                                59
      Relation of hypostoma to cephalon                  59
      Restoration of _Ceraurus pleurexanthemus_          60
    The appendages of _Acidaspis trentonensis_ Walcott   61

    The appendages of _Cryptolithus_ (see also Part IV)  61
      _Cryptolithus tessellatus_ Green                   61
      Restoration of _Cryptolithus_                      62

    Summary on the ventral anatomy of trilobites         64
      Comparison of appendages of different genera       64
        Coxopodite                                       64
        Cephalon                                         64
        Thorax                                           66
        Pygidium                                         67
        Caudal rami                                      68
      Homology of cephalic appendages with those of
        other Crustacea                                  69
      Functions of the appendages                        70
        Antennules                                       70
        Exopodites                                       70
        Endopodites                                      71
        Use of the pygidium in swimming                  72
        Coxopodites                                      74
      Position of the appendages in life                 74


  Part II. Structure and habits of trilobites            77

    Internal organs and muscles                          77
      Alimentary canal                                   77
        _Ceraurus pleurexanthemus_                       79
        _Calymene senaria_                               80
        _Cryptolithus goldfussi_                         80
        Summary                                          81
      Gastric glands                                     82
        Summary                                          84
      Heart                                              85
        _Illænus_                                        85
        _Ceraurus_ and _Calymene_                        85
        The median "ocellus" or "dorsal organ"           86
      Nervous system                                     89
      Various glands                                     89
        Dermal glands                                    89
        Renal excretory organs                           90
        Reproductive organs                              90
        Panderian organs                                 90
      Musculature                                        91
        Flexor muscles                                   92
        Extensor muscles                                 92
        Hypostomial muscles                              94

    Eyes                                                 96
      Summary                                            97

    Sex                                                  98

    Eggs                                                 98

    Methods of life (See also under "Functions of
      the Appendages")                                   98
        Habits of locomotion                             99
        Food and feeding methods                        103
        Tracks and trails                               104


  Part III. Relationship of the trilobites to other
    Arthropoda                                          106

    Crustacea                                           106
      Branchiopoda                                      106
        _Burgessia bella_ Walcott                       108
        _Waptia fieldensis_ Walcott                     108
        _Yohoia tenuis_ Walcott                         109
        _Opabina regalis_ Walcott                       109
        Summary                                         109
      Copepoda                                          110
      Archicopepoda                                     111
      Ostracoda                                         112
      Cirripedia                                        113
      Malacostraca                                      113
        Phyllocarida                                    113
        Syncarida                                       114
        Isopoda                                         114
      _Marrella splendens_ Walcott                      115
        Restoration of _Marrella_                       116

    Arachnida                                           117
      Trilobites not Arachnida                          117
      Merostomata                                       119
        _Sidneyia inexpectans_ Walcott                  119
        _Emeraldella brocki_ Walcott                    119
        _Molaria_ and _Habelia_                         120
      Araneæ                                            121

    Insecta                                             122

    Chilopoda                                           123

    Diplopoda                                           124

    Primitive characteristics of trilobites             125
      Trilobites the most primitive arthropods          125
      Limbs of trilobites primitive                     125
        Summary                                         128
      Number of segments in the trunk                   128
      Form of the simplest protaspis                    132
      Origin of the pygidium                            134
      Width of the axial lobe                           137
      Presence or absence of a "brim"                   137
      Segmentation of the glabella                      137
      Summary                                           138

    The simplest trilobite                              138
      _Naraoia compacta_ Walcott                        139

    The ancestor of the trilobites, and the descent
      of the Arthropoda                                 140
      Evolution within the Crustacea                    142
        Summary                                         144
      Evolution of the Merostomata                      146
      Evolution of the "Tracheata"                      147
      Summary on lines of descent                       147

    Final summary                                       151


  Part IV. Description of the appendages of
    individual specimens                                152

    _Triarthrus becki_ Green                            152
    _Cryptolithus tessellatus_ Green                    158


  Bibliography                                          163




LIST OF ILLUSTRATIONS.


   1. _Triarthrus becki_ Green. Diagram of limb to show
        nomenclature employed                                       20

   2. _Neolenus serratus_ (Rominger). Two thoracic appendages       24

   3. The same. An exopodite                                        26

   4. The same. A so-called "epipodite"                             26

   5. The same. The so-called "exites"                              29

   6. The same. A cephalic limb                                     29

   7. The same. Restoration of a transverse section                 30

   8. The same. Restoration of the ventral surface                  31

   9. _Isotelus_. Restoration of the ventral surface                38

  10. _Triarthrus becki_ Green. Restoration of the ventral surface  41

  11. The same. Median appendage                                    44

  12. _Ceraurus pleurexanthemus_ Green. Slice showing an exopodite  49

  13. _Calymene senaria_ Conrad. Slice showing cephalic coxopodites 53

  14. The same. Another similar slice                               53

  15. The same. Slice showing method of articulation of
        the appendages                                              53

  16. The same. Restoration of the ventral surface                  55

  17. _Ceraurus pleurexanthemus_ Green. Slice showing the method
        of articulation of the appendages                           58

  18. The same. Slice showing an exopodite above an endopodite      58

  19. The same. Restoration of a transverse section                 60

  20. _Cryptolithus tessellatus_ Green. Restoration of the
        ventral surface                                             63

  21. _Ceraurus pleurexanthemus_ Green. Slice showing the
        abdominal sheath                                            79

  22. The same. Slice showing the large alimentary canal            79

  23. _Calymene senaria_ Green. Slice showing the large
        alimentary canal                                            79

  24. _Ceraurus pleurexanthemus_ Green. Restoration of a
        longitudinal section                                        81

  25. _Cryptolithus tessellatus_ Green. Cheek showing the
        genal cæca                                                  84

  26. _Illænus._ Volborth's figure of the heart                     85

  27. Heart of _Apus_                                               85

  28. _Isotelus gigas_ Dekay. The Panderian organs                  91

  29. _Ceraurus pleurexanthemus_ Green. Restoration, showing
        heart, alimentary canal, and extensor muscles               93

  30. The same. Longitudinal section of cephalon                    95

  31. _Nileus armadillo_ Dalman. Moberg's figure of the
        muscle-scars                                                95

  32. _Marrella splendens_ Walcott. Restoration of the
        ventral surface                                            116

  33. _Triarthrus becki_ Green. Appendage of the anterior part
        of the thorax                                              126

  34. _Apus._ Appendage from the anterior part of the trunk        127

  35. _Weymouthia nobilis_ (Ford)                                  138

  36. _Naraoia compacta_ Walcott                                   145

  37. _Pagetia clytia_ Walcott                                     145

  38. _Asaphiscus wheeleri_ Meek                                   145

  39. _Pædeumias robsonensis_ Burling                              145

  40. _Robergia_ sp.                                               145

  41. Diagram showing possible lines of descent of the Arthropoda  150

  42. _Triarthrus becki_ Green. Thoracic appendages                155

  43. The same. Pygidial appendages                                157

  44. The same. Pygidial appendages                                158

  45. _Cryptolithus tessellatus_ Green. Drawing of the best
        single specimen                                            159

  46. The same. Part of the thorax and pygidium, with appendages   162

  _Frontispiece._ Charles Emerson Beecher, 1896.

  Plates 1-5. Photographs of _Triarthrus becki_, made by C. E. Beecher.

  Plate 6. Photographs of _Triarthrus becki_ (figs. 1-3), _Acidaspis
    trentonensis_ (fig. 6), and _Cryptolithus tessellatus_ (fig. 7),
    made by C. E. Beecher. Photographs of the endopodites of a probable
    species of _Calymene_ (figs. 4, 5)

  Plates 7-8. Photographs of _Cryptolithus tessellatus_, made by C. E.
    Beecher.

  Plate 9. Drawings of _Cryptolithus tessellatus_, made by C. E.
    Beecher or under his direction.

  Plate 10. Photographs of _Isotelus latus_ and _I. maximus_, made by
    C. E. Beecher.

  Plate 11. Drawing of a restoration of _Ceraurus pleurexanthemus_,
    made by Elvira Wood.






HISTORICAL REVIEW.


The beginning of the search for the limbs of trilobites was coeval
with the beginning of scientific study of the group, knowledge of the
appendages being essential to the proper systematic allocation of the
animals.

The early search was so barren of results that negative evidence came
to be accepted as of positive value, and it was for many years
generally believed that such organs as may have been present beneath
the dorsal test were so soft as to be incapable of preservation. This
view is best expressed by Burmeister (1846, p. 43):

     There is good proof that the feet of trilobites must have been soft
     membranous organs, for the absence of the slightest remains of
     these organs in the numerous specimens observed is of itself
     evidence of the fact, and it can indeed scarcely be supposed that
     hard horny extremities should be affixed to a soft membranous
     abdominal surface; since they would not have possessed that firm
     basis, which all solid organs of locomotion require, in order that
     they may be properly available.

Very well reasoned, and were it not for the discovery of new material
in American localities, Burmeister's views would probably never have
been proved incorrect. One can not escape the suspicion that some of
the accepted hypotheses of today, founded on similar "proof," may
yield in time to the weight of bits of positive evidence.

The history of the study of appendages of trilobites may be divided
into two periods. The first, in which there was a general belief that
the appendages were soft organs, but during which numerous "finds" of
limbs were reported, extended from the time of Linné to the year
(1876) in which Walcott demonstrated the fact that the animals
possessed jointed ambulatory and breathing organs.

The second, much more fruitful period, began with Walcott's
publication of 1881, descriptive of the appendages of _Ceraurus_ and
_Calymene_, and for the purposes of this memoir, closes with his great
contribution on the anatomy of _Neolenus_ (1918). Beecher's brilliant
productions came in the middle of the second period.

In the first period, there were at least two authentic discoveries of
appendages, those of Eichwald (1825) and Billings (1870), but since
neither of these men convinced his confreres of the value of his
finds, the work of neither can be considered as having marked an
especial epoch in the history.

As all the authentic finds will be treated in detail on later pages,
only a brief résumé of the first period will be given here. This has
already been done by Burmeister (1843, 1846) and Barrande (1852,
1872), whose works have been my primary sources of information, but I
have looked up the original papers, copies of nearly all of which are
to be seen in the libraries in Cambridge and Boston. Brig.-Gen. A. W.
Vogdes, U. S. A. (retired), has very kindly placed at my disposal a
number of references and notes.

Linné (1759) was the first to report the discovery of appendages of
trilobites. Törnquist (1896) has pressed for a recognition of the
contribution of the great Swedish naturalist to this problem, but
Beecher (1896 B) doubted the validity of the find. Linné figured a
specimen of _Parabolina spinulosa_ (Wahlenberg), with what he
interpreted as a pair of antennæ attached. He states (translation
quoted from Törnquist): "Most remarkable in this specimen are the
antennæ in the front, which I never saw in any other sample, and which
clearly prove this fossil to belong to the insects." Beecher has shown
as conclusively as can be shown without access to the original
specimen that the supposed antennæ were really only portions of the
thickened anterior border, the appearance being due to imperfect
preservation. Brünnich as early as 1781 called attention to the
imperfection of this specimen, and it is also referred to by
Wahlenberg (1821, p. 39), Brongniart (1822, p. 42), Dalman (1828, p.
73), and Angelin (1854, p. 46).

Audouin (1821) seems to have been the first naturalist with sufficient
knowledge of the Arthropoda to be competent to undertake the study of
the trilobites. He concluded that the absence of ventral appendages
was probably a necessary consequence of the skeletal conformation, and
thought if any were discovered, they would prove to be of a branchial
nature.

Wahlenberg (1821) in the same year expressed his belief that the
trilobites were nearly allied to _Limulus_ and in particular tried to
show that the trilobites could have had masticatory appendages
attached about the mouth as in that modern "insect" (p. 20).
Wahlenberg was also the first to describe an hypostoma of a trilobite
(p. 37, pl. 1, fig. 6), but did not understand the nature of his
specimen, which he described as a distinct species.

Brongniart (1822, p. 40) devoted five pages of his monograph to a
discussion of the affinities of trilobites, concluding that it was
very probable that the animals lacked antennæ and feet, unless it
might be that they had short soft feet which would allow them to creep
about and fix themselves to other bodies.

Schlotheim (1823) thought that the spines on _Agnostus pisiformis_
were segmented and compared them with the antennæ of _Acarus_.

Stokes (1823) was the first who, with understanding, published an
illustration of the ventral side of a trilobite, having figured the
hypostoma of an _Isotelus_. He was followed in the next year (1824) by
Dekay, who also figured the hypostoma of an _Isotelus_, and added some
observations on the structure of trilobites. The researches of
Barrande, Novak, Broegger, Lindstroem, and others have dealt so fully
with the hypostoma that further references to that organ need not be
included here.

Dalman (1826, 1828) reviewed the opinions of his predecessors, and
thought it not impossible that organs of mastication may have been
present under the head shield of the trilobite as in _Limulus_ (1828,
p. 18). In this he of course followed Wahlenberg.

Goldfuss (1828) figured sections of _Dalmanites hausmanni_, _Phacops
macrophthalma_, and _Calymene tristani_, which remind one of some of
Doctor Walcott's translucent slices. So far as one can judge from the
illustrations, it is probable that what he took for limbs were really
fragments of other trilobites. Such is certainly the case in his
figures 9 and 10, where a number of more or less broken thoracic
segments are present. The section of _Encrinurus punctatus_ shown in
figure 7 may possibly exhibit the position and folds of the ventral
membrane beneath the axial lobe, and also, perhaps, the appendages.
His figures 4, 5 and 8 show the hypostoma in section.

Pander (1830) described the hypostoma in greater detail than had been
done by previous authors, but otherwise added nothing to the subject.

Sternberg (1830) thought he had individuals showing appendages, but
judging from his poor figures, he was deceived by fragmentary
specimens.

Green (1839 A, B, C) described specimens of _Phacops_ from Berkeley
Springs, West Virginia, which had the hypostoma in position, and
appear to have had a tubular opening under the axial lobe. While
appendages were not actually present, these specimens suggested fairly
correct ideas about the swimming and breathing organs of trilobites.
They were similar to the ones which Castelnau obtained, and all were
perhaps from the same locality.

It is not worth while to do more than enumerate the other authors of
this period: Hisinger 1837, Emmrich 1839, Milne-Edwards 1841, for they
all shared the same views, and added nothing to what was already
known.

Castelnau (1843) described and figured a _Phacops_ said to come from
Cacapon Springs, West Virginia, which he thought possessed remains of
appendages. There is nothing in the description or figures to indicate
exactly what was present, but it is very unlikely that any limbs were
preserved. The broad thin "appendage" figured may have been a fragment
of a thoracic segment. This specimen was evidently described by
Castelnau before 1843, as is inferred from a reference in the Neues
Jahrbuch, 1843, P. 504, but I have not seen the earlier publication.

Burmeister (1843-1846), in his "Organization of the Trilobites,"
reviewed in _extenso_ the history of the search for appendages, and
concluded that they must have been so soft as to preclude the
possibility of their being preserved as fossils. "Their very absence
in fossils most distinctly proves their former real structure" (p.
10). In figures 7 and 8 on plate 6 he gave a restoration of the
ventral surface of an _Asaphus_, the first restoration of the ventral
anatomy to be attempted. Since he chose modern branchiopods as his
model, he did not go so far wrong as he might have done. Still, there
is little in the figure that would now be accepted as correct. The
following quotation will serve to give the opinion of this zoologist,
who from his knowledge of the Crustacea, was the most competent of the
men of his time to undertake a restoration of the appendages of the
trilobites:

     ... in giving a certain form to the feet in the restored figure, I
     have done so rather intending to indicate what they might have
     resembled, than with any idea of assuming their actual form. I
     merely assert that these organs were soft, membranous, and fringed,
     adapted for locomotion in water, placed on the abdominal portion of
     the body, and extending sidewise beneath the lateral lobes of the
     rings, as shown in the ideal transverse section. These feet were
     also indented, and thus divided into several lobes at the open
     lower side, and each separate lobe was furnished at the margin with
     small bristles serving as fins. The last and external lobe was
     probably longer, smaller, and more movable, and reached to the
     termination of the projecting shell lobe, bearing a bladder-shaped
     gill on the inner side (1846, p. 45).

McCoy (1846) observed in several trilobites a pair of pores situated
in the dorsal furrows near the anterior end of the glabella. He showed
that the pits occupy precisely the position of the antennæ of insects
and suggested that they indicated the former presence of antennæ in
these trilobites (chiefly _Anipyx_ and "_Trinucleus_"). The evidence
from _Cryptolithus_, set forth on a later page, indicates the
correctness of McCoy's view.

Richter (1848, p. 20, pl. 2, fig. 32) described and figured what he
took to be a phyllopod-like appendage found in a section through a
_Phacops_. Without the specimen it is impossible to say just what the
structure really was. The outline figure is so obviously modeled on an
appendage of _Apus_ that one is inclined to think it somewhat
diagrammatic. In calling attention to this neglected "find," Clarke
(1888, p. 254, fig.) interprets the appendage as similar to the spiral
branchiæ of _Calymene senaria_, and adds that he himself has seen
evidence of spiral branchiæ in the American Phacops rana.

Beyrich (1846) described a cast of the intestine of "_Trinucleus_,"
and Barrande (1852) further elaborated on this discovery.

Corda (1847) made a number of claims for appendages, but all were
shown by Barrande (1852) to be erroneous.

Barrande (1852, 1872) gave a somewhat incomplete summary of the
various attempts to describe the appendages of trilobites, concluding
that none showed any evidence of other than soft appendages, until
Billings' discovery of 1870.

Volborth (1863) described a long chambered tubular organ in _Illænus_
which he believed to represent a cast of the heart of a trilobite, but
which has since been likened by writers to the intestinal tract in
"_Trinucleus_."




PART I.

THE APPENDAGES OF TRILOBITES.




Terminology.


The terminology employed in the succeeding pages is essentially the
same as that used by Beecher, with two new terms added. Beecher
assigned to the various segments of the limbs the names suggested by
Huxley, but sometimes used the name protopodite instead of coxopodite
for the proximal one. It is obvious that he did not use protopodite in
the correct sense, as indicating a segment formed by the fusion of the
coxopodite and basipodite. The usage employed here is shown in figure
1.

[Illustration: Fig. 1.--_Triarthrus becki_ Green. Diagram of one of
the limbs of the thorax, viewed from above, with the endopodite in
advance of the exopodite. 1, coxopodite, the inner extension being
the endobase (gnathobase on cephalon); 2, basipodite, springing from
the coxopodite, and supporting the exopodite, which also rests upon
the coxopodite; 3, ischiopodite; 4, meropodite; 5, carpopodite; 6,
propodite; 7, dactylopodite, with terminal spines.]

The investigation of _Ceraurus_ showed that the appendages were
supported by processes extending downward from the dorsal test,
and on comparison with other trilobites it appeared that the same was
true in _Calymene_, _Cryptolithus_, _Neolenus_, and other genera. Thin
sections showed that these processes were formed by invagination of
the test beneath the dorsal and glabellar furrows. While these
processes are entirely homologous with the entopophyses of _Limulus_,
I have chosen to apply the name _appendifer_ to them in the
trilobites.

The only other new term employed is the substitution of _endobase_ for
gnathobase in speaking of the inner prolongation of a coxopodite of
the trunk region. The term gnathobase implies a function which can not
in all cases be proved.

The individual portions of which the limbs are made up are called
_segments_, and the articulations between them, _joints_. Such a
procedure is unusual, but promotes clearness.




The Appendages of Neolenus.


HISTORICAL.

The first mention of _Neolenus_ with appendages preserved was in
Doctor Walcott's paper of 1911, in which two figures were given to
show the form of the exopodites in comparison with the branchiæ of the
eurypterid-like _Sidneyia_. In 1912, two more figures were presented,
showing the antennules, exopodites, and cerci. The specimens were
found in the Burgess shale (Middle Cambrian) near Field, in British
Columbia. This shale is exceedingly fine-grained, and has yielded a
very large fauna of beautifully preserved fossils, either unknown or
extraordinarily rare elsewhere. It was stated in this paper (1912 A)
that trilobites, with the exception of _Agnostus_ and _Microdiscus_,
were not abundant in the shale.

In discussing the origin of the tracks known as _Protichnites_,
Walcott presented four figures of _Neolenus_ with appendages, and
described the three claw-like spines at the tip of each endopodite.

Three new figures of the appendages were also contributed to the
second edition of the Eastman-Zittel "Text-book of Paleontology"
(1913, p. 701). Later (1916, pl. 9) there was published a photograph
of a wonderful slab, bearing on its surface numerous Middle Cambrian
Crustacea. Several of the specimens of _Neolenus_ showed appendages.

Finally, in 1918, appeared the "Appendages of Trilobites," in which
the limbs of _Neolenus_ were fully described and figured (p. 126),
and a restoration presented. Organs previously unknown in trilobites,
epipodites and exites, attached to the coxopodites, were found.


=Neolenus serratus= (Rominger).

(Text fig. 2-8.)

     Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1911, p. 20,
     pl. 6, figs. 1, 2 (exopodites of thorax and cephalon);--Ibid., vol.
     57, 1912, p. 191, pl. 24, figs. 1, la (antennules, caudal rami, and
     endopodites of thorax);--Ibid., vol. 57, 1912, p. 277, pl. 45,
     figs. 1-4 (antennules, endopodites of cephalon and thorax, caudal
     rami);--Text-book of Paleontology, edited by C. R. Eastman, 2d ed.,
     vol. 1. 1913, p. 701, fig. 1343 (exopodites), p. 716, fig. 1376
     (abdominal appendages), fig. 1377 (appendages of thorax and
     pygidium);--Ann. Rept. Smithson. Inst. for 1915, 1916, pl.
     9;--Smithson. Misc. Coll., vol. 67, 1918, pp. 126-131 et al., pl.
     14, fig. 1; pls. 15-20; pl. 21, fig. 6; pls. 22, 23; pl. 31
     (restoration); pl. 34, fig. 3 (restored section); pl. 35, fig. 4;
     pl. 36, fig. 3 (hypostoma).

The following description of the appendages of _Neolenus_ is
summarized from Walcott's paper of 1918, and from a study of the eight
specimens mentioned below.

_Cephalon._

The antennules are long, slender, and flexible, and lack the formal
double curvature so characteristic of those of _Triarthrus_. There are
short fine spines on the distal rims of the segments of the proximal
half of each, thus giving great sensitiveness to these organs. In the
proximal portion of each, the individual segments are short and wider
than long, and in the distal region they are narrow and longer than
wide.

There are four pairs of biramous cephalic appendages, which differ
only very slightly from the appendages of the thorax. All are of
course excessively flattened, and they are here described as they
appear.

The coxopodites, shown for the first time in Walcott's paper of 1918,
are broad, longer than wide, and truncated on the inner ends, where
they bear short, stout, unequal spines similar to those along the
anterior margin. The gnathobases are but slightly modified to serve as
mouth parts, much less so than in _Triarthrus_, but the coxopodites
of the cephalon are shorter and wider than those of the thorax.

At the distal end of the coxopodite arise the endopodite and
exopodite. The endopodite consists of six segments, the distal ones,
propodite and dactylopodite, more slender than the others, the last
bearing three terminal spines. The first endopodite is shorter than
the others and slightly more slender (pl. 16, fig. 1)[1] and the
anterior appendages turn forward more or less parallel to the sides of
the hypostoma (pl. 22). The basipodite, ischiopodite, meropodite, and
carpopodite are, in their flattened condition, roughly rectangular,
only a little longer than wide, taper gradually distally, each bears
small spines on the outer rim, and some of the proximal ones usually
have a row along the margin.

[Footnote 1: _Nota bene!_ All references in this section are to the
plates of Doctor Walcott's paper in 1918.]

The exopodites of the cephalon, as of the body of Neolenus, are very
different from those of any other trilobite whose appendages were
previously known. As shown in the photographs (pl. 20, fig. 2; pl.
22), each exopodite consists of a single long, broad, leaf-like blade,
not with many segments as in _Triarthrus_, but consisting of a large
basal and small terminal lobe. It bears on its outer margin numerous
relatively short, slender, flat setæ. The long axes of the exopodites
point forward, and the setæ are directed forward and outward. They
stand more nearly at right angles to the shaft on the cephalic
exopodites than on those of the thorax. This same type of broad-bladed
exopodite is also found on the thorax and pygidium.

The number of functional gnathobases on the cephalon is unknown. That
four endopodites were present on one side is shown pretty clearly
by specimen 58591 (pl. 16, fig. 3) and while no more than two well
preserved exopodites have been seen on a side, there probably were
four. Specimen 65513 (pl. 16, fig. 1) shows gnathobases on the second
and third appendages of that individual as preserved, but there is
no positive evidence that these are really the second and third
appendages, for they are obviously displaced. The hypostoma of
Neolenus is narrow but long, several specimens showing that it
extended back to the horizon of the outer ends of the last pair of
glabellar furrows. It is not as wide as the axial lobe, so that, while
gnathobases attached beneath the first pair of furrows would probably
not reach back to the posterior end of the hypostoma, they might lie
parallel to it and not extend beneath. It seems possible, then, that
there were four pairs of endobases but that the second rather than the
first pair served as mandibles, as seems to be the case in Ceraurus.

_Thorax._

The thorax of _Neolenus_ consists of seven segments, and the
appendages are well shown (pl. 17, fig. 1; pl. 18, figs. 1, 2; pl. 20,
fig. 1.), The endopodites of successive segments vary but little,
all are slender but compact, and consist of a long coxopodite with
six short, rather broad segments beyond it. In the figures, the
endopodites extend some distance in a horizontal direction beyond the
edges of the dorsal test, as many as four segments being in some cases
visible, but measurements show that the appendages tended to fall
outward on decay of the animal. The dactylopodites are provided
with terminal spines as in _Triarthrus_. The coxopodites are long,
straight, and slender. They are well shown on only one specimen (pl.
18), where they are seen to be as wide as the basipodite, and the
endobases are set with spines on the posterior and inner margins. They
are so long that those on opposite sides must have almost met on the
median line. The segments of the endopodites are mostly but little,
if any, longer than broad, and at the distal end each shows two or
more spines. The propodite and dactylopodite are notably more slender
than the others. The exopodites of the thorax are broad and flat, and
each shaft has two distinct parts with different kinds of setæ. The
posterior edge of the proximal lobe is fringed with a slender, flat,
overlapping hairs which are a little longer than the width of the
lobe, and stand at an angle of about 60 degrees with the direction of
the axis of the appendage. The outer lobe is at an angle with the main
one, and has short, very fine setæ oh the margin. One or two specimens
show some evidence of a joint between the inner and outer lobes,
but in the great majority of cases they seem to be continuous; if
originally in two segments, they have become firmly united. The
exopodites of the thorax, like those of the cephalon, are directed
diagonally forward and outward. (pl. 21, fig. 6; pl. 22.)

_Pygidium._

The pygidium of _Neolenus serratus_ is large, and usually shows five
rings on the axial lobe and four pairs of ribs on the sides. There are
five pairs of biramous appendages belonging to this shield, and behind
these a pair of jointed cerci. That the number of abdominal appendages
should correspond to the number of divisions of the axial lobe rather
than to the number of ribs on the pleural lobes is of interest, and in
accord with other trilobites, as first shown by Beecher.

The endopodites of the pygidium have the same form as those of the
thorax, are long, and very much less modified than those of any other
trilobite whose appendages are known. On some specimens, they extend
out far beyond the dorsal test, so that nearly all the segments are
visible (pl. 17, fig. 3; pl. 18; pl. 19; pl. 20, fig. 1), but in these
cases are probably displaced. The segments are short and wide, the
whole endopodite tapering gradually outward. The dactylopodite bears
terminal spines, and the individual segments also have outward-directed
spines.

The cerci appear to have been long, slender, very spinose organs much
like the antennules, but stiff rather than flexible. They are a little
longer than the pygidium (pl. 17, figs. 1, 2), and seem to be attached
to a plate on the under surface of the posterior end and in front of
the very narrow doublure. The precise form of this attachment can not
be determined from the published figures. They bear numerous fine
spines (pl. 17, fig. 3).

_Epipodites and Exiles._

Doctor Walcott has found on several specimens of _Neolenus_ remains of
organs which he interprets as epipodites and exites attached to the
coxopodites. A study of the specimens has, however, convinced me that
both the large and small epipodites are really exopodites, and that
the exites are badly preserved and displaced coxopodites. Detailed
explanation of this interpretation is given below in the description
of the several specimens involved.

_Description of Individual Specimens._

Doctor Walcott was kind enough to send me eight of the more important
specimens of _Neolenus_ figured by him, and since my interpretation
of them does not agree in all respects with his, I have thought it
fairer to the reader to present here rather full notes explaining
the position I have taken. I understand that since I communicated my
interpretation of the epipodites and exites to him, Doctor Walcott has
submitted the specimens to several palæontologists, who consider that
epipodites are really present. Since I am not able to convince myself
that their conclusion is based upon sound evidence, I give here my own
interpretation. There is of course, no a priori reason why trilobites
should not have had epipodites.

Specimen No. 58589.

     Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, pl. 45,
     fig. 2;--Zittel-Eastman Text-book of Paleontology, vol. 1, 1913,
     fig. 1377;--Smithson. Misc. Coll., vol. 67, 1918, pl. 18, fig. 1;
     pl. 20, fig. 1.

This is one of the most important of the specimens, as it shows the
coxopodites of three thoracic limbs and the well preserved endopodites
of six thoracic and five pairs of pygidial appendages.

The appendages are all shifted to the left till the articular socket
of the coxopodite is about 8 mm. outside of its proper position. The
endopodites extend a corresponding amount beyond the edge of the
dorsal test and are there so flattened that they are revealed as a
mere impression. The coxopodites, which are beneath the test, seem to
have been somewhat protected by it, and while hopelessly crushed, are
not flattened, but rather conformed to the ridges and grooves of the
thorax.

[Illustration: Fig. 2. _Neolenus serratus_ (Rominger). A sketch of the
coxopodites and endopodites of two thoracic segments. Note notch for
the reception of the lower end of the appendifer. × 3.]

The coxopodite of the appendage of the last thoracic segment is best
preserved. It is rectangular, about one third as wide as long, with a
slight notch in the posterior margin near the outer end. The inner end
is obliquely truncated and shows about ten sharp spines which do not
appear to be articulated to the segment, but rather to be direct
outgrowths from it. There are similar spines along the posterior
margin, but only two or three of what was probably once a continuous
series are now preserved. On the opposite margin of the coxopodite
from the slight depression mentioned above, there is a slight
convexity in the outline, which is better shown and explained by the
coxopodite just in front of this. That basal segment has the same form
as the one just described, but as its posterior margin is for the
greater part of its length pushed under the one behind it, the spines
are not shown. On the posterior margin, two-thirds of the length from
the proximal end, there is a shallow notch, and corresponding to it, a
bulge on the anterior side. From analogy with Ceraurus and _Calymene_
it becomes plain that the notch and bulge represent the position of
the socket where the coxopodite articulated with the appendifer. Since
these structures have not been shown in previous illustrations, a
drawing giving my interpretation of them is here inserted (fig. 2).
It is evident from the position of the notch that the row of spines
was on the dorsal (inner) side of the coxopodite and that the
truncation was obliquely downward and outward.

The endopodite of the last thoracic appendage is well preserved and
may be described as typical of such a leg in this part. The basipodite
is as wide as the coxopodite, and it and the three succeeding
segments, ischiopodite, meropodite, and carpopodite, are all
parallel-sided, not expanded at the joints, and decrease regularly in
width. The propodite and dactylopodite are also parallel-sided, but
more slender than the inner segments, and on the end of the
dactylopodite there are four little spines, three of them--one large
and two small--articulated at the distal end, and the fourth
projecting from the posterior outer angle. Each segment has one or
more spines on the outer articular end, and the ischiopodite has
several directed obliquely outward on the posterior margin. All of the
four proximal segments show a low ridge parallel to and near the
anterior margin, and several endopodites of the pygidium have a
similar ridge and a row of spines along the posterior margin of some
of the segments. These features indicate that the segments in question
were not cylindrical in life, but compressed. From the almost
universal location of the spines on the posterior side of the limbs as
preserved, it seems probable that in the natural position the segments
were held in a plane at a high angle with the horizontal, the ridge
was dorsal and anterior and the row of spines ventral and posterior.
Because the spines on the endobases are dorsal it does not follow that
those on the endopodites were, for the position of the coxopodite in a
crushed specimen does not indicate the position of the endopodite of
even the same appendage.

The endopodites of the pygidium are similar to the one just described,
except that some of them have spines on the posterior margin of the
segments, and a few on the right side have extremely fine, faintly
visible spines on the anterior side. The specimen shows fragments
of a few exopodites, but nothing worth describing. In the middle
of the right pleural lobe there is a small organ which Walcott has
interpreted as a small epipodite. It is oval in form, broken at the
end toward the axial lobe, and has exceedingly minute short setæ on
the posterior margin. From analogy with other specimens, it appears
to me to be the outer end of an exopodite.

_Measurements:_ The entire specimen is about 64 mm. long and
52 mm. wide at the genal angles. The thorax is about 41 mm. wide
(disregarding the spines) at the seventh segment, and the axial lobe
about 13 mm. wide at the same horizon. The measurements of the
individual segments of the seventh left thoracic limb are:

  Coxopodite, 9 mm. long, 3 mm. wide, the middle of the notch 8 mm.
    from the inner end, measured along the bottom, and 6 mm. measured
    along the top.
  Basipodite,    5   mm. long, 3    mm. wide
  Ischiopodite,  4    "    "   3     "    "
  Meropodite,    3.5  "    "   2.5   "    "
  Carpopodite,   3.5  "    "   2     "    "
  Propodite,     3    "    "   1.25  "    "
  Dactylopodite, 2    "    "   1.25  "    "

The five distal segments of the last pygidial endopodite are together
10.5 mm. long. The whole six segments of the endopodite of the third
thoracic segments are together 21 mm. long. The distance from the
appendifer of the third segment to the outer end of the spine is 17
mm. From the center of the notch in the coxopodite to the outer end
is 1.5 mm., which, added to the length of the endopodite, 21 mm.,
makes a distance of 22.5 mm. from the appendifer to the tip of
the dactylopodite, showing that if projected straight outward, the
endopodites of the thorax would project 5.5 mm. beyond the test,
including spines.

The distance across the axial lobe from appendifer to appendifer on
the seventh thoracic segment is 12.5 mm. Measured along the top of
the coxopodite, it is 6 mm. from the middle of the notch to the inner
end, and measured along the bottom it is 8 mm. From the truncated form
of the ends it is evident that the coxopodites extended inward and
downward from the appendifers, and with the dimensions given above,
the inner toothed ends would practically meet on the median line.

Measurements on the appendages of the pygidia show that on this
specimen they extend back about twice as far beyond the edge of the
pygidium as they should, all being displaced.

Specimen No. 65514.

     Illustrated: Walcott, Smithson. Misc. Coll., vol. 67, 1918,
     pl. 19, figs. 1-3.

This specimen is so twisted apart that it is not possible to determine
to what segments the appendages belong, but it exhibits the best
preserved exopodites I have seen. The best one is just in front of the
pygidium on the matrix, and shows a form more easily seen than
described (our fig. 3). There is a broad, flat, leaf-like shaft, the
anterior side of which follows a smooth curve, while in the curve on
the posterior side, which is convex backward, there is a re-entrant,
setting off a small outer lobe whose length is about one third the
length of the whole. This lobe seems to be a continuation of the
shaft, and the test of the whole is wrinkled and evidently very thin.
The main and distal lobes of the shaft both bear numerous delicate
setæ, but those of the outer lobe are much shorter and finer than
those on the main portion. The latter are flattened and blade-like.

[Illustration: Fig. 3. Exopodite of _Neolenus serratus_ (Rominger), to
show form of the lobes of the shaft, and the setæ. × 4.]

[Illustration: Fig. 4. _Neolenus serratus_ (Rominger). One of the
so-called epipodites of specimen 65515, showing that it has the same
outline as an exopodite (compare figure 3) and fragments of setæ on
the margin. × 3.]

The anterior edge of the shaft shows a narrow stiffening ridge and the
setæ are but little longer than its greatest width. The second segment
of the pygidium has another exopodite like this one, but shows faintly
the line between the two lobes, as though there were two segments.

This specimen also shows some very well preserved endopodites, but
they differ in no way from those described from specimen No. 58589.
Walcott mentions two large epipodites projecting from beneath the
exopodites. I judge that he has reference to the distal lobes of the
exopodites, but as these are continuous with the main shaft, there can
be no other interpretation of them than that which I have given above.


_Measurements:_ The pygidium is 19 mm. long (without the spines) and
about 34 mm. wide at the front. The exopodites show faintly beneath
the pygidial shield, but their proximal ends are too indistinct to
allow accurate measurement. Apparently they were just about long
enough to reach to the margin of the shield. The best preserved one,
that of the second segment in the pygidium, is about 11 mm. long, 2.5
mm. wide at the widest; the distal lobe is 2.5 mm. long, and the
longest setæ of the main lobe 3.5 mm. long. The pleural lobe of the
pygidium is just 11 mm. wide at this point.

The endopodites project from 8 to 12 mm. beyond the pygidium, showing
about four segments.

The thoracic exopodite described above is 11 mm. long and 2.75 mm.
wide at the widest part. The distal lobe is 3.5 mm. long and 2.25 mm.
wide, and the longest setæ on the main lobe 3 mm. long.

Specimen No. 65519.

     Illustrated: Walcott, Zittel-Eastman Text-book of Paleontology,
     vol. 1, 1913, fig. 1343;--Smithson. Misc. Coll., vol. 67, 1918,
     pl. 21, fig. 6.

This specimen is somewhat difficult to study but is very valuable as
showing the natural position of the exopodites of the anterior part of
the thorax. Walcott's figures are excellent and show the broad
leaf-like shafts, the distal lobes with the re-entrant angles in the
posterior margin, and the long fine setæ of the main lobes. None of
the distal lobes retains its setæ. All extend back to the dorsal
furrows, but the proximal ends are not actually shown.

The specimen is especially important because it shows the same distal
lobes as specimen No. 65514, and demonstrates that they are a part of
the exopodite and not of any other structure.

_Measurements:_ The exopodite belonging to the fourth thoracic segment
is 23 mm. long and 4 mm. wide at the widest part. The longest setæ are
7 mm. in length.

Specimen No. 65520.

     Illustrated: Walcott, Smithson. Misc. Coll., vol. 67, 1918, pl. 20,
     fig. 2; pl. 22, fig. 1.

This is a practically entire specimen, on two blocks, one showing the
interior of the shell, and the other the one figured by Walcott, a
cast of the interior. The first shows the low rounded appendifers at
the anterior angle of each axial tergite. They are almost entirely
beneath the dorsal furrows and do not project so far into the axial
lobe as those of Ceraurus and _Calymene_. In fact, only those at the
anterior end of the thorax project inward at all. As expected, there
are five pairs on the pygidium. The cephalon is unfortunately so
exfoliated that the appendifers there are not preserved. The doublure
of the pygidium is extremely narrow.

The cast of the interior shows, rather faintly, the exopodites of the
right side of the thorax and of the left side of the cephalon, and,
still more faintly, the caudal rami and a few pygidial endopodites.
The exopodites on the right side are in what seems to be the customary
position, directed obliquely forward and outward, and the tips of
their distal lobes project slightly beyond the edge of the test. These
lobes were interpreted by Walcott as epipodites, but after comparing
them with the terminal lobes of the exopodites of specimens No. 65519
and 65514 I think there can be no doubt that they represent the same
structure. The pleura of the individual thoracic segments on this side
of the specimen have an unusual appearance, for they are bluntly
rounded or obtusely pointed, instead of being spinose.

The interpretation of the appendages of the cephalon is somewhat
difficult. At the left of the glabella there are two large exopodites,
the anterior of which lies over and partially conceals the other.
These show by their position that they belong to the fourth and fifth
cephalic appendages. In front of these lie two appendages which may be
either endopodites or exopodites, but which I am inclined to refer to
the latter. Both are narrow and shaped like endopodites, but bear on
their outer edges close-set fine setæ. They also show what might be
considered as faint traces of segmentation. If the first of these ran
under the end of the exopodite behind it, as shown in Walcott's figure
(pl. 22), then it would be necessary to interpret it as an endopodite,
but it really continues down between the exopodite and the glabella,
and seems to be attached opposite the middle of the eye. The specimen
does not indicate clearly whether this appendage is above or below
the exopodite behind it, but one's impression is that it is above, in
which case it also must be an exopodite. The appendage in front, being
similar, is similarly interpreted. If this be correct, then the
exopodites of the second and third cephalic appendages are much
shorter and narrower than those of the fourth and fifth. All of these
appendages are obviously out of position, for the cheek has been
pushed forward away from the thorax, though still pivoting on its
inner angle at the neck-ring, till the eye has been brought up to the
dorsal furrow. In this way the anterior exopodites have been thrust
under the glabella and all the appendages have been moved to the right
of their original position. The anterior exopodite is very poorly
shown, but seems to be articulated in front of the eye. The posterior
exopodites are very similar to those on the thorax. The distal lobe is
shown only by the second from the last. It has the same form as the
distal lobes on the thoracic exopodites, and like them has much finer
setæ than the main lobe, but it does not stand at so great an angle
with the axis of the main lobe, nor yet is it so straight as shown in
Walcott's figure.

_Measurements:_ The specimen is about 72 mm. long and 54 mm. wide at
the genal angles. The pygidium is 22 mm. long and 37 mm. wide. The
doublure is 1.5 mm. wide. The exopodite of the third thoracic segment
is 19.5 mm. long. The pleural lobe at this point is 13 mm. wide
without the spines and 18.5 mm. wide with them. The third exopodite of
the cephalon was apparently about 15 mm. long when complete.

Specimen No. 65515.

     Illustrated: Walcott, Smithson. Misc. Coll., vol. 67, 1918, pl. 20,
     figs. 3, 4.

This is a small piece of the axial portion of a badly crushed
Neolenus, showing appendages on the left side as viewed from above. On
the posterior half there are three large appendages which have the
exact form of the exopodites of other specimens. There is a broad,
oval, proximal lobe and a distal one at an angle with it. The proximal
part of the shaft has fine setæ or the bases of them, and the distal
lobe faint traces of much finer ones. The form, and the setæ so far as
they are preserved, are exactly like those of the exopodites on the
specimens previously described. (See fig. 4, page 26.) Beneath them
there are slender, poorly preserved endopodites.

In front of the exopodites and endopodites lie a series of structures
which Walcott has called exites, but for which I can see another
explanation. Walcott has shown them as four broad rounded lobes, but
his figure must be looked upon as a drawing and not as a photograph,
for it has been very much retouched.

For convenience of discussion, these lobes may be called Nos. 1, 2, 3,
and 4, the last being the posterior one (fig. 5). This lobe is best
shown on the matrix, where the anterior end is seen to be margined by
stout spines, while the posterior end lies over the endopodite and
under the exopodite behind it. No. 3 is sunk below the level of the
others, and only a part of it has been uncovered. Its margin bears
strong spines of different sizes. Its full shape can not be made out,
but it has neither the shape nor the form of spines shown in figure 3,
plate 20 (1918). Lobes 2 and 1 and another lobe in front of 1 seem to
form a continuous series and to be part of a single appendage. They
are all in one plane, arc so continuous that the joints between them
can be made out with difficulty and if they do belong together, can
easily be explained.

[Illustration: Fig. 5.--A sketch of the so-called exites of _Neolenus
serratus_ (Rominger), to show the form and the character of the
spines. × 2.]

[Illustration: Fig. 6.--Endopodite of a cephalic appendage of
_Neolenus serratus_ (Rominger), showing the very broad coxopodite.
× 2.]

Before calling these structures new organs not previously seen on
trilobites, it is of course necessary to inquire if they can be
interpreted as representing any known structures. That they can not be
exopodites is obvious, since they are bordered by short stout spines
instead of setæ. The same stout spines that negate the above possible
explanation at once suggest that they are coxopodites (compare fig 6).
At first sight, the so-called exites seem too wide and too rounded to
be so interpreted, but if reference be had to the specimens rather
than the figures, it will be noted that the only well preserved
structure (No. 2) is longer than wide, has spines only on one side and
one end, and does not differ greatly from the coxopodite of specimen
No. 58589 (pl. 18, 1918). If structures 2, 1, and the segment ahead of
1 are really parts of one appendage, it can only be an endopodite, of
which No. 2 is the coxopodite, No. 1 the basipodite, and the next
segment the ischiopodite. If one looks carefully, there are no traces
of spines on either end of No. 1, but only on the margin. The extreme
width of No. 2 is against this interpretation as a coxopodite (see,
however, fig. 6), but it may be rolled out very flat, as this is an
unusually crushed specimen. No. 2 is 10 mm. long and 6 mm. wide at
the widest point. No. 1 is 5 mm. long and 3.5 mm. wide.

The crucial point in this determination is whether 2 and 1 are parts
of the same appendage. I believe they are, but others may differ.

Specimen No. 65513.

     Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, pl. 45,
     fig. 3;--Ibid., vol. 67, 1918, pl. 16, figs. 1, 2.

This is nearly all of the right half of an entire specimen, but the
only appendages of any interest are those of the cephalon. Five
endopodites emerge from beneath that shield, but as all are displaced
it is not possible to say how many belong to the head. When held at
the proper angle to the light, the second and third from the front
show faintly the partial outlines of the coxopodites. The anterior
side and end of the best preserved one shows irregular stout spines of
unequal sizes, and the inner end is truncated obliquely (fig. 6).
These coxopodites are like those on the thorax of specimen No. 58589,
but shorter and wider. This of course suggests that the "exite" No. 2
of specimen No. 65515 may be a cephalic coxopodite. The endopodite of
this appendage, like the others on this cephalon, is shorter and
stouter than the thoracic or pygidial endopodites of the others
described.

[Illustration: Fig. 7.--A restored section across the thorax of
_Neolenus serratus_, showing the probable form of attachment of the
appendages, their relation to the ventral membrane, and the jaw-like
endobases of the coxopodites.]

_Measurements:_ The cephalon is 24 mm. long and about 60 mm. wide. The
coxopodite of the third appendage is about 10 mm. long and 5.5 mm.
wide at the widest point. The corresponding endopodite is 19 mm. long
and projects 11 mm. beyond the margin, which is about 5 mm. further
than it would project were the appendage restored to its proper
position.


RESTORATION OF NEOLENUS.

(Text figs. 7, 8.)

This restoration is based upon the information obtained from the
studies which have been detailed in the preceding pages, and differs
materially from that presented by Doctor Walcott. The appendages are
not shown in their natural positions, but as if flattened nearly into
a horizontal plane. The metastoma is added without any evidence for
its former presence.

The striking features of the appendages are the broad unsegmented
exopodites which point forward all along the body, and the strong
endopodites, which show practically no regional modification. Although
the exopodites have a form which is especially adapted for use in
swimming, their position is such as to indicate that they were not so
used. The stout endopodites, on the other hand, probably performed the
double function of natatory and ambulatory legs.

[Illustration: Fig. 8.--_Neolenus serratus_ (Rominger). A restoration
of the ventral surface, with the endopodites omitted from one side, to
permit a better exposition of the exopodites. The position and number
of the appendages about the mouth are in considerable doubt. Restored
by Doctor Elvira Wood under the supervision of the writer. About
one-half larger than the average specimen.]


=Nathorstia transitans= Walcott.

Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, pl. 28,
fig. 2.

The badly preserved specimen on which this genus and species was
based is undoubtedly a trilobite, but for some reason it does not
find a place in Walcott's recent article on "Appendages" (1918). The
preservation is different from that of the associated trilobites,
being merely a shadowy impression, indicating a very soft test. The
general outline of the body, the position of the eye, and even a
trace of spines about the pygidium (in the figure) are similar
to those of _Neolenus_, and I would venture the suggestion that
_Nathorstia transitans_ is a recently moulted _Neolenus serratus_,
still in the "soft-shelled" condition. Even if not a Neolenus, it is
probable, from the state of preservation, that it is an animal which
had recently cast its shell.

Walcott describes such fragments of appendages as remain, as follows:

     Head. A portion of what may be an antenna projects from beneath the
     right anterior margin; from near the left posterolateral angle a
     large four-jointed appendage extends backward. I assume that this
     may be the outer portion of the large posterior appendage (maxilla)
     of the head.

     Thorax. Traces of several slender-jointed thoracic legs project
     from beneath the anterior segments and back of these on the right
     side more or less of six legs have been pushed out from beneath the
     dorsal shield; these are composed of three or four long slender
     joints; fragments of the three proximal joints indicate that they
     are shorter and larger and that they have a fringe of fine setæ.
     Indications of a branchial lobe (gill) are seen in two specimens
     where the legs are not preserved. This is often the case both among
     the Merostomata (pl. 29, fig. 3, _Molaria_) and Trilobita (pl. 24,
     fig. 2, _Ptychoparia_).

     Two caudal rami project a little distance beneath the posterior
     margin of the dorsal shield.

This latter feature of course suggests _Neolenus_. The other
appendages are too poorly preserved to allow comparison without seeing
the specimen.

The specific name was given "on account of its suggesting a transition
between a Merostome-like form, such as _Molaria spinifera_, and the
trilobites." In what respect it is transitional does not appear.

Formation and locality: Same as that of _Neolenus serratus_. One
nearly complete specimen and a few fragments were found.




The Appendages of Isotelus.


HISTORICAL.

The first specimen of _Isotelus_ with appendages was described orally
by Billings before the Natural History Society of Montreal in 1864,
and in print six years later (1870, p. 479, pls. 31, 32). The specimen
is described in detail on a later page. Billings recognized the
remains of eight pairs of legs on the thorax, a pair for each segment,
and he inferred from the fact that the appendages projected forward
that they were ambulatory rather than natatory organs. He was unable
to make out the exact number of the segments in the appendages, but
thought each showed at least four or five.

Having examined the individual sent to London by Billings, Woodward
(1870, p. 486, fig, 1) reviewed the collection from the American
Trenton in the British Museum and found a specimen in the "Black
Trenton limestone," from Ottawa, Ontario, in which, alongside the
hypostoma, was a jointed appendage, which he described as the "jointed
palpus of one of the maxillæ." This has always been considered an
authentic "find," but I am informed by Doctor Bather that the specimen
does not show any real appendage. For further discussion, see under
_Isotelus gigas_.

In 1871, Billings' specimen was examined by Professors James D. Dana
(1871, p. 320), A. E. Verrill, and Sydney I. Smith, who agreed
that the structures identified by Billings as legs were merely
semicalcified arches of the membrane of the ventral surface, which
opinion seems to have been adopted by zoologists generally in spite of
the fact that the most elementary consideration of the structure of
the thorax of a trilobite should have shown its falsity. While the
curvature of the thoracic segments was convex forward, that of the
supposed ventral arches was convex backward, and the supposed arches
extended across so many segments as to have absolutely prevented any
great amount of motion of the segments of the thorax on each other.
Enrollment, a common occurrence in _Isotelus_, would have been
absolutely impossible had any such calcified arches been present.

Walcott, in his study of trilobites in thin section (1881, pp. 192,
206, pl. 2, fig. 9), obtained eleven slices of _Isotelus gigas_ which
showed remains of appendages. He figured one of the sections, stating
that it "shows the basal joint of a leg and another specimen not
illustrated gives evidence that the legs extended out beneath the
pygidium, as indicated by their basal joints."

The second important specimen of an _Isotelus_ with appendages was
found by Mr. James Pugh in strata of Richmond age 2 miles north of
Oxford, Ohio, and is now in the U. S. National Museum. It was first
described by Mickleborough (1883, p. 200, fig. 1-3). In two successive
finds, a year apart, the specimen itself and its impression were
recovered. Since I am redescribing the specimen in this memoir (see
p. 35), it only remains to state here that Mickleborough interpreted
the structures essentially correctly, though not using the same
terminology as that at present adopted. His view that the anterior
appendages were chelate can not, however, be supported, nor can his
idea that the sole appendages of the pygidium were foliaceous
branchial organs.

Walcott (1884, p. 279, fig. 1) studied the original specimens and
presented a figure which is much more detailed and clear than those of
Mickleborough. By further cleaning the specimen he made out altogether
twenty-six pairs of appendages. He stated that one of these belonged
to the cephalon, nine to the thorax,[1] and the remaining sixteen to
the pygidium. He showed that the endopodites of the pygidium were of
practically the same form as those on the thorax, and stated that the
"leg beneath the thorax of the Ohio trilobite shows seven joints in
two instances; the character of the terminal joint is unknown." His
figure shows, and he mentions, markings which are interpreted as
traces of the fringes of the exopodites.

[Footnote 1: The posterior one of these he believed to have been
crowded forward from beneath the pygidium.]

In the same year Woodward (1884, p. 162, fig. 1-3) reproduced all of
Mickleborough's figures, and suggested that the last seven pairs of
appendages on the pygidium of _Calymene_ and _Isotelus_ were probably
"lamelliform branchiferous appendages, as in _Limulus_ and in living
Isopoda."

Professor Beecher published, in 1902, an outline taken from
Mickleborough's figure of this specimen, to call attention to certain
discontinuous ridges along the axial cavity of the anterior part of
the pygidium and posterior end of the thorax. These ridges are well
shown in Mickleborough's figure, though not in that of Walcott, and
their presence on the specimen was confirmed by a study by Schuchert,
who contributed a diagrammatic cross-section to Beecher's paper (1902,
p. 169, pl. 5, figs. 5, 6). Beecher summarized in a paragraph his
interpretation of this specimen:

     The club-shaped bodies lying within the axis are the gnathobases
     attached at the sides of the axis; the curved members extending
     outward from the gnathobases are the endopodites; the longitudinal
     ridges in the ventral membrane between the inner ends of the
     gnathobases are the buttresses and apodemes of the mesosternites;
     the slender oblique rod-like bodies shown in the right pleural
     region in Walcott's figure are portions of the fringes of the
     exopodites.

In 1910, Mr. W. C. King of Ottawa, Ontario, found at Britannia, a few
miles west of Ottawa, the impression in sandstone of the under surface
of a large specimen of _Isotelus arenicola_, described on a later page
(p. 39).

Finally (1918, p. 133, pl. 24, figs. 3, 3a; pl. 25), Walcott has
redescribed the specimen from Ohio, presenting a new and partially
restored figure. He refers also to the specimen from Ottawa under the
name _Isotelus covingtonensis?_ Foerste (not Ulrich). He advances the
view, which I am unable to share, that the cylindrical appearance of
the segments of the appendages of _Isotelus_ is due to post-mortem
changes.


=Isotelus latus= Raymond.

(pl. 10, fig. 1.)

     Illustrated: _Asaphus platycephalus_ Billings, Quart. Jour. Geol.
     Soc., London, vol. 26, 1870, pl. 31, figs. 1-3; pl. 32, figs. 1,
     2.--Woodward, Geol. Mag., vol. 8, 1871, pl. 8, figs. 1,
     1a.--Gerstäcker, in Bronn's "Klassen u. Ordnungen d. Thier-Reichs,"
     1879, pl. 49, fig. 1.--von Koenen, N. Jahrb. f. Min., etc., vol. 1,
     1880, pl. 8, fig. 8.--Milne-Edwards, Ann. Sci. Nat., Zoologie, ser.
     6, vol. 12, 1881, pl. 12, fig. 45.

     _Isotelus latus_ Raymond, Bull. Victoria Mem. Mus., Geol. Survey
     Canada, No. 1, 1913, p. 45 (species named).

     _Isotelus covingtonensis?_ Walcott (not Foerste), Smithson. Misc.
     Coll., vol. 67, 1918, p. 134.

Knowledge of the appendages of this species is derived from the
specimen which Billings described in 1870. It was found in the
Trenton, probably the Middle Trenton, near Ottawa, Ontario, and is
preserved in the Victoria Memorial Museum at Ottawa.

Viewed from the upper surface, it shows a large part of the test,
but is broken along the sides, so that parts of the free cheeks,
considerable of the pleural lobes of the thorax, and one side of the
pygidium are missing. Viewed from the lower surface, the appendages
are practically confined to the cephalon and thorax.

A short time before his death, Professor Beecher had this specimen and
succeeded in cleaning away a part of the matrix so that the appendages
show somewhat more clearly than in Billings' time, but they are not so
well preserved as on the Mickleborough specimen, found in Ohio
somewhat later.

The hypostoma is in place and well preserved; the posterior points are
but 3 mm. in advance of the posterior margin of the cephalon. Behind
the hypostoma there are only two pairs of cephalic appendages, the
first of which is represented by the coxopodite and a trace of the
endopodite. The outer end of the coxopodite is close to the outer
margin of one of the prongs of the hypostoma and about 3 mm. in front
of its posterior end. The gnathobase curves backward and inward, and
appears to pass under the tip of the hypostoma. There were probably
two appendages in front of this, whose gnathobases projected under the
hypostoma, but the specimen shows nothing of them unless it be that
one small fragment about 2 mm. back of the center is really a part of
a gnathobase.

The specimen retains only the coxopodite and basipodite of the
posterior cephalic appendage on the left side. The coxopodite is
long and apparently cylindrical, the cross-section being of uniform
diameter throughout the length. The inner portion is nearly straight,
while the outer part is curved gently forward.

It is possible to make out remains of eight pairs of appendages on the
thorax, some of them represented by coxopodites only, but most with
more or less poorly preserved endopodites as well. No exopodites are
visible. The coxopodites of the thorax seem to be of the same form
as the last one on the cephalon, but slightly less curved. All are
long and heavy, and there seems to be no decrease in size toward the
pygidium. The endopodites are very imperfectly shown. They seem to be
longer than those of _Isotelus maximus_, and the segments, while of
less diameter than the coxopodites, do not show so great a contrast to
them as do those of that species. The direction of the endopodites is
diagonally forward, and the outer portions do not appear to be curved
backward as in _Isotelus maximus_. It would appear also that the
endopodites were nearly or quite long enough to reach the outer margin
of the dorsal test. On no endopodite can more than three segments be
definitely distinguished, but the longest ones are the most obscurely
segmented.

No appendages are preserved on the pygidium, but at one side of the
median groove there are two projections which may be processes to
which the appendages were attached.

_Measurements:_ Total length of specimen, 109 mm. Probable length when
complete, 116 mm. Length of cephalon, 40 mm.; width at genal angles,
restored, about 62 mm. (Billings' restoration). Width of doublure of
front of cephalon on median line, 17 mm.; length of hypostoma, 20 mm.
Length of coxopodite of last appendage on left side of cephalon,
10.5 mm.; length of basipodite of the same appendage, 5 mm. Diameter
of coxopodite, 2 mm.; diameter of basipodite, 1.5 mm. Length of
coxopodite on left side of the second segment of the thorax, 11 mm.;
diameter, about 2.5 mm. Length of basipodite of the same, 5 mm.;
diameter, about 1.5 mm. Length of ischiopodite, 3.5 mm.; diameter,
about 1.5 mm. Length of meropodite, 2.5 mm. (this may be less than
the total length as the segment is not completely exposed.) Distance
between proximal ends of gnathobases of the fifth thoracic segment,
about 7 mm. Distance between outer ends of the coxopodites of the
first thoracic segment (estimated from measurements on the left side),
27 mm Distance apart of the dorsal furrows at the first thoracic
segment, 27 mm. Length of the longest exopodite which can be traced,
about 20 mm.


=Isotelus maximus= Locke.

(pl. 10, fig. 2.)

     Illustrated: Mickleborough, Jour. Cincinnati Soc. Nat. Hist., vol.
     6, 1883, p. 200, figs. 1-3 (endopodites and coxopodites). Walcott,
     Science, vol. 3, 1884, p. 279, fig. 1 (endopodites, coxopodites,
     and traces of exopodites). Woodward, Geol. Mag., dec. 3, vol. 1,
     1884, p. 162, figs. 1-3 (copies of Mickleborough's figures).
     Bernard, The Apodidæ, 1892, text fig. 49. Beecher, Amer. Jour.
     Sci., vol. 13, 1902, p. 169, pl. 5. figs. 5, 6 (outline from one of
     Mickleborough's figures and an original figure). Walcott, Smithson.
     Misc. Coll., vol. 67, 1918, p. 133, pl. 24, figs. 3, 3a; pl. 25,
     fig. 1.

This specimen, which conies from the Richmond strata 2 miles north of
Oxford, Ohio, is the best preserved of the specimens of _Isotelus_
with appendages which has so far been found. The individual consists
of two parts, the actual specimen, and the impression of the ventral
side.

To describe it I am using very skillfully made plaster reproductions
of both parts, presented to the Museum of Comparative Zoology by
Doctor Charles D. Walcott, and presumably made after he cleaned the
specimen as described in Science (1884). I have also an enlarged
photograph (pl. 10, fig. 2) which seems to have been made after some
later period of cleaning, probably by Professor Beecher, and I have
examined the original specimens in Washington.

Viewed from the dorsal side, it is seen that the individual is very
imperfect, the greater part of the cephalon being removed by a
diagonal break which cuts off the anterior third of the left eye and
extends to the front of the second thoracic segment on the right side.
The ends of the pleura of both sides of the thorax are broken away, as
are also the greater parts of the pleural lobes and the posterior end
of the pygidium. On the ventral side, merely the posterior tips of the
hypostoma remain, but the distal ends of the appendages were so far
within the outer margin that the appendagiferous area is quite fully
retained.

The most conspicuous feature of this specimen is the presence of nine
pairs of large coxopodites behind the hypostoma, and of the remains of
ten pairs of endopodites, making in all ten pairs of appendages which
are easily seen. The apportionment of these segments to cephalon,
thorax, and pygidium is not agreed upon by the people who have
examined the specimens, but if one remembers that it is the outer
and not the inner end of the coxopodite which articulates with the
appendifer, it at once becomes evident that the first two pairs of
appendages on the specimen are the last two pairs belonging to the
cephalon, and that the next eight pairs are those of the thorax.

The impressions of fourteen pairs of coxopodites are readily counted
on the pygidium, and as Doctor Walcott noted sixteen pairs on the
actual specimens, his number was probably correct.

_Cephalon._

Projecting the line of the back of the cephalon through from the
dorsal side, it is found that the posterior tips of the hypostoma are
7 mm. in front of the posterior margin of the cephalon, and that the
points of attachment of the posterior pair of cephalic appendages
(the second pair shown on the specimen) are just within the posterior
margin. The gnathobases of this pair of appendages extend back some
distance beneath the thorax, and so give the impression that they
belong to that part of the body. So far as can be determined, the
cephalic appendages do not differ in any way from those of the thorax.
On the mould of the ventral surface, just outside of the lateral edge
of the right lobe of the hypostoma, is the somewhat imperfectly shown
impression of the endopodite of the third cephalic appendage. The
point of junction of the endopodite and coxopodite is about 2 mm. in
front of the tip of the adjacent branch of the hypostoma, and the
gnathobase is curved around just behind it. This accounts for three of
the pairs of cephalic appendages. The second cephalic appendages must
have thrust their gnathobases under the prongs of the hypostoma, and
the endopodites were probably close to its edge. No trace of this pair
appears on the specimen.

_Thorax._

The thoracic appendages are the best preserved of any, and show the
large coxopodites and the more slender endopodites which do not extend
to the outer margin of the test. The latter extend forward and outward
for about one half their length, then turn backward in a graceful
curve.

Walcott's figure in Science shows hair-like markings on the under
side of the right half of the thorax. These were interpreted by both
Walcott and Beecher as fringes of the exopodites, but since the
setæ of those organs on all other trilobites are always above the
endopodites, while these are represented as below them, it would seem
doubtful if this interpretation can be sustained. Furthermore, I find
no trace of them on either cast or mould, and the actual specimen does
not now show them.

_Pygidium._

The coxopodites and endopodites of the pygidium seem to be similar
to those on the thorax, but both are shorter and more slender, and
the former decrease in length rapidly toward the posterior end. As
mentioned above, it is not perfectly plain how many appendages are
present, but I have accepted Doctor Walcott's count of sixteen pairs.
Of the endopodites only the barest traces are seen, and of exopodites
nothing.

One point of considerable interest in this specimen is the thickness,
as it probably gives some measure of the space occupied by the animal.
In _Triarthrus_ and other trilobites from Rome, New York, the
appendages are pressed directly against the dorsal test, but in this
specimen a considerable space intervenes between the plane of the
appendages and the shell. Between the central furrow and the inner
surface of the dorsal test at the anterior end of the thorax is a
distance of 13 mm. and under the dorsal furrows the thickness is about
7 or 8 mm., no accurate measurement being possible in the present
state of the specimen.

_Measurements:_ Length of specimen on median line, 121 mm.; probable
original length, about 195 mm. (Walcott's restoration). Length of
thorax, 58 mm.[1] Width of axial lobe at the first thoracic segment,
45 mm.; total width as preserved, 92 mm.; width as estimated from the
mould of the ventral surface, no mm.; Walcott's restoration, 105 mm.

[Footnote 1: If this specimen had the same proportions as specimens of
_Isotelus maximus_ from Toronto, the total length would be only 174
mm. The cephalon would be about 52 mm. long, the thorax 58 mm., and
the pygidium about 64 mm. long.]

Length of coxopodite of fourth left cephalic appendage, about 18 mm.;
diameter, about 2.5 mm. Length of coxopodite of last left cephalic
appendage, about 18.5 mm. Distance apart of inner ends of gnathobases
of fourth cephalic appendages, about 4 mm. Distance apart of inner
ends of endobases of first thoracic segment, about 6 mm. Distance
apart of outer ends of coxopodites of first thoracic segment, about 43
mm.

Length of coxopodite of seventh left thoracic appendage 16 mm.,
diameter about 3.5 mm.; length of basipodite of the endopodite of the
same appendage 6 mm.; diameter about 2 mm.; length of ischiopodite 5
mm.; length of meropodite 4.5 mm.; length of carpopodite 4.5 mm.;
length of propodite 3 mm.; length of dactylopodite 2.75 mm.; total
length of endopodite 25.75 mm.

Length of coxopodite of fourth left thoracic appendage 20 mm.,
diameter 4 mm.; length of five proximal joints of the endopodite 25
mm.; diameter of basipodite about 2 mm.


RESTORATION OF ISOTELUS.

(Text fig. 9.)

The exopodites have been omitted from this restoration since nothing
is known of their actual form. The chief reason for the figure is to
contrast the greatly developed coxopodites of the posterior part of
the cephalon and thorax with those of other trilobites. The antennules
and first two pairs of biramous appendages of the cephalon are more or
less hypothetical, and less is known of the appendages of the pygidium
than is shown here. The restoration is based somewhat upon Walcott's
figure in Science. The outline is that of a specimen of _Isotelus
maximus_ from Toronto, Ontario.


=Isotelus gigas= Dekay.

     Illustrated: Woodward, Quart. Jour. Geol. Soc., London, vol. 26,
     1870, text fig. 1; Geol. Mag., dec. 3, vol. 1. 1884, p. 78, text
     fig. Milne-Edwards, Ann. Sci. Nat, Zoologie, ser. 6, vol. 12, 1881,
     pl. 12, fig. 46. Walcott, Bull. Mus. Comp. Zool., Harvard Coll.,
     vol. 8, 1881, pl. 2, fig. 9; Geol. Mag., dec. 4, vol. 1, 1894, pl.
     8, fig. 9; Proc. Biol. Soc. Washington, vol. 9, 1894, pl. 1, fig.
     9.

The specimen in the British Museum which Woodward called _Asaphus
platycephalus_, is, in all probability, an _Isotelus gigas_. Woodward
says of it:

     I was at once attracted by a specimen of _Asaphus_, from the Black
     Trenton Limestone (Lower Silurian), which has been much eroded on
     its upper surface, leaving the hypostoma and what appear to be
     the appendages belonging to the first, second, and third somites,
     exposed to view, united along the median line by a longitudinal
     ridge. The pseudo-appendages, however, have no evidence of any
     articulations. But what appears to me to be of the highest
     importance, as a piece of additional information afforded by
     the Museum specimen, is the discovery of what I believe to be
     the _jointed palpus_ of one of the maxillæ, which has left its
     impression upon the side of the hypostoma--just, in fact, in that
     position which it must have occupied in life, judging by other
     Crustaceans which are furnished with an hypostoma, as _Apus_,
     _Serolis_, etc.

     The palpus is 9 lines in length, the basal joint measures 3 lines,
     and is 2 lines broad, and somewhat triangular in form.

     There appear to be about 7 articulations in the palpus itself,
     above the basal joint, marked by swellings upon its tubular stem,
     which is 1 line in diameter.


[Illustration: Fig. 9.--A restored composite of _Isotelus maximus_ and
_I. latus_. The exopodites are left out because entirely unknown.
Drawn by Doctor Elvira Wood. Natural size.]

Desiring to know more of this individual, I wrote to Doctor Bather
and was surprised to learn that the specimen which was the basis of
Woodward's observations is so badly preserved as to be of no real
value. With his permission, I append a note made by Doctor Bather
some years ago when selecting fossils to be placed on exhibition:

     _Asaphus gigas_ Dekay. Ordovician, Trenton Limestone. N. America,
     Canada. Descr. H. Woodward, 1870, Q. J. G. S., XXVI, pp. 486-488,
     text fig. 1, as _Asaphus platycephalus_. Coll. and presd. J. J.
     Bigsby, 1851. Regd. I 14431.

     This specimen is in the Brit. Mus. Geol. Dept. I 14431. The
     supposed hypostome is exceedingly doubtful; it lies dorsad of the
     crushed glabellar skeleton. The "appendage" is merely the edge of
     a part in the head-shield; the maxilla is some calcite filling,
     between two such laminæ.

  13 Sept. 1911.                                (Signed) F. A. BATHER.

Walcott figured a slice of _Isotelus gigas_ from Trenton Falls, New
York, which shows a few fragments of appendages, but is of particular
importance because it shows the presence of well developed appendifers
beneath the axial lobe.


=Isotelus arenicola= Raymond.

     Illustrated: Ottawa Nat, vol. 24, 1910, p. 129, pl. 2, fig. 5.

The following quotations from my paper are inserted here to complete
the record of appendage-bearing specimens:

     A rather remarkable specimen of this species was found by W. C.
     King, Esq., on the shore of Lake Deschenes at Britannia [near
     Ottawa, Ontario]. This specimen is an impression of the lower
     surface of the trilobite, and shows a longitudinal ridge
     corresponding to the central furrow along the axis of the ventral
     side of the animal, ten pairs of transverse furrows, and the
     impression of the hypostoma. The doublure of the pygidium has
     also left a wide smooth impression, but in the cephalic region
     the hypostoma is the only portion of which there are any traces
     remaining. The specimen was found on a waterworn surface of the
     beach, partially covered by shingle....

     The transverse furrows are the impressions left by the gnathobases
     of the basal joints of the legs. They were evidently long and very
     heavy, but the specimen has been so abraded that all details are
     obscured. The first six pairs of impressions are longer and deeper
     than the four behind. The first eight pairs seem to pertain to the
     thoracic appendages, while the last two belong to the pygidium.
     From the posterior tips of the hypostoma to the first gnathobases
     of which traces are present there is a distance of about 22 mm.
     without impressions. In _Isotelus gigas_ the hypostoma normally
     extends back to the posterior margin of the cephalon, so that it
     seems that in this specimen the impressions of the first two pairs
     of gnathobases under the thorax may not have been preserved. In
     that case, the six pairs of strong impressions may represent the
     last six pairs of thoracic segments, and the pygidium might begin
     with the first of the fainter ones.

_Horizon and locality:_ From the sandstone near the base of the Aylmer
(Upper Chazy) formation at Britannia, west of Ottawa, Ontario.
Specimen in the Victoria Memorial Museum, Geological Survey of Canada,
Ottawa.




The Appendages of Triarthrus.


=Triarthrus becki= Green.

(Pls. 1-5; pl. 6, figs. 1-3; text figs. 1, 10, 11, 33, 42.)

(Also see Part IV.)

     Illustrated: Matthew, Amer. Jour. Sci., vol. 46, 1893, pl. 1, figs.
     1-7;--Trans. N. Y. Acad. Sci., vol. 12, pl. 8, figs. 1-7.--Beecher,
     Amer. Jour. Sci., vol. 46, 1893, text figs. 1-3;--Amer. Geol., vol.
     13, 1894, pl. 3;--Amer. Jour. Sci., vol. 47, pl. 7, text fig.
     1;--Amer. Geol., vol. 15, 1895, pls. 4, 5;--Ibid., vol. 16, 1895,
     pl. 8, figs. 12-14; pl. 10. fig. 1;--Amer. Jour. Sci.,
     vol. 1, 1896, pl. 8; Geol. Mag., dec. 4, vol. 3, 1896, pl.
     9;--Eastman-Zittel Text-book of Paleontology, vol. 1, 1900, text
     figs. 1267-1269;--2d ed., 1913, fig. 1375; Studies in Evolution,
     1901, reprint of all previous figs.;--Amer. Jour. Sci., vol. 13,
     1902, pl. 2, figs. 1-5; pl. 3, fig. 1; pl. 4, fig. 1; pl. 5, figs.
     2-4;--Geol. Mag., dec. 10, vol. 9, 1902, pls. 9-11, text figs.
     1-3.--Walcott, Proc. Biol. Soc. Washington, vol. 9, 1894, pl. 1
     figs. 1-6;--Geol. Mag., dec. 4, vol. 1, 1894, pl. 8;--Smithson.
     Misc. Coll., vol. 67, 1918, pl. 29, figs. 1-11; pl. 30, figs.
     17-20; pl. 32; pl. 34, figs. 4-7; pl. 35, fig. 5.--Bernard, Quart.
     Jour. Geol. Soc., London, vol. 50, 1894, text figs. 11,
     12.--Oehlert, Bull. Soc. Géol. France, ser. 3, vol. 24, 1896,
     text figs. 1-17, 34.--Jaekel, Zeits. d. d. geol. Gesell., vol. 53,
     1901, text fig. 24. Moberg, Geol. Fören. Förhandl., vol. 29, pl. 5,
     1907, pl. 4, fig. 2; pl. 5, fig. 1.--Handlirsch, Foss. Insekten,
     1908, text fig. 6.--Tothill, Amer. Jour. Sci., vol. 42, 1916, p.
     380, text fig. 5.--Crampton, Jour. N. Y. Entomol. Soc., vol. 24,
     1917, pl. 2, fig. 20.




Historical.


Specimens of _Triarthrus_ retaining appendages were first obtained by
Mr. W. S. Valiant from the dark carbonaceous Utica shale near Rome,
New York, in 1884, but no considerable amount of material was found
until 1892. The first specimens were sent to Columbia University, and
were described by Doctor W. D. Matthew (1893). This article was
accompanied by a plate of sketches, showing for the first time the
presence of antennules in trilobites and indicating something of the
endopodites and exopodites of the appendages of the cephalon, thorax,
and pygidium. Specimens had not yet been cleaned from the lower side,
so that no great amount could then be learned of the detailed
structure. Matthew concluded that "The homology with _Limulus_ seems
not to be as close in _Triarthrus_ as in the forms studied by Mr.
Walcott; but the characters seem to be of a more comprehensive type,
approaching the general structure of the other Crustacea rather than
any special form."

Professor Beecher's first paper, dated October 9, 1893, merely
mentioned the fact that the Yale University Museum had obtained
material from Valiant's locality, but was quickly followed by a paper
read before the National Academy of Sciences on November 8, and
published in December, 1893. This paper described particularly the
thoracic appendages.

This was followed in January (1894 A) by an article in which some
information about the mode of occurrence of the specimens was added,
and in April (1894 B), the limbs of the pygidium were described and
figured. The determination of the structure of the appendages of the
head evidently presented some difficulty, for the article describing
this portion of the animal did not appear until the next February
(1895 A). This cleared up the ventral anatomy of _Triarthrus_, and was
followed by a short article (1896 A) accompanied by a restoration of
the trilobite showing all the appendages.

This ended Professor Beecher's publications on _Triarthrus_ until his
final paper in 1902, although he contributed some of his results and
figures to his chapter on the trilobites in the Eastman-Zittel
Text-book of Paleontology in 1900.

The discovery of these excellent specimens had of course excited very
great interest. Doctor Walcott also studied a number of specimens from
Valiant's locality, and published in 1894, with some original figures,
the results of his comparison of the appendages of _Triarthrus_ with
those of _Calymene_ and _Ceraurus_.

In his article on the "Systematic Position of the Trilobites," Bernard
(1894) used the results of Professor Beecher's studies of 1893, and
also quoted the papers by Matthew (1893) and Walcott (1894), though
the article by the latter appeared too late to be used except for a
note added while Bernard's paper was in press. A final footnote quoted
from Professor Beecher's paper of April, 1894 (1894 B).

Oehlert (1896) gave an excellent summary in French of the work of
Beecher and Walcott on _Triarthrus_, with reproductions of many of
their figures.

Valiant (1901) in a non-technical article described his long search
for trilobites with antennas. The discovery of the wonderful pyritized
trilobites at Cleveland's Glen near Rome was not the result of a lucky
accident, but the culmination of eight years of labor in a locality
especially selected on account of the fineness of grain of the shale.

[Illustration: Fig. 10.--_Triarthrus becki_ Green. A new restoration,
modified from Professor Beecher's, to incorporate the results of his
later work. The inner ends of the endobases are probably too far
apart, as it was not discovered until after the drawing had been made
that the appendifers projected within the dorsal furrows. Drawn by
Doctor Elvira Wood. × about 3.8.]

After 1896, Professor Beecher turned his attention largely to the
problem of the classification of trilobites, and while he continued
the arduous task of cleaning the matrix from specimens of
_Triarthrus_ and _Cryptolithus_ he did not again publish upon the
subject of appendages until forced to do so by the doubts cast by
Jaekel (1901) upon the validity of his earlier conclusions. Because of
certain structures which he thought he had interpreted correctly from
a poorly preserved specimen of _Ptychoparia_, Jaekel came to the
conclusion that Beecher's material was not well preserved. Professor
Beecher would have taken much more kindly to aspersions upon his
opinions than to any slight upon his beloved trilobites, and his
article on the "Ventral Integument of Trilobites" of 1902 was designed
not only as an answer to Jaekel, but also to show by means of
photographs the unusually perfect state of preservation of the
specimens of _Triarthrus_. This article, like so many describing the
appendages of trilobites, beginning with Matthew's, was published in
two places (Beecher 1902).

Most of Beecher's papers, except the last one, were reprinted in
the volume entitled "Studies in Evolution," published by Charles
Scribner's Sons at the time of the Yale Bicentennial in 1901. The
part pertaining particularly to _Triarthrus_ is on pages 197 to 219.

Moberg (1907), in connection with a specimen of _Eurycare angustatum_
which he thought preserved some appendages, described and illustrated
some of the appendages of _Triarthrus_.

The most recent discussion of _Triarthrus_, with some new figures,
is by Walcott (1918, p. 135, pls. 29, 30). He gives a summary of
Beecher's work with numerous quotations. The principal original
contribution is a discussion of the form and shape of the appendages
before they were flattened out in the shale. He found also what
he thought might possibly be the remains of epipodites on three
specimens, one of which he illustrated with a photograph. I have seen
nothing which could be interpreted as such an organ in the many
specimens I have studied.

A point in which Walcott differs from Beecher in the interpretation of
specimens is in regard to the development of the endopodites of small
pygidia. Beecher (1894 B, pl. 7, fig. 3) illustrated a series of
endopodites which he likened to the endites of a thoracic limb of
_Apus_. Doctor Walcott finds that specimens in the United States
National Museum show slender endopodites all the way to the back of
the pygidium, and thinks that Beecher mistook a mass of terminal
segments of exopodites for a series of endopodites. On careful
examination, however, the specimen shows, as Beecher indicated, a
series of endopodites in undisturbed condition (No. 222, our pl. 4,
fig. 5).

_Restoration of Triarthrus._

One of the more important points noted in the later studies of
_Triarthrus_ is that the gnathites of the cephalic appendages are much
less like the endobases under the thorax than Beecher earlier thought,
and showed in his restored figures and in his model. The four
gnathites of each side are curved, flattened, not club-shaped, and
so wide and so close together that they overlap one another. The
metastoma is somewhat larger and more nearly circular than Beecher's
earlier preparations led him to suppose.

The restoration here presented is modified only slightly from the
one designed by Professor Beecher, and the modifications are taken
principally from figures published by him. The gnathites are drawn in
form more like that shown by the specimens and his figures in the
American Geologist (1895 A), and the metastoma is taken from one of
the specimens. On the thorax the chief modification is in the addition
of a considerable number of spines to the endopodites. In spite of the
trivial character of most of these changes, they emphasize one of the
important characteristics of _Triarthrus_ the regional differentiation
of the appendages.

It should be pointed out that although _Triarthrus_ is usually
considered to be a very primitive trilobite, its appendages are more
specialized than those of any of the others known. This is shown in
their great length, the double curvature of the antennules, the
differentiation of four pairs of endobases on the cephalon as
gnathites, and the flattening of the segments of the posterior
endopodites. These departures from the uniformity existing among the
appendages of the other genera lead one to question whether the genus
is really so primitive as has been supposed.

_Relation of the Cephalic Appendages to the Markings on the Dorsal
Surface of the Glabella._

_Triarthrus becki_ is usually represented as having four pairs of
glabellar furrows, but the two pairs at the front are exceedingly
faint and the first of them is hardly ever visible, though that it
does exist is proved by a number of authentic specimens. The neck
furrow is narrow and sharply impressed, continuing across the glabella
with a slightly backward curvature. In front of it are two pairs of
linear, deeply impressed furrows which in their inward and backward
sweep are bowed slightly forward, the ends of the corresponding
furrows on opposite sides nearly meeting along the crest of the
glabella. In front of these, near the median line, is a pair of slight
indentations, having the appearance and position of the inner ends of
a pair of furrows similar to those situated just behind them.

In front of and just outside this pair are the exceedingly faint
impressions of the anterior pair of furrows, these, as said above,
being but seldom seen. They are short, slightly indented linear
furrows which have their axes perpendicular to the axis of the
cephalon, and do not connect with each other or with the dorsal
furrows. The latter are narrow, sharply impressed, and merge into a
circumglabellar furrow at the front. In front of the circumglabellar
furrow is a very narrow rounded ridge, but the anterior end of the
glabella is very close to the margin of the cephalon.

Specimen No. 214, which was cleaned from the dorsal side, shows the
posterior tip of the hypostoma, apparently in its natural position,
3.5 mm. back from the anterior margin. The entire length of the
cephalon is 6 mm., so that the hypostoma reaches back slightly over
one half the length (0.583). The greater part of it has been cleaned
off, and one sees the proximal portions of the antennules, which are
apparently attached just at the sides of the hypostoma, 2.5 mm. apart
and 2.25 mm. back from the anterior edge of the cephalon. This
position is distinctly within the outline of the glabella and
corresponds approximately to the location of the second pair of
glabellar furrows. Specimens 214, 215, 216, 217, and 219 all seem to
show the same location for the bases of the antennules. Specimen 220
is the one in which the basal shafts are best preserved and the points
of attachment seem to be further apart in it than in any of the
others. This specimen is 38 mm. long, and the bases of the antennules
are 5.5 mm. apart and 4 mm. behind the anterior margin. As the
specimen is cleaned from the ventral side, the dorsal furrows do not
show distinctly, but another specimen of about the same size (No. 228,
38.5 mm. long) has the dorsal furrows 8 mm. apart 4 mm. back of the
anterior margin.

On the same slab with specimens 209 and 210 there is an individual
which, although retaining the test, has had the proximal ends of the
antennules so pressed against it that the course of the one on the
left side is readily visible. It originates in a small oval mound
whose posterior margin impinges upon the third glabellar furrow near
the middle of its course, and just outside the outer end of the second
glabellar furrow. The cephalon of this specimen is 5 mm. long, and the
point of origin of the left antennule is 2.75 mm. in front of the
posterior margin and 0.75 mm. from the dorsal furrow.

It is therefore evident that the antennules in this species are not
attached beneath the dorsal furrows, but within them and opposite the
second pair of glabellar furrows.

All cephalic appendages behind the antennules are attached somewhat
within the dorsal furrows, the first pair as far forward as the
antennules and the last pair apparently under the anterior edge of
the neck ring. They do not appear to correspond in position to the
posterior glabellar furrows and neck ring, being more crowded. The
last pair is attached to appendifers beneath the nuchal segment, and
the first pair beneath the third glabellar furrows. There are no
depressions on the dorsal surface corresponding to the points of
attachment of the mandibles.

Anal Plate.

Professor Beecher, during his first studies of _Triarthrus_, found no
appendages pertaining to the anal segment, but later evidently came
upon a spinose anal plate which he caused to be figured. The specimen
(No. 201) on which this appendage is preserved is cleaned from the
dorsal side, and the anal plate is a small, bilaterally symmetrical,
nearly semicircular structure margined with small spines. Specimen 202
also shows the same plate (pl. 5, fig. 6), but it is imperfectly
preserved. It has a large perforation in the anterior half. Both of
these specimens are in the Yale University Museum.

[Illustration: Fig. 11.--_Triarthrus becki_ Green. Anal plate of
specimen 65525 in the U. S. National Museum. Drawn by Doctor Wood. ×
20.]

The anal plate is especially well shown by specimen 65525 in the
United States National Museum (fig. 11). This specimen is from Rome,
New York, and two photographs of it have been published by Walcott
(1918, pl. 29, fig. 6; pl. 30, fig. 19). It is developed from the
dorsal side, and the anal plate is displaced, so that it projects
behind the end of the pygidium. It is semicircular in shape, with a
hemispheric mound at the middle of the anterior half. Two furrows
starting from the anterior edge on either side of the mound border its
sides, and, uniting back of it, continue as an axial furrow to the
posterior margin. The mound is perforated for the opening of the
posterior end of the alimentary canal. The lateral borders of the
plate bear five pairs of short, symmetrically placed spines. The plate
is 1 mm. wide and 0.5 mm. long, and the entire trilobite is 11.5 mm.
long.




THE APPENDAGES OF PTYCHOPARIA.


=Ptychoparia striata= (Emmrich).

     Illustrated: Jaekel, Zeits. d. d. geol. Gesell., 1901, vol. 53,
     part 1, pls. 4, 5.

Jaekel has described a specimen of this species obtained from the
Middle Cambrian near Tejrovic, Bohemia, which on development showed
beneath the test of the axial lobe, certain structures which he
believed represented the casts of proximal segments of appendages.
On the basis of this specimen he produced a new restoration of the
ventral surface of the trilobite, in which he showed three short wide
segments in the place occupied by the coxopodite of an appendage of
_Triarthrus_. He also made the mouth parts considerably different from
those of the latter genus. Beecher (1902) showed that the structures
which Jaekel took for segments of appendages were really the fillings
between stiffening plates of chitin on the ventral membrane, and
demonstrated the fact that similar structures existed in _Triarthrus_.
It cannot be said, therefore, that any appendages are really known in
_Ptychoparia striata_, but some knowledge of the internal anatomy of
the species is supplied by the specimen.


=Ptychoparia cordilleræ= (Rominger).

     Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 192,
     pl. 24, fig. 2;--Ibid., vol. 67, 1918, pl. 21, figs. 3-5 (corrected
     figure).

Walcott has figured a single individual of this species showing
appendages, the accompanying description being as follows (1918, p.
144):

     Ventral appendages. Only one specimen has been found showing the
     thoracic limbs. This indicates very clearly the general character
     of the exopodite and that it is situated above the endopodite,
     although there are only imperfect traces of the latter....

     The exopodites are unlike those of any trilobite now known. They
     are long, rather broad lobes extending from the line of the union
     of the mesosternites and the pleurosternites. At the proximal end
     they appear to be as wide as the axial lobe of each segment, and to
     increase in width and slightly overlap each other nearly out to the
     distal extremity.... They are finely crenulated along both the
     anterior and dorsal margins, which indicates the presence of fine
     setæ.

The specimen is quite imperfectly preserved, but seems to indicate
that the exopodite of Ptychoparia had a long, rather narrow
unsegmented shaft.

_Measurements_ (from Walcott's figure): The specimen is a small one,
about 9.5 mm. long, an individual exopodite is about 2 mm. long and
the shaft 0.33 mm. wide.

_Horizon and locality:_ Middle Cambrian, Burgess shale, between Mount
Field and Wapta Peak, above Field, British Columbia.


=Ptychoparia permulta= Walcott.

     Illustrated: Walcott, Smithson. Misc. Coll., vol. 67, 1918, p. 145,
     pl. 21, figs. 1, 2.

Walcott figured one individual of this species showing long slender
antennules projecting in front of the cephalon. It is of especial
interest because one of the antennules shows almost exactly the same
sigmoid curvature which is so characteristic of the related
_Triarthrus_. The individual segments are not visible.

_Measurements:_ The specimen is 23 mm. long and the direct distance
from the front of the head to the anterior end of the more perfect
antennule is 9.5 mm. Measured along the curvature, the same antennule
is about 11 mm. long.

_Horizon and locality:_ Same as the preceding.




The Appendages of Kootenia.


=Kootenia dawsoni= Walcott.

     Illustrated: Walcott, Smithson. Misc. Coll., vol. 67, 1918, pl. 14,
     figs. 2, 3.

One specimen figured by Doctor Walcott shows the distal ends of some
of the exopodites and endopodites of the right side. He compares the
exopodites with those of Neolenus, stating that the shaft consists
of two segments, the proximal section being long and flat, fringed
with long setæ, while the distal segment has short fine setæ. The
endopodite best shown is very slender, and the segments are of uniform
width and only slightly longer than wide.

Measurements (from Walcott's figures): Length of specimen, about 41
mm. Length of five distal segments of an endopodite, 7.5 mm. Since
the pleural lobe is only 7 mm. wide, the endopodites, and probably
the exopodites also, must have projected a few millimeters beyond the
dorsal test when extended straight out laterally.

Formation and locality: Burgess shale, Middle Cambrian, on the west
slope of the ridge between Mount Field and Wapta Peak, above Field,
British Columbia.




The Appendages of Calymene and Ceraurus.


HISTORICAL.

All of the work on these species has been done by Doctor Walcott, who
summarized his results in 1881.

In the first of his papers (1875, p. 159), Walcott did not describe
any appendages but paved the way for further work by a detailed and
accurate description of the ventral surface of the dorsal shell of
Ceraurus. He demonstrated the presence in this species of strongly
buttressed processes which extend directly downward from the test just
within the line of the dorsal furrows. One pair of these is seen
beneath each pair of the glabellar furrows, each segment of the thorax
has a pair, and there are four pairs on the pygidium. He pointed out
also that these projections were but poorly developed on that part of
the glabella which is covered by the hypostoma. He called them axial
processes, the only name which appears to have been suggested thus
far.

The first announcement of the discovery of actual appendages in
_Ceraurus_ and _Calymene_ was made by the same investigator in a
pamphlet published in 1876 in advance of the 28th Report of the New
York State Museum of Natural History, the publication of the whole
report being delayed till 1879. The results were obtained by the
process of cutting translucent slices of enrolled trilobites derived
from the Trenton limestone at Trenton Falls, New York. Since he
summarized all the results of this study in one paper at a later
date, it is not necessary to follow the stages of the work.

A second preliminary paper was published in pamphlet form in
September, 1877, and in final form in 1879, when the first figures
were presented.

In his important paper of 1881, Walcott reviewed all that was known of
the appendages of trilobites to that time, and gave the results of
seven years of study of sections of enrolled specimens. Slices had
been made of 2,200 individuals from Trenton Falls, which resulted in
obtaining 270 which were worthy of study. Of these, 205 were from
_Ceraurus pleurexanthemus_, 49 from _Calymene senaria_, 11 from
_Isotelus gigas_, and 5 from _Acidaspis trentonensis_.

Walcott's views on certain portions of the anatomy can best be set
forth in the form of a few extracts (1881, pp. 199-208):

_The Ventral Membrane._--In those longitudinal sections in which the
ventral membrane is most perfectly preserved, it is shown to have been
a thin, delicate pellicle or membrane, strengthened in each segment by
a transverse arch, to which the appendages were attached. These arches
appear as flat bands separated by a thin connecting membrane, somewhat
as the arches in the ventral surface of some of the Macrouran
Decapods....

In by far the greater number of sections, both transverse and
longitudinal, the evidence of the former presence of an exterior
membrane, protecting the contents of the visceral cavity, rests on the
fact that the sections show a definite boundary line between the white
calcspar, filling the space formerly occupied by the viscera, and the
dark limestone matrix. Even the thickened arches are rarely seen.

The mode of attachment of the leg to the ventral surface is shown [in
transverse and longitudinal sections of _Ceraurus_ and _Calymene_].
These illustrations are considered as showing that the point of
articulation was a small, round process projecting from the posterior
surface of the large basal joint, and articulating in the ventral arch
somewhat as the legs of some of the Isopods articulate with the arches
in the ventral membrane. The arches of the ventral membrane in the
trilobite ... afford a correspondingly firm basis for the attachment
of the legs.

Branchial appendages.--The branchiæ have required more time and labor
to determine their true structure than any of the appendages yet
discovered. They were first regarded as small tubes arranged side by
side, like the teeth in a rake; then as setiferous appendages, and
finally as elongate ribbon-like spirals and bands attached to the side
of the thoracic cavity, the epipodite being a so-called branchial arm.
All of these parts are now known to belong to the respiratory system,
but from their somewhat complex structure, and the various curious
forms assumed by the parts when broken up and distorted, it was a long
time before their relations were determined.

The respiratory system is formed of two series of appendages, as found
beneath the thorax. The first is a series of branchiæ attached to the
basal joints of the legs, and the second, the branchial arms, or
epipodites.

The branchiæ, as found in _Calymene_, _Ceraurus_, and _Acidaspis_,
have three forms. In the first they bifurcate a short distance from
the attachment to the basal joint of the leg, and extend outward and
downward as two simple, slender tubes, or ribbon-like filaments.
In the second form they bifurcate in the same mariner, but the two
branches are spirals. These two forms occur in the same individual
but, as a rule, the more simple ribbon-like branchia is found in the
smaller or younger specimens, and the spiral form in the adult.... The
spiral branchiæ of Ceraurus are usually larger and coarser than those
of _Calymene_.

The third type of the branchiæ [consists of rather long straight
ribbons arranged in a digitate manner on a broad basal joint]. As far
as yet known, this is confined to the anterior segments of the thorax.

The epipodite or branchial arm was attached to the basal joints of the
thoracic legs and formed of two or more joints. This has been called a
branchial arm, not that it carried a branchia, but on account of its
relation to the respiratory system. It is regarded as an arm or
paddle, that, kept in constant motion, produced a current of water
circulating among the branchiæ gathered close beneath the dorsal
shell. . . .

Of the modification the respiratory apparatus underwent beneath the
pygidium, we have no evidence.

In his latest publication (1918, pp. 147-153, pls. 26-28, 33), Walcott
has reviewed his earlier work on _Calymene_ and _Ceraurus_, and
presented a new restoration of the former. The coxopodites are now
interpreted as being similar to those of _Triarthrus_ and Neolenus,
but the exopodites are still held to be spiral and the setiferous
organs labelled as epipodites rather than exopodites.




Comparison of the Appendages of Calymene and Ceraurus with those of
Triarthrus.


As one may see by reading the above quotations from Doctor Walcott's
descriptions, he found certain branchial organs in _Ceraurus_ and
_Calymene_ which have not been found in other trilobites but otherwise
the essential features of the appendages of all are in agreement.

Spiral Branchiæ.

It is now necessary to inquire if the thin sections can not be
interpreted on the basis of trilobites with the same organs as
_Triarthrus_. The interpretation of the structures seen in these
translucent slices is exceedingly difficult, and Doctor Walcott
deserves the utmost praise for the acumen with which he drew his
deductions. Even with the present knowledge of _Triarthrus_,
_Isotelus_, and _Neolenus_ as a guide, I do not think it is safe to
speak dogmatically about what one sees in them.

Walcott has summarized his results in his restoration of the
appendages of _Calymene_ (1918, pl. 33). The coxopodite supports a
slender six-jointed endopodite as in _Triarthrus_, dorsal to which is
a short setiferous epipodite which differs from the exopodite of
_Triarthrus_, in being less long, unsegmented, and in having shorter
setæ. Arising from the same part of the coxopodite with this epipodite
is the bifurcate spiral branchia which has not been seen in this form
in other trilobites. The evidence on which the existence of this organ
is postulated consists of a series of sections across the thorax, the
best of them figured by Walcott in his plates 2 and 3 (1881) and plate
27 (1918).

The specimens sliced were all partially or quite enrolled, and in that
position one would expect to find the appendages so displaced that it
would be only rarely that a section would be cut, either by chance or
design, in such a direction as to show any considerable part of any
one appendage. This expectation has proved true in regard to the
endopodites, the sections rarely showing more than two or three
consecutive segments. Sections like those shown in figures 1 and 2
in plate 2 (1881) seem to be unique. On the other hand, there are
numerous slices showing the so-called spiral branchiæ. They show for
the most part as a succession of rectangular to kidney-shaped spots
of clear calcite.[1] Usually these clear spots are isolated, not
confluent, but in a small number of specimens, perhaps three or four,
the spots are connected in such a way as to show a zig-zag band which
suggests a spiral. Such an explanation is of course entirely
reasonable, but it would be surprising if so slender a spiral should
be cut in such a way as to exhibit the large series of successive
turns shown in many of these thin sections. Continuous sections of
such organs should be no more common than continuous sections of
endopodites.

[Footnote 1: In looking at Walcott's figures of 1881, it should be
remembered that the dark portions of the figures are clear calcite in
the specimens, while the light part is the more or less opaque
matrix.]

One of the arguments against the interpretation of these series of
spots as sections across spiral arms is that of probabilities. It
is known from flattened specimens that _Neolenus_, _Kootenia_,
_Ptychoparia_, _Triarthrus_, and _Cryptolithus_ all have a single type
of exopodite, consisting of a simple setiferous shaft. All these
genera have been examined in a way that permits no doubt about the
structure, and no trace of spiral arms has been detected. On the other
hand, Walcott found spiral arms in three unrelated genera, _Calymene_,
_Ceraurus_, and _Acidaspis_, all of the trilobites in which he found
exopodites by the method of sectioning. What are the probabilities
that genera of three different families, studied by means of sections,
should agree in having a type of exopodite different from that of the
five genera about whose interpretation there can be no doubt?

Another argument against the interpretation of the sections as spirals
is that in any one line the individual spots are of roughly uniform
size. This means of course that the spiral has been cut by a plane
parallel to the tangent plane. This might happen once, just as once
Doctor Walcott cut all six segments of a single endopodite, but that
it should happen repeatedly is highly improbable. Moreover, there is
a limit to the diameter of the section which may be made from these
slender spirals. Most of the spots have one diameter about one half
greater than the other, but others are from three to six times as long
as wide. These last could obviously be cut only from a very large
spiral, and they are therefore interpreted by Walcott as setæ of
epipodites. Yet all gradations are found among the sections, from the
long setæ to the short dots. (See pl. 27, 1918.) In referring to one
slice, Walcott says (1918, p. 152):

In the latter figure and in figure 13, plate 27, the setæ of several
epipodites appear to have been cut across so as to give the effect
of long rows of setæ. The same condition occurs in specimens of
_Marrella_ when the setæ of several exopodites are matted against each
other.

[Illustration: Fig. 12.--A slice of _Ceraurus pleurexanthemus_ in
which the exopodite happened to be cut in such a way as to show a part
of the shaft and some of the setæ in longitudinal section. Specimen
80. × 4.]

This is certainly an apt comparison, and equally true if _Neolenus_,
_Triarthrus_, or _Cryptolithus_ were substituted for _Marrella_.

Now consider the "epipodites." They are well shown in _Calymene_ in
the specimens illustrated on plate 27, figure 11 (1918), and plate 3,
figure 3 (1881), and less clearly in one or two others. Slices 22 (pl.
27, fig. 12, 1918) and 80 (our fig. 12) show what is called the same
organ in Ceraurus. It will be noted that all of these slices are cut
in the same way, that is, more or less parallel to the under surface
of the head, or, at any rate, on a plane parallel to a plane which
would be tangent to the axial portion of the coiled shell. The
sections which show the spirals best are those which are cut by a
plane perpendicular to the long axis of the body. If one were to
attempt to cut an enrolled _Triarthrus_ in such a way as to get a
section showing the length of the setæ, one would not cut a section
perpendicular to the axis of the animal, nor, in fact, would he cut
one parallel to the ventral plane, but it is obvious that in this
latter type of section he would stand a better chance of finding a
part of the plane of the exopodite coincident with the plane of his
section than in the former. And that seems to be what has happened in
these sections of _Calymene_ and _Ceraurus_. If the exopodites were
preserved, transverse sections were bound to cut across many sets of
fringes, and the resultant slice would show transverse sections of the
setæ as a series of overlapping spots. A few fortunately located
sections in a more nearly horizontal plane might cut the setæ and
occasionally the shaft of one or more exopodites in the longitudinal
plane, and the resulting effect would produce the so-called
"epipodites." A careful study has shown that no one of these
epipodites is complete, and they do not have the palmate form shown in
Walcott's figures.

And the last and most important argument against the spiral appendages
is that certain slices, of both _Calymene_ and _Ceraurus_, show
definitely exopodites of exactly the type found in other trilobites.
These are discussed later in the detailed description of the various
slices.

If these series of spots are interpreted on the basis of the known
structure of _Triarthrus_, they are of course a series of sections
through the setæ of the exopodites. It will be shown in Part IV
that these setæ are not circular in section, but flattened, in
_Cryptolithus_ even blade-like, and that they overlap one another. A
section across them would give the same general appearance as, for
instance, that shown in figures 4, 6, 9, and 10 of Walcott's plate 3
(1881).

When both endopodites and the "spiral branchiæ" are present in the
same section (pl. 1, fig. 4; pl. 2, figs. 1, 2), the "spiral branchiæ"
are dorsal to the endopodites, as the setæ of the exopodites would be
expected to be. The specimens which show the clear spots connected,
and which suggest a spiral (pl. 3, fig. 5), may seem at first sight to
bear evidence against this interpretation, but one has only to think
of the effect of cutting a section along the edge where the setæ are
attached to the shaft of the exopodite of _Triarthrus_ to see that
such a zig-zag effect is entirely possible. One would expect to cut
just this position only rarely, and, in fact, the zig-zags are seen in
only three or four sections. The bifurcation of the basal segment of
the "spiral branchiæ" (pl. 3, fig. 10, 1881) is probably more apparent
than real, if indeed these basal segments have anything to do with the
succeeding one.

A second peculiarity of _Calymene_, shown in Walcott's restoration, is
the great enlargement of the coxopodites and of the distal segments of
the endopodites of the fifth pair of appendages of the cephalon. This
is based on the sections of plate 3, figures 6, 7, 8, 9, 10 (1881).
After a study of the specimens I regret to find myself still
unconvinced that the posterior cephalic appendages were any larger
than those in front.

Ventral Membrane.

The most striking value of the thin sections of _Ceraurus_ and
_Calymene_, and therein they have a great superiority over all the
other forms so far investigated, is that they show the extent of the
body cavity and the position, though not the substance, of the ventral
membrane. Transverse sections through _Ceraurus_ (Walcott's pl. 1.
figs. 1-5; pl. 2, figs. 1, 3, 1881) and _Calymene_ (pl. 3, figs. 9,
10, 1881) show that the body cavity was almost entirely confined to
the axial lobe. The longitudinal sections of _Ceraurus_ (pl. 2, figs.
6, 8; pl. 4, fig. 8) and of _Calymene_ (pl. 2, figs. 5, 7; pl. 5,
figs. 1-4) show that the ventral membrane was exceedingly thin and was
wrinkled transversely when the shell was enrolled.

The specimens of figures 1-3, plate 5 (1881) show the form of the
ventral membrane more distinctly than any of the others. The section
of figure 1 was cut just inside the dorsal furrow on the right side,
and figure 2, which is on the opposite side of the same slice, is
almost exactly on the median line. Figure 3 shows a section just
inside the left dorsal furrow. Section 2 did not cut any of the
appendages, and the ventral membrane is shown as a thickened,
probably chitinous sheet thrown into low sharply crested folds equal
in number to, and pointing in a direction just the reverse of, the
crests of the segments of the thorax. Under the pygidium, where there
would of course be less wrinkling, the folds are hardly noticeable. In
the actual specimens one sees more plainly than in the figures the
line of separation between the ventral membrane and the appendages,
but the state of preservation of everything beneath the dorsal shell
is so indefinite that one does not feel sure just what the connection
between the appendages and the membrane was. In the original of figure
5, plate 2, which seems to have been cut so as to cross the appendages
at their line of junction with the ventral membrane, there appear to
be narrow chitinous (?) plates extending from the ventral membrane to
the dorsal test.

Appendifers.

In Ceraurus there are regular calcareous processes which extend down
from the dorsal test just inside the line of the dorsal furrow, and
which undoubtedly serve as points of attachment of the appendages.
These processes, which for convenience I have designated as
"appendifers," are broken off in most specimens showing the lower
surface of _Ceraurus pleurexanthemus_, but on certain ones cleaned
with potash they are well preserved. Doctor Walcott showed them well
in his figures of the lower surface of this species (1875, pl. 11;
1881, pl. 4, fig. 5), while the attempt of Raymond and Barton (1913,
pl. 2, fig. 7) to show them by photography was not so successful.

There is one pair of appendifers on each of the thoracic segments and
four pairs on the pygidium. On the cephalon there is one pair under
the neck furrow, and a pair under the posterior glabellar furrows.
These are not concealed by the hypostoma. Further forward, and
completely covered by the hypostoma, are two much less strongly
developed but similar ones, so that there are in all four pairs of
appendifers on the cephalon, though it is extremely doubtful if the
appendages were articulated directly to all of them. On a specimen of
_Ceraurus pleurexanthemus_ 30 mm. long on the median line, the dorsal
furrows are 7.5 mm. apart at the anterior end of the thorax, and the
tips of the appendifers of this segment are only 4 mm. apart. Each
consists of a straight slender rod with a knoblike end projecting
directly downward from the dorsal test, and supported by a thin
calcareous plate which runs diagonally forward to the anterior edge of
the segment directly under the dorsal furrow. On the pygidium three
pairs of the appendifers have this form, while the fourth pair consist
of low rounded tubercles which are concealed by the doublure. These
appendifers are probably cut in many of Walcott's sections of
Ceraurus, but owing to the state of preservation it is not always
possible to determine what part is appendage, what part is body
cavity, and what part is appendifer.

Nearly forty years ago Von Koenen (1880, p. 431, pl. 8, figs. 9, 10)
described and figured the appendifers of Phacops latifrons. He found
them to be calcareous projections on the hinder margin of each
segment, converging inward, and about 1.5 mm. long. He correctly
considered them as supports (Stützpunkte) for the feet.

Appendifers are well developed also in Pliomerops, and in well
preserved specimens of _Calymene senaria_ from Trenton Falls they are
present, but instead of being rod-like processes, they are rather
thick, prominent folds of the shell. They are also well shown in some
of the thin sections. A specimen of _Triarthrus_ (No. 229, our pl. 5,
fig. 2) has broad processes extending downward from the lower side of
the test below the dorsal furrows, much as in _Calymene_, and the
individual of _Cryptolithus_ shown in plate 8, figure 1, possesses
slender appendifers. Two other specimens (Nos. 237 and 242) show them
quite well. They were probably present in all trilobites, but seldom
preserved. The appendifers have the same origin as the entopophyses of
_Limulus_, and like them, may have relatively little effect on the
dorsal surface.

_Calymene senaria_ Conrad.

(Text figs. 13-16, 23.)

     Illustrated: Walcott, Bull. Mus. Comp. Zool., Harvard Coll., vol.
     8, 1881, pl. 1, figs. 6-10; pl. 2, figs. 5-7, 10; pl. 3, figs. 1,
     3, 8-10; pl. 4, figs. 3, 7; pl. 5, figs. 1-6; pl. 6, figs. 1
     (restoration), 2;--Proc. Biol. Soc. Washington, vol. 9, 1894, pl.
     1. fig. 7 (restoration);--Geol. Mag., dec. 4, vol. 1. 1894, pl. 8,
     figs. 7, 8;--Smithson. Misc. Coll., vol. 67, 1918, pl. 26, figs.
     1-7, 9-13; pl. 27, figs. 4, 5 (not 5a), 11 (not 12, _Ceraurus_),
     13, 14, 15 (not _Ceraurus_); pl. 28, figs. 7, 8; pl. 33, fig. 1
     (restoration); pl. 34, fig. 2; pl. 35, fig. 6.--Dames, N. Jahrb. f.
     Min., etc., vol. 1, 1880, pl. 8, figs. 1-5.--Milne-Edwards, Ann.
     Sci. Nat., Zoologie, ser. 6, vol. 12, 1881, pl. 11, figs. 19-32;
     pl. 12, figs. 33-41.--Packard, Amer. Nat., vol. 16, 1882, p. 796,
     fig. 12.--Bernard, The Apodidæ, 1892, text figs. 50, 52,
     54;--Quart. Jour. Geol. Soc., London, vol. 50, 1894, text figs. 13,
     15, 17.--Oehlert, Bull. Soc. Géol. France, ser. 3, vol. 24, 1896,
     fig. 12.--Beecher, Amer. Jour. Sci., vol. 13, 1902, pl. 5, fig. 7.

In both of Walcott's accounts (1881, 1918) of the appendages of
_Calymene_ and _Ceraurus_, he has described them together, so that
those who have not taken time to study the illustrations and
disentangle the descriptions are very apt to have a confused notion in
regard to them. I have therefore selected from the original specimens
those slices of _Calymene_ which are most instructive, and bearing in
mind the probable appearance of the appendages of an enrolled
_Triarthrus_, have tried to interpret them. In such a method of study,
I have of course started with a pre-formed theory of what to expect,
but have tried to look for differences as well as likenesses.

_Cephalic Appendages._

_Antennules._--The evidence of antennules rests on a single slice (No.
78). The appendage in question is exceedingly slender and arises at
the side of the hypostoma near its posterior end. It shows fine,
slender segments, and curves first outward and then forward. If it is
in its natural position, it is not an antennule, but the endopodite of
the second or third pair of cephalic appendages. It is short, only
about one-third the length of the hypostoma, but is doubtless
incomplete. The two distal segments show a darker filling, indicating
that they were hollow. Judging from analogy with other trilobites, the
appendage is probably an endopodite and not an antennule. There can be
no reasonable doubt, however, that _Calymene_ possessed antennules.

Some idea of the form of the coxopodites of the cephalic appendages
may be obtained from sections which cut in approximately the plane of
the hypostoma. Such sections are shown in Walcott's photographs (pl.
26, figs. 4, 6, 11, 1918). Specimens 50 (fig. 4, our fig. 13), 51
(fig. 6), 6 (fig. 11), and 40 (our fig. 14) agree in showing two
pairs of slender coxopodites which are attached at the sides of the
hypostoma and run backward parallel and close to it, and two pairs of
larger coxopodites which are behind the hypostoma, although the point
of attachment of the third pair is in front of its tip. The anterior
pair are apparently under-developed and no longer function as mouth
parts, while the posterior two pairs are large and armed on their
inner ends with spines. Specimen 78, which has already been mentioned
in connection with the antennules, shows a second very slender
appendage back of the so-called antennule, which is equally slender,
but is directed outward instead of forward. It seems not improbable,
from their position and similarity, that these two are the endopodites
of the first two appendages on one side of the hypostoma. Specimen 6
shows rather inadequately the endopodites of the second and third
cephalic appendages. I have not found other slices showing endopodites
of the cephalon. Walcott, in both his restorations, has shown
enlarged, paddle-shaped dactylopodites on the distal ends of the
fourth cephalic endopodites. The evidence for this rests principally
on three slices, No. 38 (pl. 26, figs. 9, 10), 53 (pl. 26, fig. 12),
and 43 (pl. 26, fig. 13). Of these, No. 43 may be dismissed at once as
too poorly preserved to be interpreted. No. 53 does show a section of
an appendage which seems to have an unusually wide dactylopodite, but
this slice presents no evidence at all as to the appendage to which
the dactylopodite appertains, nor can one even be sure that there has
not been a secondary enlargement. Specimen 43 shows this feature
much less definitely than is indicated by the published photograph
and drawing. The segment in question is strongly curved, with a
constriction possibly dividing it into two. If it is in its natural
position in this section, it obviously belongs to one of the thoracic
segments and not to the cephalon. With evidence of difference so
unsatisfactory, I prefer to reconstruct the posterior cephalic
endopodites on the same plan as those of the thorax.

[Illustration: Fig. 13.--Slice through _Calymene senaria_ in the plane
of the hypostoma, showing the very slender coxopodites beside that
organ, the spines on the inner end of one of the maxillulæ, and the
anterior position of the attachment of all these appendages. From a
photographic enlargement. Specimen 50. × 4.]

[Illustration: Fig. 14.--Slice through the hypostoma and thorax of
_Calymene senaria_ Conrad, showing the small size of the coxopodites
nearest the hypostoma. Shell in black, appendages and filling of
abdominal cavity dotted. From a photographic enlargement. Specimen 40.
× 3.8.]

[Illustration: Fig. 15.--Transverse section of _Calymene_, showing
method of articulation with the appendifer. The shell is in solid
black, the filling of the appendage and appendifer stippled. Traced
from a photographic enlargement of the slice. Specimen 63. × 7.]

_Exopodites._--Walcott admits that there is no direct evidence of spiral
exopodites in the cephalon of _Calymene_. No one of the sections
cutting through the plane of the hypostoma shows any trace of
appendages which could be interpreted as exopodites.

_Thoracic Appendages._

The large coxopodites of the anterior thoracic appendages are well
shown in many specimens cut longitudinally, of which Nos. 23, 50, and
55 may be mentioned, since photographs of them have been published by
Walcott (pl. 26, figs. 1-4, 1918). The endobases of all taper toward
the proximal ends. Transverse slices show sections of the coxopodites
which are no wider than those in longitudinal sections, indicating
that they were not compressed but probably cylindrical. This is borne
out by an individual (pl. 28, fig. 7, 1918) which is not a slice but
an actual specimen, the body cavity of which was hollow, and, opened
from above, shows the impressions of the last two coxopodites of the
cephalon, and the first four of the thorax.

One transverse section (No. 63, see our fig. 15) is especially
valuable, as it shows the method of articulation of the coxopodites
with the dorsal skeleton. Another specimen (No. 73) shows that
appendifers are present in _Calymene_, and while the appendifer does
not retain its original form in slice No. 63, the section does show
clearly that there was a notch in the inner (upper) side of the
coxopodite into which the lower end of the appendifer fitted, thus
giving a firm, articulated support for the appendage. This notch
appears to be slightly nearer the outer than the inner end of the
coxopodite, and since it must have made a kind of ball-and-socket
joint, considerable freedom of movement was allowed. The appendage
must have been held in place by muscles within the coxopodite and
attached to the appendifer.

No slice which I have seen shows a continuous section through all the
segments of an endopodite, but many, both longitudinal and transverse,
show one, two, or as many as three segments.

Such sections as No. 120 show that the endopodites of the thorax
were slender and composed of segments of rather uniform diameter.
Other sections, notably No. 83, 154, and in, show that they tapered
distally, and bore small spines at the outer end of each segment.

The exopodites of course furnish the chief difficulty in
interpretation. Doctor Walcott finds two sets of structures attached
to the coxopodite, a long, slender, spiral exopodite, and a short,
broad epipodite with a fringe of long setæ. Since he has given the
same interpretation for _Calymene_, _Ceraurus_, and _Acidaspis_, I
have considered the question of all three together on a preceding page
(p. 48), and given my reasons for regarding both structures as due to
sections in different directions across setiferous exopodites.

Sections like those shown in figures 11, 13, and 14 of plate 27 (1918)
happen to be cut in or near the plane of the setæ of an exopodite, and
so show hairs of considerable length. Such sections are, as would be
expected, very few in number, while sections like those shown on
figures 4, 5, 7, and 9 of plate 27, which cut the setæ more nearly at
right angles, are very common. Slices which give any definite idea of
the form of the shaft of the exopodite are exceedingly rare. Perhaps
the most satisfactory one is No. 23 (pl. 3, fig. 3, 1881), which shows
the proximal part of a long, slender, unsegmented shaft, with the
bases of a number of slender setæ. The organ is not complete, as would
be inferred from the published figure, but the section cuts diagonally
across it, and the total length is unknown. It is directed forward,
like the exopodites of Neolenus, but whether or not this is a natural
position is yet to be learned.

The proximal, non-setiferous portion of the exopodite is evidently
at an angle with the setiferous part. Another similar exopodite is
apparently shown by specimen 29 (pl. 3, fig. 9, 1881), which has a
similar angulated shaft and just a trace of the bases of the setæ.

_Pygidial Appendages._

That appendages were present under the pygidium is shown by
longitudinal sections, but nothing is known of the detail of
structure.

[Illustration: Fig. 16. Restoration of _Calymene senaria_ Conrad,
based upon data obtained from the study of the translucent sections
made by Doctor Walcott. Prepared by Doctor Elvira Wood, under the
supervision of the author. About twice natural size.]

_Relation of Hypostoma to Cephalon in Calymene._

In _Calymene_ the shape of the hypostoma bears little relation to the
shape of the glabella, and it is relatively smaller, both shorter and
narrower, than in Ceraurus. In shape, neglecting the side lappets at
the front, it is somewhat rectangular, but rounded at the back, where
it is bifurcated by a shallow notch. The anterior edge has a narrow
flange all across, which is turned at almost right angles to the plane
of the appendage, and which fits against the doublure of the free
cheeks at the sides and against the epistoma in the middle. The side
lappets show on their inner (upper) surface shallow pits, one on each
lappet, which fit over projections that on the dorsal surface show as
deep pits in the bottom of the dorsal furrows in front of the anterior
glabellar furrows. The appendifers on the head in _Calymene_ take the
form of curving projections of shell underneath the glabellar and neck
furrows, and owing to the narrowness of the hypostoma, all these are
visible from the ventral side, even with it in position. This shield
extends back about 0.6 of the length of the cephalon, and to a point
a little behind the second glabellar furrow from the back of the head.

In Doctor Walcott's restoration of _Calymene_ he has represented
all four pairs of biramous appendages as articulating back of the
posterior end of the hypostoma. I think his sections indicate that
the gnathobases of two pairs of these appendages rested alongside or
beneath it, and in particular, the longitudinal sections (1881, pl. 5)
would appear to show that the mouth was some distance in advance of
its posterior end.

_Restoration of Calymene._

(Text fig. 16.)

From what has been said above, it is evident that for a restoration of
the appendages of _Calymene_ considerable dependence must be placed
upon analogy with other trilobites. Nothing is positively known of the
antennules, the exopodites of the cephalon, or any appendages, other
than coxopodites, of the pygidium, but all were probably present. It
is inferred from the slices that the first two pairs of cephalic
appendages were poorly developed, the endopodites short and very
slender, the coxopodites lying parallel to the sides of the hypostoma
and nearly or quite functionless. The gnathites of the last two pairs
of cephalic appendages are large, closely approximated at their inner
ends, and bear small tooth-like spines. The endopodites are probably
somewhat better developed than the anterior ones and more like those
on the thorax.

The coxopodites of the thorax appear to have had nearly cylindrical
endobases which tapered inward. The endopodites were slender, tapering
gradually outward, and probably did not extend beyond the dorsal test.
Small spines were present on the distal end of each segment. Each
exopodite had a long, slender, unsegmented shaft, to which were
attached numerous long, overlapping, flattened setæ. The shaft may
have been angulated near the proximal end, and may have been directed
somewhat forward and outward as in Neolenus, but the evidence on this
point is unsatisfactory. The number of pairs of appendages is that
determined by Walcott from longitudinal sections, namely, four pairs
on the cephalon beside the antennules, thirteen pairs in the thorax,
and nine pairs on the pygidium.


=Calymene= sp. ind.

(pl. 6, figs. 4, 5.)

     Illustrated: Walcott, Bull. Mus. Comp. Zool., Harvard Coll., vol.
     8, 1881, pl. 6, figs. 5a, b;--Proc. Biol. Soc. Washington, vol. 9,
     1894, pl. 1, fig. 10;--Geol. Mag., dec. 4, vol. 1, 1894, pl. 8,
     fig. 10;--Smithson. Misc. Coll., vol. 67, 1918, pl. 36, figs. 1, 2,
     2a-d.--Milne-Edwards, Ann. Sci. Nat., Zoologie, ser. 6, vol. 12,
     1881; pl. 12, figs. 44a, b.

In the United States National Museum there is a thin piece of
limestone, about 3 inches square, which has on its surface eight
jointed objects that have been called legs of trilobites. Two of these
were figured by Walcott (1881, pl. 6, fig. 5). The slab contains
specimens of _Dalmanella_ and _Cryptolithus_, in addition to the
appendages of trilobites, and is said by Doctor Ulrich to have come
from the tipper part of the Point Pleasant formation (Trenton) on the
bank of the Ohio River below Covington, Kentucky.

The specimens are all endopodites of long slender form, similar to
those of _Triarthrus_, but since that genus does not occur in the
Point Pleasant, it is necessary to look upon some other trilobite as
the former possessor of these organs. Both _Isotelus_ and _Calymene_
occur at this horizon, and as the specimens obviously do not belong
to _Isotelus_ or _Cryptolithus_, it is probable that they were
formerly part of a _Calymene_.

All the endopodites are of chitinous material, and the various
specimens show, according to the perfection of their preservation,
from four to six segments. The endopodite as a whole tapers but
slightly outward, and the individual segments are of nearly equal
length. They appear to be but little crushed, and are oval in section,
with a crimped anterior and posterior margin. One or two show a median
longitudinal ridge, such as is seen in some appendages of
_Triarthrus_. Each segment is parallel-sided, with a slight expansion
at the distal end, where the next segment fits into it.

Under the heading "Ordovician Crustacean Leg," Walcott (1918, p. 154,
pl. 36, figs. 1,2) has recently redescribed these specimens, and
thinks that they do not belong to _Calymene_, nor, indeed, to any
trilobite. He concludes that they were more like what one would expect
in an isopod. Passing over the fact that the oldest isopod now known
is Devonian, the fossils in question seem to me quite trilobite-like.
Walcott says:

     The legs are associated with fragments of _Calymene meeki_ but it
     is not probable that they belong to that species; if they did, they
     are unlike any trilobite leg known to me. The very short coxopodite
     and basopodite are unknown in the trilobites of which we have the
     legs, as they are fused into one joint forming the long protopodite
     in the trilobite. The distal joint is also unlike that of the
     trilobite legs known to us.

A great deal of Doctor Walcott's difficulty probably arises from his
homology of the coxopodite of the trilobite with the protopodite of
the higher Crustacea. The coxopodite of the trilobite is not fused
with the basipodite, this latter segment always remaining free.
Indeed, Walcott himself says of _Neolenus_ (1918, p. 128):

     Each thoracic leg (endopodite) is formed of a large elongate
     proximal joint (protopodite), four strong joints each about 1.5
     times as long as wide (basopodite, ischiopodite, meropodite and
     carpopodite); two slender elongate joints (propodite and
     dactylopodite) and a claw-like, more or less tripartite
     termination.

Walcott's drawing (pl. 36, fig. 1) is a composite one, and while it
shows eight segments, I was not able to count more than seven on any
of the specimens themselves. In regard to the terminal segment,
the dactylopodite of the limb shown in his plate 36, figure 2, is
unusually long, and a comparison with other photographs published on
the same plate shows that such long segments are unusual.

Proof that these are appendages of a _Calymene_ is of course wanting,
but there is no particular reason so far to say that they are not.

_Measurements:_ Two of the more complete specimens, each showing six
segments, are each 8 mm. long.

Somewhat similar to the specimens from Covington are the ones
described by Eichwald (1825, p. 39, 1860, pl. 21), the specimens being
from the Silurian of Gotland. The figure copied by Walcott (1881, pl.
6, fig. 4) has never been looked upon as entirely satisfactory
evidence of the nature of the specimen, and so far as I know, the
fossil has not been seen by any modern investigator.


=Ceraurus pleurexanthemus= Green.

(pl. 11; text figs. 12, 17-19, 21, 22, 24, 29, 30.)

     Illustrated: Walcott, Ann. Lye. Nat. Hist. New York, vol. II, 1875,
     pl. 11;--31st Ann. Rept. New York State Mus. Nat. Hist, 1879, pl.
     1, fig. 3;--Bull. Mus. Comp. Zool., Harvard Coll., vol. 8, 1881,
     pl. 1, figs. 1-5; pl. 2, figs. 1-4, 6-8; pl. 3, figs. 2, 4-7; pl.
     4, figs. 1, 2, 4-6, 8; pl. 6, fig. 3; Smithson. Misc. Coll., vol.
     67, 1918, pl. 26, figs. 8, 14, 15; pl. 27, figs. 1-3, 5a, 6-9, 12
     (not _Calymene_), (not 15, _Calymene_); pl. 28, figs. 1-5; pl. 34,
     fig. 1; pl. 35, fig. 7.--Milne-Edwards, Ann. Sci. Nat., Zoologie,
     ser. 6, vol. 12, 1881, pl. 10, figs. 1-18.--Bernard, The Apodidæ,
     1892, text figs. 46, 51.

_Cephalic Appendages._

No trace of antennules has yet been found.

I find only three sections cut through the plane of the hypostoma of
Ceraurus which show anything of the cephalic appendages, and no one of
them is very satisfactory. The best is No. 22, the one figured by
Walcott (pl. 3, fig. 2, 1881; pl. 27, fig. 12, 1918), but one should
remember that this section is not actually cut in the plane of the
hypostoma but is a slice diagonally through the head, cutting through
one eye and the posterior end of the hypostoma. It shows what seem to
be the coxopodites of the second, third, and fourth pairs of cephalic
appendages, the exopodites of the third and fourth pairs, and the
metastoma. If this interpretation is correct, the first pair of
gnathites lay alongside the hypostoma or under its edge, and were
feebly developed, the second pair were attached in front of the tip of
the hypostoma, curved back close to it, and their inner ends reached
the sides of the metastoma. The third and fourth pairs were back of
the metastoma, the third pair was stronger than the second, and the
fourth probably like the third.

[Illustration: Fig. 17. Transverse section of _Ceraurus
pleurexanthemus_, showing the relation of the coxopodite to the
appendifer. Traced from a photographic enlargement of the slice.
Specimen 128. × 4/5.]

[Illustration: Fig. 18. Slice of _Ceraurus pleurexanthemus_, showing a
nearly continuous section of an endopodite and an exopodite above it.
The latter is so cut as to show only the edge of the shaft and the
bases of a few setæ. Traced from a photographic enlargement. Specimen
in. × 4.]

Specimen 92 shows traces of the slender endopodites belonging to the
cephalon, but no details. Specimen 22 shows on one side exopodites
(epipodites of Walcott) belonging to the third and fourth cephalic
appendages. That belonging to the third shows some long setæ and a
trace of the shaft, while the one on the fourth appendage (third
coxopodite) has a portion of a broad shaft and a number of long setæ.
It should again be remembered that the slice does not cut through the
plane of the exopodite, but across it at a low angle, so that a part
but not all of the shaft is shown. On the other side of this slice
there is a fairly good section of one of the thoracic exopodites. It
is, however, turned around in the opposite direction from the others,
as would be expected in an enrolled specimen.

Specimens 4 and 5 (pl. 1, figs. 4, 5, 1881) are slices cut diagonally
through the head of Ceraurus, in front of the posterior tip of the
hypostoma. They show fragments of endopodites and exopodites which may
be interpreted as practically identical in form with those of the
thorax. Due to the diagonal plane in which the section is cut, slice 5
shows the coxopodites of two pairs of appendages, one lying nearer
the median cavity than the other. It is extremely difficult to
visualize the interpretation of such sections.

_Thoracic Appendages._

A transverse section through a thoracic segment (No. 128, our fig. 17)
shows the relation of coxopodite to appendifer to be the same as in
_Calymene_, the upper side of the coxopodite having a notch a little
outward from the middle. After seeing that specimen, it is possible to
understand slice No. 168, which shows longitudinal sections through a
number of coxopodites of the thorax, with fragments of both exopodites
and endopodites articulated at the distal ends. These and longitudinal
vertical sections like No. 18 (pl. 2, fig. 8, 1881) show that the
endobases taper inward, and the general uniformity in width in
sections taken at various angles indicates that the coxopodites were
not greatly flattened.

A unique slice (No. 111, pl. 2, fig. 2, 1881; pl. 27, fig. 1, 1918;
our fig. 18) shows a nearly complete thoracic endopodite, and above it
a part of the proximal end of the exopodite of the same segment. When
one considers that out of over two thousand sections only this one
shows the six successive segments of an endopodite, one realizes how
futile it is to expect that dozens of the equally slender "spirals"
should be cut so as to show practically all their turns.

This endopodite is slender, all the segments have nearly the same
length and diameter, though there is a slight taper outward, each
segment is expanded distally for the articulation of the next, and
there are small spines on the distal ends of some of them. There is
probably a terminal spine present, though it is neither so long nor so
plainly visible as in Walcott's photograph.

The exopodite on this same specimen was evidently cut diagonally
across near the setiferous edge, showing a section through the shaft
and the bases of seven setæ (fig. 18). This section is so exactly what
would be obtained by cutting similarly an exopodite of either Neolenus
or _Triarthrus_ that it should in itself dispose of the
"spiral-exopodite" theory.

Several sections have already been illustrated showing sections across
the setæ of the exopodites (pl. 3, figs. 4-6, 1881; pl. 27, figs. 3,
4, 9, 1918), and similar sections are not uncommon. Only a very few,
however, show sections in the plane of the exopodite. If only No. 111,
described above, were known, it would be inferred that the exopodite
had a slender shaft as in _Calymene_, but another good slice, No. 80
(fig. 12, ante) shows that the blade was rather broad, though not so
broad as in Neolenus. The other specimen is No. 22, which has already
been discussed. The thoracic exopodite of this specimen has been very
incorrectly figured by Walcott, as it shows no such palmate shaft as
he has indicated, but a long blade-like one is outlined, though its
entire width is not actually shown.

_Pygidial Appendages._

Sections 14 and 18 (pl. 2, figs. 4, 8, 1881) prove the presence under
the pygidium of three pairs of appendages, the coxopodites and
fragments of endopodites of which are shown. Nothing is known of the
exopodites.

_Relation of Hypostoma to Cephalon._

In Ceraurus the body portion and posterior end of the hypostoma are
roughly oval, about as wide as the glabella at its broadest part, and
the posterior edge extends back to within 0.5 to 1 mm. of the neck
furrow. The posterior pair of appendifers are behind the hypostoma,
while the second pair are in front of its posterior end but escape
being covered by it on account of its oval shape. At the anterior end
the hypostoma is widened by the presence of two side lappets which
extend beyond the boundaries of the glabella. In both Ceraurus and
Cheirurus the anterior edge of the hypostoma fits against the doublure
at the anterior margin of the head and the epistoma is either entirely
absent or is so narrow as not to be seen in specimens in the ordinary
state of preservation. A section across the cephalon of _Ceraurus
pleurexanthemus_ at the horizon of the eyes shows the sides of the
hypostoma fitting closely against the sides of the glabella (Walcott's
pl. 1, fig. 1). Further back on the head it is not in contact with the
dorsal test, and the gnathobases extend beneath it.

Restoration of _Ceraurus pleurexanthemus_. (pl. 11; text fig. 19.)

The restoration of the appendages of _Ceraurus pleurexanthemus_ is a
tentative one, based upon a careful study of the translucent sections
prepared by Doctor Walcott. In no case among these sections is the
actual test of any appendage preserved, and the real form of each part
is generally obscured by the crystallization of the calcite which
fills the spaces formerly occupied by animal matter.

[Illustration: Fig. 19. Restoration of a transverse section of the
thorax of _Ceraurus pleurexanthemus_ Green, showing the relation of
the appendages to the appendifers and the ventral membrane. The
probable positions of the heart and alimentary canal are indicated.]

No section shows anything which can be identified as any part of the
antennules, so that these organs have been supplied from analogy with
_Triarthrus_.

There are undoubtedly four pairs of biramous Cephalic appendages, but
their points of attachment are not so obvious. There are two pairs of
conspicuous appendifers on the posterior part of the cephalon and
another pair almost concealed by the hypostoma. It is probable that
the appendages of the cephalon were not attached directly beneath
them, as the four pairs have to be placed within the space occupied by
the three pairs of appendifers. As the mouth is in front of the
posterior end of the hypostoma, the gnathites of the first pair of
biramous appendages may have extended beneath that organ, or they may
have lain beside it, and only become functional when the hypostoma was
dropped down in the feeding position. The second pair of gnathites
reached just to the tip of the hypostoma, and the other two pairs
seemingly curved backward behind it.

The points of attachment on the thorax, as shown clearly in sections,
were directly beneath the lower ends of the appendifers. The
endopodites were long enough to reach to or a little beyond the outer
extremities of the pleural spines, while the exopodites were
apparently somewhat shorter. Each endopodite consisted of six short,
fairly stout segments, each with at least two spines on the somewhat
expanded distal ends. The exact form of the exopodites could not be
made out. The shaft was apparently rather short, unsegmented, and
fairly broad. The setæ appear from the sections to have been more or
less blade-shaped and to have overlapped, as do those of the
exopodites of _Cryptolithus_. Judging from their position in the
sections, the setæ not only bordered the posterior side of the shaft,
but radiated out from the end as well.

The pygidium shows three pairs of functional appendifers, hence three
pairs of appendages have been supplied. There is a fourth pair of
rudimentary appendifers, but as they are beneath the doublure they
could not have borne ambulatory appendages.


The Appendages of Acidaspis trentonensis Walcott.

(pl. 6, fig. 6.)

A single individual of _Acidaspis trentonensis_, obtained from the
same locality and horizon as the specimens of _Triarthrus_ and
_Cryptolithus_, when cleaned from the ventral side shows a number
of poorly preserved endopodites which seem very similar in shape and
position to those of _Triarthrus_. One endopodite on the right side
of the head and the first five on the right side of the thorax are the
best shown. All are slender, are directed first forward at an angle of
about 45 with the axis, then, except in the case of the cephalic
appendage, turn backward on a gentle curve and extend a little
distance beyond the margin of the test, but not as far as the tips of
the lateral spines of the thoracic segments.

The individual segments of the endopodites can not be seen clearly
enough to make any measurements. On the fourth and fifth endopodites
of the thorax, some of the segments seem to be broad and triangular as
in _Triarthrus_. All that can be seen indicates that _Acidaspis_ had
appendages entirely similar to those of _Triarthrus_, but perhaps not
quite so long, as they seem not to have projected beyond the limits of
the lateral spines. There are no traces of antennules nor,
unfortunately, of exopodites.

_Measurements:_ Length 8 mm.

Walcott (1881, p. 206) stated that his sections had shown the presence
in this species of legs "both cephalic and thoracic" and also the
"spiral branchiæ." His specimens were from the Trenton at Trenton
Falls, New York.




The Appendages of Cryptolithus.


=Cryptolithus tessellatus= Green.

(pl. 6, fig. 7; pls. 7-9; text figs. 20, 25, 45, 46.)

(See also Part IV.)

     Illustrated: Beecher, Amer. Jour. Sci., vol. 49, 1895, pl. 3.

When Professor Beecher wrote his short article on the "Structure
and Appendages of _Trinucleus_" (1895), he had only three specimens
showing appendages. In his later work he cleaned several more, so that
there are now thirteen specimens of _Trinucleus_ = _Cryptolithus_
available for study, though some of these do not show much detail. In
his last and unpublished study, Beecher devoted the major part of his
attention to this genus, and summarized his findings in the drawings
which he himself made of the best individuals (text figs. 45, 46).
Valiant (1901) stated that he had found a _Trinucleus_ with antennæ in
the Frankfort shale south of Rome, New York. The specimen has not been
figured.

None of the specimens shows much more of the appendages of the
cephalon than, the hypostoma and the antennules, so that we are still
in ignorance about the mouth parts.

The most striking characteristics of the appendages are as follows:
the antennules are long, and turn backward instead of forward; none
of the limbs projects beyond the margin of the dorsal test; the
exopodites extend beyond the endopodites, reaching very nearly to the
margin of the test; the endopodites are not stretched out at right
angles to the axis, but the first three segments have a forward and
outward direction as in _Triarthrus_, while the last four turn back
abruptly so that they are parallel to the axis; the limbs at the
anterior end of the thorax are much more powerful than the others; the
dactylopodites of the endopodites show a fringe of setæ instead of
three spines as in _Triarthrus_ and _Neolenus_. All these would, as
Beecher has already suggested, seem to be adaptations to a burrowing
habit of life, the antennules being turned backward and the other
appendages kept within the shelter of the dorsal test in order to
protect them, and the anterior endopodites enlarged and equipped with
extra spines to make them more efficient digging and pushing organs.

_Restoration of Cryptolithus._

(Text fig. 20.)

It should be definitely understood that the present figure is a
restoration and not a drawing of a specimen, and that there are many
points in the morphology of _Cryptolithus_ about which no information
is available, especially about the appendages under the central
portion of the cephalon. The information afforded by all the figures
published in this memoir is combined here. As gnathites are preserved
on none of the specimens, those represented in the figure are purely
conventional.

A person who is acquainted only with _Cryptolithus_ preserved in
shale, or with figures, usually has a very erroneous idea of the
fringe It is not a flat border spread out around the front of the
head, but stands at an angle about 45 in uncrushed specimens of most
species. When viewed from the lower side, there is a single outer,
concentric row of the cup-shaped depressions, bounded within by a
prominent girder. This row is in an approximately horizontal plane,
while the remainder of the doublure of the fringe rises steeply into
the hollow of the cephalon. Since the front of the hypostoma is
attached to this doublure, it stands high up within the vault and
under the glabella. Two specimens, Nos. 231 and 233, show something of
the hypostoma, and they are the only ones known of any American
trinucleid. That of specimen 233, the better preserved, is very small,
straight across the front, and oval behind. It seems that it is
abnormally small in this specimen and I should not be surprised if in
other specimens it should be found to be larger.

In the Bohemian _Trinucleoides reussi_ (Barrande), the oldest of the
trinucleids, the hypostoma is very commonly present, and is of the
proper size to just cover the cavity of the glabella, seen from the
lower side, and has, toward the anterior end, side flaps which reach
out under the prominent glabellar lobes. This large size of the
hypostoma would cause the antennules to be attached outside the dorsal
furrows, and the position in which they are attached in the American
species of _Cryptolithus_ may be explained as an inherited one, since
with the small hypostoma they might have been within the glabella, as
in _Triarthrus_.

The antennules are seen in three specimens, and in all cases are
directed backward. The particular course in which they are drawn in
the restoration is purely arbitrary. The second pair of cephalic
appendages are represented as directed downward and forward, since in
one or two specimens fragments of forward-pointing endopodites were
seen near the front of the cephalon, and because in other trilobites
the second pair of appendages is always directed forward. The
remaining three pairs have a more solid basis in observed fact, for
the two or three specimens retaining fragmentary remains of them
indicate that they turn backward like those on the thorax, and that
the individual segments are longer and more nearly parallel-sided than
those of the more posterior appendages. The gnathites of all the
cephalic appendages are admittedly purely hypothetical. None of the
specimens shows them. As drawn, they are singularly inefficient as
jaws, but if, as is suggested by the casts of the intestines of
trinucleids found in Bohemia, these trilobites were mud-feeders,
inefficient mouth-parts would be quite in order.

[Illustration: Fig. 20. _Cryptolithus tessellatus_ Green. A
restoration of the appendages drawn by Doctor Elvira Wood from the
original specimens and from the photographs made by Professor Beecher.
× 9.]

The appendages of the thorax and pygidium can fortunately be taken
quite directly from the photographs of the dorsal and ventral sides of
well preserved specimens. There is of course a question as to the
number and the exact form of those on the pygidium, but I think the
present restoration is fairly well justified by the specimens. As
would be expected from the narrow axial lobe, the gnathobases of the
coxopodites are short and small.




Summary on the Ventral Anatomy of Trilobites.


COMPARISON OF APPENDAGES OF DIFFERENT GENERA.

Since the appendages of _Triarthrus_, _Cryptolithus_, _Neolenus_,
_Calymene_, and _Ceraurus_ are now known with some degree of
completeness, those of _Isotelus_ somewhat less fully, and something
at least of those of _Ptychoparia_, _Kootenia_, and _Acidaspis_, these
forms being representatives of all three orders and of seven different
families of trilobites, it is of some interest to compare the
homologous organs of each.

All in which the various appendages are preserved prove to have a pair
of antennules, four pairs of biramous limbs on the cephalon, as many
pairs of biramous limbs as there are segments in the thorax, and
a variable number of pairs on the pygidium, with, in the case of
_Neolenus_ alone, a pair of tactile organs at the posterior end. Each
limb, whether of cephalon, thorax, or pygidium, consists of a
coxopodite, which is attached on its dorsal side to the ventral
integument and supported by an appendifer, an exopodite, and an
endopodite. The exopodite is setiferous, and the shaft is of variable
form, consisting of one, two, or numerous segments. The endopodite
always has six segments, the distal one armed with short movable
spines.

_Coxopodite._

The coxopodite does not correspond to the protopodite of higher
Crustacea, the basipodite remaining as a separate entity. The inner
end of the coxopodite is prolonged into a flattened or cylindrical
process, which on the cephalon is more or less modified to assist in
feeding, and so becomes a gnathobase or gnathite. The inner ends of
the coxopodites of the thorax and pygidium are also prolonged in a
similar fashion, but are generally somewhat less modified. These
organs also undoubtedly assisted in carrying food forward to the
mouth, but since they probably had other functions as well, I prefer
to give them the more non-committal name of endobases.

In _Triarthrus_ and _Neolenus_ the endobases are flattened and taper
somewhat toward the inward end. In _Isotelus_, _Calymene_ and
_Ceraurus_, they appear to have been cylindrical. In other genera they
are not yet well known. In all cases, particularly about the mouth,
they appear to have been directed somewhat backward from the point of
attachment. As it is supposed that these organs moved freely forward
and backward, the position in which they occur in the best preserved
fossils should indicate something of their natural position when
muscles were relaxed.

_Cephalon._

_Antennules._--Antennules are known in _Triarthrus_, _Cryptolithus_,
_Neolenus_, and _Ptychoparia_. In all they are long, slender, and
composed of numerous segments, which are spiniferous in _Neolenus_,
and very probably so in the other genera.

In _Triarthrus_, _Neolenus_, and _Ptychoparia_ they project ahead of
the cephalon, emerging quite close together under the front of the
glabella, one on either side of the median line. In _Cryptolithus_
they turn backward beneath the body, but since only three or four
specimens are known which retain them, it is possible that other
specimens would show that these organs were capable of being turned
forward as well as backward. The proximal ends of the antennules being
ball-like, it is probable, as Doctor Faxon has suggested to me, that
these "feelers" had considerable freedom of motion. The antennules of
_Triarthrus_ are apparently somewhat less flexible than those of the
other genera, and have a double curvature that is seen among the
others only in Ptychoparia. The proximal end of an antennule in
_Triarthrus_ is a short cylindrical shaft, apparently articulating in
a sort of ball-and-socket joint. The proximal end in the other genera
is still unknown. The points of attachment in _Triarthrus_ seem to be
under the inner part of the second pair of glabellar furrows. In
_Cryptolithus_ they appear to be beside the anterior lobe of the
glabella under what have long been known as the antennal pits. In the
other genera the location is not definitely known, but in _Neolenus_
it seems to be under the dorsal furrows near the anterior end of the
glabella. Viewed from the under side, the point of attachment is
probably always beside the middle or anterior part of the hypostoma,
just behind the side wings.

_Paired biramous appendages._--Behind the antennules all the appendages
except those on the anal segment are biramous, consisting of a
coxopodite with an inward-directed endobase and an outward-directed
pair of branches, the exopodite above, and the six-jointed endopodite
beneath. The basipodite really bears the exopodite, but the latter
also touches the coxopodite. This structure has been seen in
_Triarthrus_, _Cryptolithus_, _Neolenus_, _Kootenia_, _Calymene_,
_Ceraurus_, and _Ptychoparia_. In _Triarthrus_, _Neolenus_,
_Acidaspis_, _Ptyclioparia_, and Kootenia, the appendages extend
beyond the margins of the dorsal test. In _Cryptolithus_ and
_Isotelus_ none (other than antennules) does so. In _Isotelus_ and
_Acidaspis_ only the endopodites have been seen. In _Triarthrus_,
_Calymene_, _Ceraurus_, and _Neolenus_ there are four pairs of
appendages behind the antennules. The other genera probably had the
same number, but the full structure of the under part of their cephala
is not known. In _Triarthrus_ the endopodites of the cephalon are
slender, the individual segments parallel-sided, the inner ones
flattened, the outer ones cylindrical in section. They project
slightly beyond the edge of the cephalon when fully extended, and each
terminates in three small spines. In _Cryptolithus_ the endopodites of
the cephalon are longer than those of the thorax, but with the
possible exception of the first pair, are bent backward at the
carpopodite, and do not ordinarily project beyond the brim of the
test. In _Neolenus_ the endopodites of the cephalon are rather thick
and wide, but are long, project forward, and extend beyond the brim.
The individual segments are flattened, probably compressed oval in
section. The terminal segment of each is furnished with three strong
spines at its distal end. In _Calymene_ and _Ceraurus_ the endopodites
appear to consist of slender segments which are oval or circular in
section. In _Calymene_ Walcott believed the three distal segments of
the last endopodites of the head to be greatly enlarged, giving these
appendages a paddle-like form similar to some of the appendages of
eurypterids. The evidence for this does not seem to me to be good. The
cephalic endopodites of _Isotelus_ are entirely similar to those of
the thorax, and are rather short, consisting of a series of short
cylindrical segments which do not taper greatly toward the distal end.
The endopodites of the cephalon of _Acidaspis_, _Kootenia_, and
_Ptychoparia_ are still unknown.

The exopodites of the cephalon seem in all known cases (_Triarthrus_,
_Cryptolithus_, _Neolenus_, and Ceraurus) to be like those of the
thorax. They point more directly forward in most cases, project beyond
the margin of the head normally only in Triarthrus, and usually occupy
the region under the cheeks (fixed and free).

The endobases of the coxopodites of the appendages of the cephalon
probably in all cases function as mouth-parts (gnathites), and are
especially modified for this purpose in Triarthrus, being flattened,
shoe-shaped in outline, and so arranged that they work over one
another in a shearing fashion. While the more anterior of the
coxopodites are attached in front of the posterior tip of the
hypostoma, the gnathites of Triarthrus bend backward so that all are
behind the hypostoma. In _Calymene_ and _Ceraurus_, two or three pairs
of the gnathites are back of the hypostoma, and one or more pairs may
be beside or under the hypostoma. In these genera the mouth is
probably in front of the tip of the upper lip. In _Isotelus_, the
mouth seems to have been situated in the notch between the two
branches of the hypostoma, and the gnathites of two or three pairs of
the appendages probably worked under its forks. Since the length of
the hypostoma differs in the various species of _Isotelus_, there
would be a variable number of gnathites projecting under its forks,
according to the species. In this genus the gnathites are of the same
long form, cylindrical in cross-section, as the endobases of the
thoracic segments, but each is bowed back considerably from the point
of attachment.

The gnathites of _Neolenus_ are like the endobases of the thorax, but
broader. The great length of the hypostoma makes it probable that the
mouth was far back and that some of the gnathites were in front of it.
The gnathites of _Cryptolithus_ are unknown. Professor Beecher in his
drawing shows some fragments with toothed ends near the hypostoma, and
it may be that they are inner ends of gnathites, but I see nothing
to substantiate such an interpretation. If, as some suppose,
_Cryptolithus_ was a mud feeder, the gnathites were probably poorly
developed. Of the gnathites of _Kootenia_, _Ptychoparia_, and
_Acidaspis_ also nothing is known.

_Thorax._

In each genus there is a pair of appendages for each segment of
the thorax. When the axial lobe is narrow, the endobases of the
coxopodites are small and short (_Cryptolithus_, _Ceraurus_,
_Calymene_). When the axial lobe is wide, the endobases are long and
stout (_Isotelus_, _Triarthrus_). The exopodites always lie above
and in front of the corresponding endopodites. In Triarthrus the two
branches are of practically equal length. In _Cryptolithus_ the
exopodites are much the longer. In _Neolenus_, _Calymene_, _Ceraurus_,
_Kootenia_, and _Ptychoparia_, the exopodites are shorter than the
endopodites.

The exopodites in Triarthrus consist of a proximal shaft, succeeded by
numerous short segments, and ending distally in a long, grooved,
somewhat spatula-shaped segment. Along the anterior margin of the
shaft there are many small spines. Along the posterior margin there
are numerous flattened setæ, which all lie in one plane and which seem
to be more or less united to one another like the barbs of a feather.
The setæ are short, not much longer than the width of one of the
thoracic segments, and point backward and outward. In _Cryptolithus_
the shaft does not seem to be made up of small segments, and is
narrow, with a decided backward curve. The setæ are considerably
longer and much more flattened than in Triarthrus. In _Calymene_ the
state of preservation does not allow a very full knowledge of the
exopodites, but they appear to have a slender, unjointed shaft and
short and delicate setæ. The coiled branches of the exopodites as
described by Walcott seem to me to be only ordinary Triarthrus-like
organs, and this, as I understand from Schuchert, was also the view of
Beecher. In _Ceraurus_ the exopodite seems to have been somewhat
paddle-shaped, expanded at the distal end, and to have had rather
thick, blade-like setæ.

The exopodite of _Neolenus_ is decidedly leaf-like, and reminds one
somewhat of the exites of some of the phyllopods. The shaft is a
broad unsegmented blade. The setæ are slender, delicate, flattened,
and a little longer than the width of the shaft. The exopodites
of this genus point forward all along the body. In _Kootenia_ the
exopodites are like those of _Neolenus_, but with a narrower shaft.
The exopodites of _Ptychoparia_ appear to be very much like those of
Triarthrus, but the shaft is probably not segmented.

The endopodites of the thorax of _Triarthrus_, _Cryptolithus_, and
_Acidaspis_ show progressive modification from front to back in the
broadening of the individual segments and the assumption by them of
a triangular form. Not only do the individual segments become more
triangular from front to back, but more of the segments of each
endopodite become triangular. This modification has so far been seen
in these three genera only. The individual segments, except the distal
ones, seem to be flattened in all these genera. The distal end of the
terminal segment of each endopodite of _Triarthrus_ bears three small
movable spines, and each of the segments usually bears three or
more spines, located in sockets along the dorsal surface and at
the anterior distal angle of each segment. The endopodite of
_Cryptolithus_ is bent backward at the carpopodite and this segment
is always thickened. At the distal end of the dactylopodite there
is a tuft of spines, the triangular segments have tufts of spines on
their posterior corners, and there are groups of spines also in the
neighborhood of the articulations.

The endopodites of _Ceraurus_, _Calymene_, and _Isotelus_ are all
relatively slender, the segments are parallel-sided, and there seems
to be no particular modification from front to back of the thorax. The
endopodites of _Isotelus_ are short, the entire six segments of one
being but little longer than the coxopodite of the same appendage. The
segments of the endopodites of _Neolenus_ are mostly short and wide,
and at the distal end of the terminal segment there are three stout
spines. In _Kootenia_ the endopodites are long and very slender. The
endopodites of Ptychoparia are too poorly preserved to show details,
and those of the thorax of _Acidaspis_ likewise reveal little
structure, but they seem to have the triangular modification, and to
turn back somewhat sharply at about the position of the carpopodite.

_Pygidium._

Beecher showed that in _Triarthrus_ there was a pair of appendages on
the pygidium for every segment of which it is composed except the last
or anal segment (protopygidium). Walcott has since shown that in
_Neolenus_ this segment bears a pair of cerci, and Beecher's drawings
show that in his later studies he recognized a spinous plate, the
possible bearer of cerci, on the anal segment of _Triarthrus_. The
appendages of the anal segment have not yet been seen on other species
of trilobites.

The appendages of the pygidium do not show any special modifications,
but seem in all cases to be similar to those of the posterior part of
the thorax. In _Cryptolithus_ all the pygidial appendages are short
and remain beneath the cover of the dorsal test, while in _Triarthrus_
and _Neolenus_ they extend behind it.

In the latter genus the endopodites of the pygidial appendages appear
to be practically identical in form with those of the thorax, the
individual segments being perhaps a little more nearly square in
outline. Like those of the thorax, the segments of the pygidial
endopodites bear numerous short spines. The caudal cerci are richly
segmented, slightly flexible, spinous tactile organs. They are
symmetrically placed, nearly straight when in their natural position,
and make an angle of about 75 with one another. They appear to be
attached to a narrow rim-like plate which seems to fit in just ahead
of the doublure of the pygidium, or perhaps over it.

In _Ceraurus_, _Calymene_, and _Isotelus_, the endopodites of the
pygidium are similar to those of the thorax, but seemingly more
slender, with less well developed coxopodites, and with, in the
last-named genus, slender cylindrical segments. Exopodites are not
known on the pygidia of any of these genera, but since they are
present and like those of the thorax in _Triarthrus_, _Cryptolithus_,
_Neolenus_, and _Ptychoparia_, there is little reason to think that
they were absent in _Ceraurus_ or _Calymene_, though there is some
question about _Isotelus_.

The limbs are largest and longest on the anterior part of the thorax
of a trilobite, and diminish regularly in length and strength to the
posterior end of the pygidium. This regular gradation shows, as
Beecher was the first to point out, that the growing point of the
trilobites is, as in other arthropods, in front of the anal segment.
New _free_ segments are introduced into the thorax at the anterior end
of the pygidium, and this has led to some confusion between the
growing point and the place of introduction of free segments.

If a new segment were introduced at a moult in front of the pygidium,
that segment would probably have less fully developed appendages than
those adjacent to it, and so make a break in the regular succession.
The condition of the appendages corroborates the evidence derived from
the ontogeny of the pygidium, and proves that the new segments are
introduced at the same growing point as in other Arthropoda.

_Caudal Rami._

Bernard, who believed that the Crustacea had been derived through an
_Apus_-like ancestor (1892, pp. 20, 85, 274), pointed out that four or
less than four anal cirri were to be expected. Two well developed
cirri and two rudimentary ones are present in _Apus_, and they are
also to be found in other phyllopods and some isopods. It is, however,
characteristic of the Crustacea as a whole to lack appendages on the
anal segment. Caudal cirri (cerci) are much more freely developed in
the hexapods than in the Crustacea, particularly in the more primitive
orders, Palæodictyoptera, Apterygota, Archiptera, and Neuroptera. They
are supposed, in this case, to be modified limbs, and therefore not
homologous with the bristles on the anal segment of an annelid. Doctor
W. M. Wheeler of the Bussey Institution has kindly allowed me to quote
the following excerpt from a letter to me, as expressing the opinion
of one who has made an extensive study of the embryology of insects:

     I would say that I have no doubt that the cerci of insects are
     directly inherited from the insect ancestors. They are always
     highly developed in the lower insects, and only absent or vestigial
     in a few of the most highly specialized orders such as the
     Hemiptera, Diptera, and Hymenoptera. I have further no doubt
     concerning their being originally ambulatory in function. They are
     certainly not developed independently in insects. Embryologically
     they arise precisely like the legs, and each cercus contains a
     diverticulum of the mesoblastic somite precisely as is the case
     with the ambulatory legs and mouth parts.

The "pygidial antennæ" seem to be as fully developed in _Neolenus_ as
in any of the other arthropods, and may suggest a common ancestry of
the phyllopods, isopods, and hexapods, in the trilobites. They were
doubtless tactile organs, and while the evidence is chiefly negative,
it would seem that they proved useless, and were lost early in the
phylogeny of this group. Possibly the use of the pygidium as a
swimming organ proved destructive to them.


HOMOLOGY OF THE CEPHALIC APPENDAGES WITH THOSE OF OTHER CRUSTACEA.

The head of the typical crustacean bears five pairs of appendages,
namely, the antennules, antennas, mandibles, and first and second
maxillæ, or, as they are more properly called, the maxillulæ and
maxillæ.

As Beecher has pointed out, the "antennæ" of the trilobites, on
account of their pre-oral position and invariably uniramous character,
are quite certainly to be correlated with the antennules.

The second pair of appendages, the first pair of biramous ones,
Beecher homologized with the antennæ of other crustaceans, and that
homology has been generally accepted, though Kingsley (1897) suggested
that it was possible that no representatives of the true antennæ were
present.

In preparing the restorations in the present study, the greatest
difficulty has been to adjust the organs about the mouth. In
_Triarthrus_, numerous specimens show that without question there are
four pairs of gnathites back of the hypostoma, and that all four
belong to the cephalon. In forms with a long hypostoma, however, there
was no room on the cephalon for the attachment of four pairs of
gnathites, neither were there enough appendifers to supply the
requisite fulcra. At first I supposed I had solved the difficulty by
assuming the mouth to be in front of the posterior tip of the
hypostoma, as it really is in Ceraurus and _Calymene_, and allowing
the gnathites to play under the hypostoma as Walcott (1912) has shown
that they do in _Marrella_. Finally, when I came to study in greater
detail the slices of _Calymene_ and _Ceraurus_, they seemed to show
that the anterior one or two pairs of appendages became degenerate and
under-developed. This was probably a specialization due to the great
development of the hypostoma in trilobites, that organ being much
more prominent in this than in any other group. As the hypostoma
lengthened to accommodate the increasing size of sub-glabellar organs
(stomach, heart, etc.), the mouth migrated backward, leaving the
anterior appendages ahead of it, with their gnathobases, at least,
functionless. That such migration has taken place, even in Triarthrus,
is shown by the fact that the points of articulation of the first
biramous appendages are pre-oral, and it is more obviously true of
_Ceraurus_. Correlated with the weakening of the appendages on the
lower surface is the loss of glabellar furrows on the upper surface.
The glabellar furrows mark lines of infolding of the test to form the
appendifers and other rugosities for the attachment of tendons and
muscles. It is conceivable that this migration backward of the mouth
began very early in the history of the race, and that even before
Cambrian times, the antennæ, probably originally biramous appendages
like those on the remainder of the body, had dwindled away and become
lost. If this is the case, then the first pair of biramous appendages
of _Triarthrus_ would be mandibles, the second pair maxillulæ, and the
third pair maxillæ.

There remain the last pair of cephalic appendages, and they bring up
the whole head problem of the trilobites. Beecher has stated (1897 A,
p. 96) his conviction that the head of the trilobite is made up of
five segments, representing the third, fourth, fifth, sixth, and
seventh neuromeres of the theoretical crustacean. As a matter of fact,
he really made up the head of seven segments, since he stated that the
first neuromere was represented by the hypostoma and the second by the
epistoma and free cheeks.

Jaekel (1901, p. 157) nearly agreed with Beecher, but made eight
segments, as he saw five segments in the glabella of certain
trilobites. In his table (p. 165) he has listed the segments with
their appendages as follows: 1. Acron, with hypostoma; 2, rostrum
(epistoma), with free cheeks; 3, first frontal lobe, with (?)
antennules; 4, second frontal lobe, with antennæ; 5, mandibles; 6,
first, or pre-maxillæ; 7, second maxillæ; 8, occipital segment with
maxillipeds.

Jaekel refused to believe that the antennæ of trilobites were really
entirely simple, and so homologized them with the antennæ and not the
antennules of other Crustacea. In this he was obviously incorrect, but
it accounts for his homology of the remainder of the cephalic
appendages.

It is, at present, impossible to demonstrate the actual number of
somites in the cephalon of the trilobite, but I believe that Beecher
was correct in holding that the glabella was composed of four
segments. There are, it is true, a number of trilobites (Mesonacidæ,
Paradoxidæ Cheiruridæ, etc.) which show distinctly four pairs of
glabellar furrows, indicating five segments in the glabella. This is,
however, probably due to a secondary division of the first lobe.

The correspondence of the five segments on the dorsal side with the
five pairs of appendages makes it unlikely that any pair of limbs has
been lost. The condition in _Marrella_, where a trilobite-like
cephalon bears five pairs of appendages, the second pair of which are
tactile antennæ, is favorable to the above interpretation. In spite of
the apparent degeneration of the first two pairs of appendages in
_Calymene_, no limbs are actually missing, and if some are dropped out
in the later trilobites it would not affect the homology of those now
known. I therefore agree with Beecher in homologizing the appendages,
pair for pair, with those of the higher Crustacea.


FUNCTIONS OF THE APPENDAGES.

_Antennules._

The antennules were obviously tactile organs, probably freely movable
in most trilobites, but in the case of Triarthrus perhaps rather
rigid, judging from the great numbers of specimens which show the
characteristic sigmoid curve made familiar by Professor Beecher's
restoration. The proximal end of the shaft of each antennule of
Triarthrus is hemispheric and doubtless fitted into a socket, thus
suggesting great mobility of the whole organ. In spite of this, I have
seen no specimens in which they did not turn in toward each other and
cross the anterior margin very near the median line. In front of the
margin, various specimens show evidence of flexibility, but from the
proximal end to the margin the position is the same in all specimens.

In all the few specimens of _Cryptolithus_ retaining the antennules,
these organs are turned directly backward, but it is entirely within
the range of probabilities that while its burrowing habits made this
the more usual position, the animal had the power of turning them
around to the front when they could be used to advantage in that
direction.

_Exopodites._

It has been the opinion of most observers that the exopodites of
trilobites were swimming organs, while others have thought that they
functioned also in aerating the blood. To the present writer it seems
probable that the chief function was that of acting as gills, for
which the numerous thin, flattened or blade-like setæ are particularly
adapted. That they were also used in swimming is of course possible,
but that was not their chief function. It should be remembered that
the exopodites are always found dorsal to or above the endopodites,
and in a horizontal plane. For use in swimming it would have been
necessary to rotate each exopodite into a plane approximately
perpendicular to or at least making a considerable angle with the
dorsal test. In this position, the exopodites would have been thrust
down between the endopodites, and one would expect to find some
specimens in which a part at least of the exopodites were ventral to
the endopodites. Specimens in this condition have not yet been seen
among the fossils. To avoid having the exopodites and endopodites
intermingled in this way, the animal would have to bring all the
endopodites together along the axial line in a plane approximately
perpendicular to the dorsal test, in which case the exopodites would
be free to act as swimming organs. The fact that the setæ of an
exopodite stay together like the barbs on a feather would of course
tend to strengthen the idea that the exopodites could be used in
swimming, but that is not the only possible explanation of this
condition. The union of the basipodite and exopodite shows that the
two branches of the appendage acted together. Every movement of one
affected the other, and the motion of the endopodites in either
swimming or crawling produced a movement of the exopodites which
helped to keep up a circulation of water, thus insuring a constant
supply of oxygen.

Although _Neolenus_ is usually accounted a less primitive form than
_Ptychoparia_ or _Triarthrus_, it has much the most primitive type
of exopodite yet known. It would appear that the exopodites were
originally broad, thin, simple lamellæ, which became broken up, on the
posterior side, into fine cylindrical setæ. As development progressed,
more and more of the original lamella was broken up until there
remained only the anterior margin, which became thickened and
strengthened to support the delicate filaments. The setæ in turn
became modified from their original simple cylindrical shape to form
the wide, thin, blade-like filaments of _Cryptolithus_ and _Ceraurus_.

Another possible use of the exopodites is suggested by the action of
some of the barnacles, which use similar organs as nets in gathering
food and the endopodites as rakes which take off the particles and
convey them to the mouth. The exopodites of the trilobite might well
set up currents which would direct food into the median groove, where
it could be carried forward to the mouth.

_Endopodites._

The endopodites were undoubtedly used for crawling; in some
trilobites, probably most of them, for swimming; in the case of
_Cryptolithus_, and probably others, for burrowing; and probably in
all for gathering food, in which function the numerous spines with
which they are arrayed doubtless assisted.

Various trails have been ascribed to the action of trilobites, and
many of them doubtless were made by those animals (see especially
Walcott, 1918). Some of these trails seem to indicate that in crawling
the animal rested on the greater part of each endopodite, while
others, notably the _Protichnites_ recently interpreted by Walcott
(1912 B, p. 275, pl. 47), seem to have touched only the spinous tips
of the dactylopodites to the substratum. The question of the tracks,
trails, and burrows which have been ascribed to trilobites is discussed
briefly on a later page; but can not be taken up fully, as it would
require another monograph to treat of them satisfactorily.

The flattened, more or less triangular segments of the endopodites
of the posterior part of the thorax and pygidium in _Triarthrus_,
_Cryptolithus_, and _Acidaspis_ probably show an adaptation of the
endopodites of the posterior part of the body both as more efficient
pushing organs and as better swimming legs. The fact that these
segments are pointed below enabled them to get a better grip on
whatever they were crawling over, and the flattening allowed a much
greater surface to be opposed to the water in swimming. In this
connection it might be stated that it seems very probable that the
trilobites with large pygidia at least, perhaps all trilobites, had
longitudinal muscles which allowed them to swim by an up and down
motion of the fin-like posterior shield, the pygidium acting like the
caudal fin of a squid. Such a use would explain the function of the
large, nearly flat pygidia seen in so many of the trilobites beginning
with the Middle Cambrian, and of those with wide concave borders. It
should be noted here that it is in trilobites like _Isotelus_, with
pygidia particularly adapted to this method of swimming, that the
endopodites are most feebly developed, and show no flattening or
modification as swimming organs.

The relatively strong, curved, bristle-studded endopodites of
_Cryptolithus_, combined with its shovel-shaped cephalon, indicate
_Limulus_-like burrowing habits for the animal, and the mud-filled
casts of its intestine corroborate this view. That it was not,
however, entirely a mud groveller is indicated by its widespread
distribution in middle Ordovician times.

_Use of the Pygidium in Swimming._

The idea that the use of the pygidium as a swimming organ is a
possible explanation of that caudalization which is so characteristic
of trilobites has not been developed so far as its merits seem to
deserve. Two principal uses for a large pygidium of course occur
to one: either it might form a sort of operculum to complete the
protection when the trilobite was enrolled, or it might serve as a
swimming organ. That the former was one of its important functions is
shown by the nicety with which the cephalon and pygidium are adapted
to one another in such families as the Agnostidæ, Asaphidæ, Phacopidæ,
and others. That a large pygidium is not essential to perfect
protection on enrollment is shown by an equally perfect adjustment of
the two shields in some families with small pygidia, notably the
Harpedidæ and Cheiruridæ That the large pygidial shields are not for
protective purposes only is also shown by those forms with large
pygidia which are not adjusted to the conformation of the cephalon, as
in the Goldiidæ and Lichadidæ. It is evident that a large pygidium,
while useful to complete protection on enrollment, is not essential.

It would probably be impossible to demonstrate that the trilobites
used the pygidium in swimming. The following facts may, however, be
brought forward as indicating that they probably did so use them.

1. The appendages, both exopodites and endopodites, are relatively
feebly developed as swimming organs. This has been discussed above,
and need not be repeated. It must in fairness be observed, however,
that many modern Crustacea get about very well with limbs no better
adapted for swimming than those of the trilobites.

2. The articulations of the thoracic segments with each other and with
the two shields are such as to allow the pygidium to swing through an
arc of at least 270, that is, from a position above the body and at
right angles to it, around to the plane of the bottom of the cephalon.
Specimens are occasionally found in which the thorax and pygidium are
so flexed that the latter shield stands straight above the body. A
well preserved _Dipleura_ in this position is on exhibition in the
Museum of Comparative Zoology, and Mr. Narraway and I have figured a
_Bumastus milleri_ in the same attitude (Ann. Carnegie Mus., vol. 4,
1908, pl. 62, fig. 3).

3. What little can be learned of the musculature (see under
musculature, seq.) indicates that the trilobites had powerful extensor
and flexor muscles, such as would be required for this method of
swimming. It may be objected that the longitudinal muscles were too
small to permit the use of a caudal fin. In the lobster, where this
method of progression is most highly developed, there is a large
mass of muscular tissue which nearly fills the posterior segments.
Trilobites have not usually been thought of as powerfully muscled, but
it may be noted that in many cases broad axial lobes accompany large
pygidia. As the chief digestive region appears to have been at the
anterior end, and other organs are not largely developed, it seems
probable that the great enlargement of the axial lobe was to
accommodate the increased muscles necessary to properly operate the
pygidium. It may be noted that in all these genera the axial lobe of
the pygidium is either short or narrow.

4. The geological history of the rise of caudalization favors this
view. With the exception of the Agnostidæ and Eodiscidæ, all Lower
Cambrian trilobites had small pygidia, and the same is true of
those of the Middle Cambrian of the Atlantic realm (except for the
_Dorypyge_ of Bornholm). In Pacific seas, however, large-tailed
trilobites of the families Oryctocephalidæ, Bathyuridæ, and Asaphidæ
then began to be fairly common, though making up but a small part of
the total fauna of trilobites. In the Upper Cambrian of the Atlantic
province the Agnostidæ were the sole representatives of the isopygous
trilobites, while in the Pacific still another family, the
Dikelocephalidæ, was added to those previously existing.

With the Ordovician, caudalization reached its climax and the fashion
swept all over the world. It is shown not so much in the proportion of
families with large pygidia, as in the very great development of the
particular trilobites so equipped. Asaphidæ and Illænidæ were then
dominant, and the Proëtidæ, Cyclopygidæ Goldiidæ, and Lichadidæ had
begun their existence. A similar story is told by the Silurian record,
except that the burden of the Asaphidæ has been transferred to the
Lichadidæ and Goldiidæ. All the really old (Cambrian) families of
trilobites with small pygidia had now disappeared. In the general
dwindling of the subclass through the Devonian and later Palæozoic,
the few surviving species with small pygidia were the first to go, and
the proëtids with large abdominal shields the last.

The explanation of this history is probably to be found in the rise of
the predatory cephalopods and fishes, the natural enemies of the
trilobites, against whom they could have no other protection than
their agility in escaping. While the records at present known carry
the fishes back only so far as the Ordovician (fishes may have arisen
in fresh waters and have gone to sea in a limited way in the
Ordovician and more so in Silurian time) and the cephalopods to the
Upper Cambrian, the rise of the latter must have begun at an earlier
date, and it is probably no more than fair to conjecture that the
attempt to escape swimming enemies caused an increase in the swimming
powers of the trilobites themselves. At any rate, the time of the
great development of the straight cephalopods coincided with the time
of greatest development of caudalization; both were initiated in the
Pacific realm, and both spread throughout the marine world during the
middle Ordovician. And since, in the asaphids, a decrease in swimming
power of the appendages accompanied the increase in the size of the
pygidium, it seems probable that the swimming function of the one had
been transferred to the other. A high-speed, erratic motion which
could be produced by the sudden flap of a pygidium would be of more
service in escape than any amount of steady swiftness produced by the
oar-like appendages of an animal of the shape of a trilobite.

_Coxopodites._

The primary function of the endobases of the coxopodites was doubtless
the gathering, preparation, and carrying of food to the mouth.
Although the endobases of opposite sides could not in all cases meet
one another, they were probably spinose or setiferous and could
readily pass food from any part of the axial groove forward to the
mouth, and also send it in currents of water. The endobases of the
cephalic coxopodites were probably modified as gnathites in all cases,
but little is known of them except in Triarthrus, where they were
flattened and worked over one another so as to make excellent shears
for slicing up food, either animal or vegetable. In some cases the
proximal ends of opposed gnathites were toothed so as to act as jaws,
but a great deal still remains to be learned about the oral organs of
all species.

The writer has suggested (1910, p. 131) that a secondary function
of the endobases of the thorax of _Isotelus_ and probably other
trilobites with wide axial lobes was that of locomotion. In _Isotelus_
the endobases of the thorax are greatly over-developed, each being
much stouter and nearly as long as the corresponding endopodite, and
the explanation seemed to me to lie in the locomotor or crawling use
of these organs instead of the endopodites. Certain trails which I
figured seemed to support this view.


POSITION OF THE APPENDAGES IN LIFE.

In almost all the specimens so far recovered the appendages are either
flattened by pressure or lie with their flat surfaces in or very near
the plane of stratification of the sediment. This flattening is
extreme in Neolenus, Ptychoparia, and Kootenia, moderate in
_Triarthrus_ and _Cryptolithus_, and apparently slight or not
effective in _Isotelus_, _Ceraurus_, and _Calymene_. These last are,
however, from the conditions of preservation, least available for
study.

In Part IV, attention is called to a specimen of Triarthrus (No. 222)
in which some of the endopodites are imbedded nearly at right angles
to the stratification of the shale. This specimen is especially
valuable because it shows that the appendages in the average specimen
of Triarthrus have suffered very little compression, and it also
suggests the probable position of the endopodites when used for
crawling.

In considering the position of the appendages in life, one must always
remember one great outstanding feature of trilobites, the thinness and
flexibility of the ventral membrane. The appendages were not inserted
in any rigid test but were held only by muscular and connective
tissue. Hence we must premise for them great freedom of motion, and
also relatively little power. The rigid appendifers, and the
supporting apodemes discovered by Beecher, supplied fulcra against
which they could push, but their attachment to these was rather loose.

Considering, first, the position of the appendages in crawling, it
appears that different trilobites used their appendages in different
ways. _Neolenus_ had compact stocky legs, which allowed little play of
one segment on another, as is shown by the wide joints at right angles
to the axis of the segment. Such limbs were stiff enough to support
the body when the animal was crawling beneath the water, where of
course it weighed but little. That such a crawling attitude was
adopted by trilobites has been shown by Walcott in his explanation of
the trails known as _Protichnites_ (1912 B, p. 278). Many trilobites
probably crawled in this way, on the tips of the toes, so to speak.
In such the limbs would probably extend downward and outward, with the
flattened sides vertical.

The limb of _Triarthrus_, however, is of another type. The endopodites
are long, slender, flexibly jointed, the whole endopodite probably too
flexible to be used as a unit as a leg must be in walking on the
"toes." The proximal segments of the thoracic and pygidial endopodites
are, however, triangular instead of straight-sided, and, the
spine-bearing apex of the triangle being ventral, it enabled the
endopodites to get a grip on the bottom and thus push the animal
forward. This method of progression was more clumsy and less rapid
than that of Neolenus, but it sufficed. The natural position of the
endopodite when used in this way would seem to be with the flattened
sides of the segments standing at an angle of 30 to 45 with the
vertical, thus allowing a good purchase on the bottom and at the same
time offering the minimum resistance to the water when moving the
appendages forward.

_Isotelus_ has endopodites different from those of either _Neolenus_
or _Triarthrus_. They are composed of cylindrical segments, the joints
indicating a certain amount of flexibility. Since there is no method
by which the segments may get a purchase on the bottom other than by
pushing with the distal ends, it would seem at first thought that
_Isotelus_, like Neolenus, crawled on its "toes." The endopodites
of _Isotelus_ are however, short and feeble when compared with
the size of the test, while the endobases of the coxopodites are
extraordinarily developed. These facts, together with certain trails,
strongly suggest the use of the coxopodites as the primary ambulatory
organs, the endopodites probably assisting. In this event, the
position of the endopodites and coxopodites would be downward, both
outward and inward from the point of attachment, and the motion both
backward and forward. The fact that in the specimens as preserved the
coxopodites point backward and the endopodites forward indicates that
the limb as a whole swung on a pivot at the appendifer. It is of
course natural to suggest that the coxopodites and endopodites of all
the trilobites with wide axial lobes, _Nileus_, _Bumastus_,
_Homalonotus_, etc., were developed in this same way.

_Cryptolithus_ presents still another and very peculiar development of
the endopodites where ability to get purchase on the sea floor is
obtained by a stout limb of slight flexibility, bowed and turned
backward in the middle, where an enlarged segment insures stiffness.
The segments are flattened, and since the greatest strength when used
in pushing and crawling is in the long axis of the oval section of
the flattened limb, it seems probable that these limbs did not hang
directly down, with their sides vertical, but that their position in
life was very much the same as that in which they are preserved as
fossils. By moving these bowed legs forward and backward in a plane at
a small angle to the surface of the body, a powerful pushing impetus
could be obtained. They may, however, have occupied much the same
position as do those of _Limulus_.

In the case of the endopodites, therefore, it is necessary to study
the structure and probable method of their use in each individual
genus before suggesting what was the probable position in life. In
the act of swimming, the position was probably more uniform. When
the endopodites were used in swimming, as they undoubtedly could be
with more or less effect in all the trilobites now known, those with
flattened surfaces probably had them at such an angle as to give the
best push against the water on the back stroke, while on the forward
stroke the appendage would be turned so that' the thin edge opposed
the water. The great flexibility of attachment would certainly permit
this, though unfortunately nothing is as yet known of the
musculature. The coxopodites of course had less freedom of movement
in this respect, and probably could not turn their faces. For this
reason, it seems to me likely that those coxopodites which are
compressed did not stand with their flattened faces vertical, but in a
position which was nearly horizontal or at least not more than 45 from
the horizontal. If the flattened faces were vertical, they would be in
constant opposition to the water during forward movements and would be
of no use in setting up currents of water toward the mouth, as every
back stroke would reverse the motion.

The position of the exopodites in life seems to have been rather
uniform in all the genera now known. I have set forth on a previous
page my reasons for thinking that they took little part in swimming,
and I look upon them as being, in effect, leaf-gills. It seems
probable that in all genera the exopodites were held rather close
to the test, the shaft more or less rigid, the filamentous setæ
gracefully pendent, but pendent as a sheet and not individually, there
having been some method by which adjoining setæ were connected
laterally. Free contact with the water was thus obtained without the
mingling of endopodites and exopodites which would have been so
disastrous to progression.




PART II.




Structure And Habits Of Trilobites.


INTERNAL ORGANS AND MUSCLES.

Granting that the trilobite is a simple, generalized, ancient
crustacean, it appears justifiable to attribute to it such internal
organs as seem, from a study of comparative anatomy, to be primitive.

The alimentary canal would be expected to be straight and simple,
curving downward to the mouth, and should be composed of three
portions, stomodæum, mesenteron, and proctodæum, the first and last
with chitinous lining. In modern Crustacea, muscle-bands run from the
gut to part of the adjacent body wall, so that scars of attachment of
these muscles may be sought. At the anterior end of the stomodæum,
they are usually especially strong. From the mesenteron there might be
pouch-like or tubular outgrowths.

The heart would probably be long and tubular, with a pair of ostia for
each somite.

In modern Crustacea, the chief organs of renal excretion are two pairs
of glands in the head, one lying at the base of the antennæ and one at
the base of the maxillæ. Only one pair is functional at a time, but
these are supposed to be survivors of a series of segmentally arranged
organs, so that there might be a pair to each somite of a trilobite.

The nervous system might be expected to consist of a supracesophageal
"brain," comprising at least two pairs of ganglionic centers, and a
double ventral chain of ganglia with a ladder-like arrangement.

Besides these organs, a variety of glands of special function might be
predicted.

Reproductive organs probably should occur in pairs, and more than one
pair is to be expected. There is little to indicate the probable
location of the genital openings, but they may have been located all
along the body back of the cephalon.

It may be profitable to summarize present knowledge of such traces of
these organs as have been found in the fossils, if only to point out
what should be sought.


ALIMENTARY CANAL.

Beyrich (1846, p. 30) first called attention to the alimentary canal
of a trilobite, (_Cryptolithus goldfussi_,) and Barrande (1852, p.
229) confirmed his observations. A number of specimens of this species
have been found which show a straight cylindrical tube or its filling,
extending from the glabella back nearly to the posterior end of the
pygidium. It lies directly under the median line of the axial lobe,
and less than its own diameter beneath the dorsal test. At the
anterior end it apparently enlarges to occupy the greater part of the
space between the glabella and the hypostoma, but was said by the
early observers to extend only a little over halfway to the front.
Beyrich thought the position of the median tubercle indicated the
location of the anterior end.

Walcott (1881, p. 200) stated that in his experience in cutting
sections of trilobites it was a very rare occurrence to find traces of
the alimentary canal. The visceral cavity was usually filled with
crystalline calcite and all vestiges of organs obliterated. There
were, however, some slices which showed a dark spot under the axial
lobe, which probably represented the canal. In his restoration he
showed it as of practically uniform diameter throughout, and extending
but slightly in front of the mouth.

Jaekel (1901, p. 168, fig. 28) has produced a very different
restoration. His discussion of this point seems so good, and has been
so completely overlooked, that I will append a slightly abridged
version of a translation made some years ago for Professor Beecher.
The idea was, however, not original with Jaekel, as it was suggested
by Bernard (1894, p. 417), but not worked out in detail.

     While considering the problem as to what organ could have lain
     beneath the glabella of the trilobite, and while studying the
     organization of living Crustacea for the purpose of comparison, I
     found in the collections of the Geological Institute preparations
     of _Limulus_ which seemed to me to directly solve the entire
     question.

     From the mouth, which lies at about the middle of the head shield,
     the oesophagus bends forward, swells out at the frontal margin of
     the animal at a sharp upward bend in order to take a straight
     course backward after the formation of an enlarged stomach. Still
     within the head shield there branch out from each' side of the
     canal two small vessels which pass over into the richly branched
     mass of liver lying under the broad lateral parts of the head
     shield. After seeing this specimen, I no longer had the least doubt
     that the head shield of the trilobites is to be interpreted in a
     similar manner. The position of the hypostoma and gnathopods makes
     it necessary to assume that the position of the mouth of the
     trilobite lay pretty far back. If, therefore, this depends upon the
     secondary ventral deflection of the oral region, as seems to be the
     case, then it is a priori probable that the anterior part of the
     canal has also shared in this ventral inflection.

     The posterior part of the canal in the region of the segmented
     thorax and pygidium is comparatively narrow, as shown long ago by
     Beyrich; he represents only a thin tube which shows no swellings
     whatever, and such are usually missing in Arthropoda.

     As the glabella of most trilobites is regularly convex, there must
     lie beneath it an organ running from front to back, which presses
     the bases of the cephalic legs away from each other and down from
     the dorsal test. An organ so extensive and unpaired, running thus
     from front to back, can, among the Arthropoda, be regarded only as
     an alimentary canal, for the swellings of the cephalic ganglia and
     the heart are by far too small to produce such striking elevations
     on the front and upper surface of the glabella. The canal might
     then have consisted of a gizzard belonging to the oesophagus,
     and astomach proper or main digestive canal.

     ... Among the trilobites there are two pairs of vessels on both
     sides of the glabella which have precisely the same position with
     reference to the supposed course of the alimentary canal as the
     ducts of the hepatic lobes in _Limulus_. One observes in numerous
     trilobites, although in different degrees of clearness and under
     various modifications, a dendritic marking of the inner surface
     of the cheeks which takes its rise at the lateral margins of the
     glabella and spreads thence like a bush over the entire surface
     of the cheeks. Exactly the same position is taken by the richly
     branched hepatic lobes of _Limulus_ on the lower surface of the
     head shield; a fact of special weight in favor of the homology
     and similar significance of the two phenomena, is that in the
     trilobites also, the anterior of the two main ducts is the larger,
     the posterior the smaller. The striking similarity of the two
     structures is shown by a comparison of the head shield of
     _Eurycare_ [_Elyx_] from the Cambrian of Sweden, in which the
     course of the canals is shown with remarkable clearness [with
     those of _Limulus_].

     I have been able to convince myself that the existence of the two
     canals on each side is also the rule in other genera, even though
     the posterior pair is frequently but feebly developed or completely
     obscured by the anterior pair. In _Dionide formosa_, for example, I
     find only the anterior pair, which is very large and divided into
     two principal branches. From all these considerations it seems to
     me no longer doubtful that the median elevation was caused by the
     stomach and gizzard, and that the cheeks have principally served to
     cover the hepatic appendages of the alimentary canal.

     The cause of the incomplete development of the glabellar lobes
     lies, hence, in the intrusion of the alimentary canal, and it makes
     naturally the most effect where the gizzard spreads out and bends
     into the stomach. This spot lies behind the frontal lobe, which is
     hence increased in size according as the stomach increases in size;
     in this way not only the foremost segments of the glabella become
     enlarged, but also the following ones more or less pressed aside.
     This process is easily followed phylogenetically and
     ontogenetically.

     From the latter point of view, the development of _Paradoxides_ is
     very instructive. In a head shield 2.5 mm. long the whole anterior
     part of the glabella is broadened, but the five pairs of lateral
     impressions are clearly marked and the six segments of the head
     bounded by them are all of about the same size. In a head shield
     about 13 mm. long, the foremost segment is very much increased in
     size, the jaw lobes pressed still further apart; in adult forms
     both anterior segments are combined into the frontal swellings
     of the glabella. In other groups this process proceeds
     phylogenetically still further, so that among the Phacopidæ and in
     _Trinucleus_, behind the frontal swelling of the glabella only the
     last cephalic segment retains a certain independence. The frontal
     lobe is thus no definite part, although it is as a rule composed of
     the mesotergites of the first two cranidial segments.


This idea of an enlarged mesenteron certainly has much to commend it,
and such actual evidence as exists seems in favor of rather than
against it. The strongest, firmest, best-protected place in the whole
body of the trilobite is the cavity between the vaulted glabella and
the hypostoma. As Jaekel has said, it is far too large a cavity for
the brain, larger than would seem to be required for a heart, and what
else could be there but a stomach? As has already been pointed out,
Beyrich and Barrande found a pear-shaped enlargement of the alimentary
canal under the glabella of _Cryptolithus_. Longitudinal sections
through the glabella of _Calymene_ and _Ceraurus_ practically always
show the cavity there filled with clear crystalline calcite. One
actual specimen of _Ceraurus_ (Walcott 1881, pl. 4, fig. 1) shows the
cavity between the glabella and hypostoma entirely empty. The vacant
spaces in these two classes of specimens do not, however, necessarily
mean anything more than imperfect preservation.

[Illustration: Fig. 21.--Transverse slice through _Ceraurus
pleurexanthemus_, to show the dorsal sheath above the abdominal
cavity. Specimen 118. Traced from a photographic enlargement. × 4.]

[Illustration: Fig. 22.--Transverse section through the cephalon of
_Ceraurus pleurexanthemus_, showing the abdominal sheath and the large
mud-filled alimentary canal (clear white). Traced from a photographic
enlargement. Specimen 97. × 3.3.]

[Illustration: Fig. 23.--Transverse section of the thorax of _Calymene
senaria_, showing the large size of the mud-filled alimentary canal
(clear white). Traced from a photographic enlargement One appendifer
(also clear white) is shown. Specimen 153. × 3.3.]

_Ceraurus pleurexanthemus._

This species is taken up first, as it is the one shown in Walcott's
often-copied figure (1881, pl. 4, fig. 6). It is to be feared that too
many have looked at this figure without reading the accompanying
explanation, and have taken it for a copy of an actual specimen and
not a mere diagram, which it admittedly is. The evidence on which it
is based is comprised in eight transverse slices, one through the
glabella and seven through the thorax. Three of these have been
figured by Walcott: No. 27, 1881, pl. 3, fig. 7; No. 13, 1881, pl. 2,
fig. 3, 1918, pl. 26, fig. 14; No. 202, 1918, pl. 27, fig. 8. In all,
as can be seen by reference to the figures, the canal is partially
collapsed, and is much larger than is indicated in Walcott's
restoration. The other sections bear out the testimony of those
figured. One of these figured specimens (No. 27) and another figured
herewith (No. 118, see fig. 21) show an exceedingly interesting
structure which has previously escaped notice. The body cavity seems
to have had, in this region at least, a chitinous sheath on the dorsal
side. As shown especially in figure 21, this sheath impinges dorsally
and laterally against the axial lobe and thus furnishes a special
protection for the soft organs beneath, probably protecting them from
the strain of the dorsal muscles.

While there is no way in which the location of these sections in the
thorax can be positively determined, it is probable that they came
from the anterior end. In sections further back, supposed to be in the
posterior region of the mesenteron, no sheath is shown, but the canal
is nearly if not quite as large in relation to the size of the axial
lobe.

The single section through the glabella (specimen 97) is of course
important and fortunately well preserved (fig. 22). It shows the
dorsal sheath pressed against the inner surface of the axial lobe
along its middle portion, but diverging from it at the sides. The
section of the canal is oval, nearly twice as wide as high, but it is
obviously somewhat depressed. The original canal evidently filled
nearly the whole of the dorsal part of the glabella in this particular
region. Unfortunately, the connection with the mouth is not shown, and
the form of the hypostoma indicates that the section cut the glabella
diagonally, either in the anterior or posterior part, probably the
latter. In all these cases it should be remembered that the specimens
were found lying on their backs, and the canal has fallen in
(dorsally) since death.

The sections show that in _Ceraurus pleurexanthemus_ the anterior part
of the alimentary canal was large, filling the part of the glabella
below the heart; that the body cavity was provided with a chitinous
dorsal sheath extending back into the thorax; and that the posterior
portion of the mesenteron was likewise large and oval in section.
Since the alimentary canal must be connected with the mouth and anus,
some such restoration as that of Jaekel is indicated. No chitinous
lining of the stomodæum or proctodæum was found, but it is not certain
that any of the sections cut either of those regions.

_Calymene senaria._

Ten transverse sections and one longitudinal slice show the form of
the alimentary canal in _Calymene_. One of these has been figured by
Walcott (1881, pl. 1, fig. 9) but without showing the organ in
question.

The only section cutting the cephalon which shows any trace of the
canal is a longitudinal one (No. 141), which is not very satisfactory.
It has a large, nearly circular, opaque spot under the anterior part
of the glabella which may or may not represent a section across the
anterior end of the mesenteron. Three sections (No. 9, 115, 143) show
the dorsal sheath, the latter having the mud-filled canal beneath it.
The sheath arches across the axial lobe as in Ceraurus, leaving room
for the dorsal muscles at the sides and above it. In this region the
canal is large and oval in section. Six slices cut the mesenteron
behind the abdominal sheath (Nos. 39, 117, 148, 153, 62, 65) (see fig.
23). In the first four of these it is oval in section and large, but
not so large as in No. 143. In the last two, it is small and circular
in section, from which it is inferred that the canal tapers
posteriorly.

_Cryptolithus goldfussi_ (Barrande).

     Illustrated: Beyrich, Untersuch. über Trilobiten, Berlin, 1846, pl.
     4, fig. 1c.--Barrande, Syst. Sil. Bohême, vol. 1 1852, pl. 30,
     figs. 38, 39.

Both Beyrich and Barrande have shown that from the posterior end of
the axial lobe to the neck-ring on the cephalon, the alimentary canal
in _Cryptolithus_ has a nearly uniform diameter of less than half the
width of the axial lobe. In front of the neck-ring, it enlarges, and
while its original describers state that it extends only about halfway
to the front of the glabella, Barrande's figure 39 shows it extending
quite to the front, and his figure 38 shows it fully two thirds of the
distance to the anterior end, as does Beyrich's figure of 1846.

The Museum of Comparative Zoology contains a single specimen of this
species from Wesela, Bohemia, which shows the course of the canal from
the middle of the pygidium to the anterior part of the glabella. The
enlargement appears to begin about halfway to the front of the
glabella and to be greatest at the anterior end. At the anterior end
of the glabella, the anterior end of the thorax, and the posterior end
of the pygidium, the canal is still packed full of a material somewhat
darker in appearance than the matrix, while the remainder of it is
open. A well defined constriction is present under the middle of the
next to the last thoracic segment, but whether this is accidental or
whether it indicates the point where the mesenteron discharges into
the proctodæum can not be determined. The inside of the canal has
somewhat of a lustre and there are three conical projections into it
on the median ventral line, a very small one in front of the neck
furrow, a larger one under the anterior part of the second segment,
and a third between the fourth and fifth segments.

_Summary._

The specimens of _Cryptolithus_ from Bohemia and of _Ceraurus_ and
_Calymene_ from New York seem to substantiate the claim of Bernard and
Jaekel that at the anterior end of the canal there was an enlarged
organ which occupied the greater part of the cavity of the glabella.
It appears that it extended into the thorax, and that above it and the
heart was a chitinous dorsal sheath. Behind the enlarged portion, the
mesenteron appears to have been of practically uniform diameter in
_Cryptolithus_, but to have tapered posteriorly in Ceraurus and
_Calymene_. The proctodæum can not yet be differentiated from the
mesenteron, and only in _Cryptolithus_ has the posterior portion of
the alimentary canal been seen. It is, there, merely a continuation of
the mesenteron. The stomodæum likewise has not been identified, but
was probably a short gullet leading up from the mouth into the
enlarged digestive cavity.

[Illustration: Fig. 24. Longitudinal section of _Ceraurus
pleurexanthemus_, showing the probable outline of the alimentary canal
and the heart above it. A restoration based on the slices described
above.]

The principle of the enlargement of the latter and its influence on
the dorsal shell once established, the significance of different types
of glabellæ becomes apparent. It will be remembered that the glabella
of the protaspis of most trilobites is narrow, and that the same is
true of the glabellæ of most ancient and all primitive trilobites. The
free-swimming larvæ and the free-swimming ancestors of the trilobites
were probably strictly carnivorous, lived on concentrated food, and
needed but a small digestive tract. As the animals "discovered the
ocean bottom" and began to be omnivorous or herbivorous, larger
stomachs were required, and so in the later and more specialized
trilobites the glabella became expanded latterally or dorsally, or
both, to meet the requirement for more space, until, in such Devonian
genera as _Phacops_, the cephalon was nearly all glabella.


GASTRIC GLANDS.

Jaekel's suggestion, quoted above, that the so-called "nervures" seen
on the under surfaces of the heads of some trilobites are really
glands for the secretion of digestive juices, is at least worthy of
consideration. Moberg, however (1902, p. 299), suggested that these
markings probably had something to do with the eyes rather than the
stomach. He says in part (translation):

     In general we can now say that such features are common to all the
     eyeless Conocoryphidæ. With the conocoryphs I include _Elyx_ and
     consider Harpides as at least closely related. Similar impressions
     are also found in forms with eyes, as, for instance, in the
     Olenidæ, but here such radiate partly from the border of the eye,
     partly from the front end of the glabella, partly from the [visual
     surface of the] eye, and sometimes from the angle between the
     occipital ring and the glabella. They therefore go out from such
     different points that they can not possibly be branches of the
     liver. It would also be very remarkable if such an important organ
     should have been developed in a few eyeless forms, but have failed
     to leave the least trace in the rest of the trilobites.

Lindstroem (1901, pp. 18, 19, 33; pl. 5. figs. 29, 31; pl. 6,
figs. 43-45) has discussed these markings and given beautiful
figures showing their appearance in _Olenus_, _Parabolina_, _Elyx_,
_Conocoryphe_, and _Solenopleura_. He decided that they were to be
explained as branches of the circulatory system, comparing them with
the veins and arteries of _Limulus_. He pointed out that there was a
coincidence between these markings and the position of the eyes, and
suggested a causal connection with the latter.

Beecher (1895 B, p. 309), also from a comparison with _Limulus_,
suggested that the eye-lines of _Cryptolithus_, _Harpes_,
_Conocoryphe_, _Olenus_, _Ptychoparia_, _Arethusina_, etc., probably
represented the optic nerves, and since the eye-lines are usually the
main trunks of the dendritic markings, it is fair to assume that he
considered the whole as due to branches of nerves.

Reed has recently (1916, pp. 122, 173) discussed these lines as
developed in the Trinucleidæ, and seems to accept Beecher's
explanation.

Three explanations of the "nervures" are thus current, and the authors
of all of them refer us to _Limulus_ as proving their claims! So far
as general appearance goes, the markings on the trilobites more
closely resemble the veins of a _Limulus_ than either the nerves or
"liver" of that animal. The veins, however, are not in contact with
the dorsal shell, but are buried in the liver and muscles, while the
arrangement of the arteries, which are dorsal in position, is quite
unlike what is seen in the trilobites.

The term nervures, as applied to these markings, is not only
misleading, but an incorrect use of one of Barrande's words, for by
nervures he meant delicate surface markings. Until the real function
of the organs which made these markings is definitely established, it
may be well to call them genal cæca, for they obviously were open
tunnels ending blindly, whatever they contained.

The question of the function of the genal cæca can not, in any case,
be settled by an appeal to _Limulus_, and it is doubtful if it can be
settled at all at the present time. Certain things tend to show that
Jacket's explanation is the most plausible, and these may be briefly
set forth.

Walcott (1912 A, pp. 176, 179, pls. 27, 28) has described specimens of
_Naraoia_ and _Burgessia_ in which similar markings are well shown,
and where they are obviously connected with the alimentary canal just
at the anterior end of the mesenteron. In _Burgessia_, which seems to
be a notostracan branchiopod, the trunk sinuses are very wide, and the
appearance is on the whole unlike that of any known trilobite. In
_Naraoia_, however, the markings are much finer and directly
comparable with those of _Elyx_. If my contention that _Naraoia_ is a
trilobite should be sustained, it might almost settle the question of
the "nervures." In _Burgessia_ these lateral trunks enter the main
canal behind the fifth pair of appendages. In the trilobites they
debouch much further forward.

The principal argument in favor of the interpretation of these
markings as nerves lies in their connection with the eyes. There is
considerable evidence to indicate that the eye-lines and the genal
cæca are two distinct structures, but because both originate from the
sides of the anterior lobe of the glabella, and both extend outward at
nearly right angles to the axis, or obliquely backward, they are, when
both present, coincident. Genal cæca occur on blind trilobites, on
trilobites with simple eyes, and on trilobites with compound eyes.
Eye-lines occur on trilobites with both simple and compound eyes, and
genal cæca may or may not be present in both cases. The morphology
of the ridge forming the eye-line in trilobites with compound eyes
is well known. It is abundantly proved by ontogeny that it is the
continuation of the palpebral lobe, and a development of the pleura of
the first dorsal segment of the cephalon. Lake, Swinnerton, and Reed
have tried to show that the eye-lines of the Harpedidæ and Trinucleidæ
are homologous with the eye-lines of the trilobites with compound
eyes, and that the ocelli on the cheeks are therefore degenerate
compound eyes.

The simplest form of the genal cæcum is seen in the blind _Elyx_
(Lindstroem 1901, pl. 6, fig. 43). The main trunk is at nearly right
angles to the axis, the increase in its width is gradual in
approaching the glabella, and an equal number of branches diverge from
both sides.

_Ptychoparia striata_ (Barrande 1852, pl. 14, figs. 1, 3) is an
excellent example of a trilobite with compound eyes and genal cæca. It
will be noted that the main trunk and the eye-line are coincident, and
that both on the free and fixed cheeks the branches are all on the
anterior side of the eye-line. Compare this with the condition in
_Conocoryphe_ (Barrande, pl. 14, fig. 8; Lindstroem, pl. 6, fig. 44),
and one sees there a main branch having the same direction as in
_Ptychoparia_ and likewise with all the branches on the anterior side.
At first sight this would seem to support the contention that these
lines do lead out to the eyes, since _Conocoryphe_ is blind, and the
main trunk leads practically to the margin. But although Conocoryphe
is blind, it has free cheeks, and the main trunk does not lead to the
point on those free cheeks where eyes are to be expected, but back
into the genal angles. And this direction holds in such diverse genera
(as to eyes and free cheeks) as _Harpes_, _Cryptolithus_, _Dionide_,
and _Endymionia_. In all these the genal cæca fade out in the genal
angles, and in none of them would compound eyes be expected in that
region. The coincidence of the eye-lines with the trunks of the
genal cæca in _Ptychoparia_ seems to be merely a coincidence. That
the markings which radiate from the eyes of _Ptychoparia_ and
_Solenopleura_ are not impressions made by nerves is obvious. That
they are of the same nature as the similar markings in the eyeless
trilobites is equally obvious. Ergo, they can not be nerves in either
case, and that they have anything to do with the eyes is highly
improbable. The eye was merely superimposed upon these structures.

The relation of the genal cæca to the ocelli on the cheeks is best
shown in the Trinucleidæ. In all species of _Tretaspis_ simple eyes
are present, and in most of them there are very narrow eye-lines. The
latter are occasionally continued beyond the ocular tubercle back to
the genal angle. A similar course is seen in _Harpes_. If the simple
eye is the homologue of the compound eye, and the eye-line here the
homologue of the eye-line in _Ptychoparia_, why does it continue
beyond the eye? In any case, it can not be interpreted as a nerve.
_Cryptolithus tessellatus_, when the cephalon is 0.45 mm. to 0.65 mm.
long, shows short eye-lines and a small simple eye on each cheek. In
some half-grown specimens, traces of the ocelli can be seen, but the
eye-lines are absent. In the adult, both the eye-lines and the ocelli
are entirely wanting. Reed states that "nervures" are also absent, and
so they are from most specimens, but well preserved casts of the
interior from the Upper Trenton opposite Cincinnati show them, and one
cheek is here figured (fig. 25). As apparent from the figure, the main
trunk is very short and gives rise to two principal branches, the
first of which in its turn sends off lines from the anterior side. It
was a specimen showing these lines which Ruedemann (1916, p. 147)
figured as showing facial sutures. The interest lies in the fact that
while the ocelli and eye-lines were lost in development, the genal
cæca are present in the adult, showing that they are different
structures.

[Illustration: Fig. 25.--_Cryptolithus tessellatus_ Green. Side view
of the cheek of a specimen from the top of the Trenton opposite
Cincinnati, Ohio, to show the branching genal cæca. These are the
"facial sutures" of Ruedemann.]

_Harpides_ is another genus in which genal cæca are strikingly shown,
and in this case they completely cover the huge cheeks, radiating from
two main trunks to the front and sides. I have seen no good specimens,
but it would appear from Angelin's figure (1854, pl. 41, fig. 7) that
the rather large, simple eyes are not situated exactly on the vascular
trunks. In the _Harpides_ from Bohemia, the main trunks extend out
with many branches beyond the simple eyes. It should be stated that
the courses of the genal cæca are not correctly figured by Barrande
(Supplement, 1872, pl. 1, fig. 11), as shown by casts of the original
specimen in the Museum of Comparative Zoology. From Barrande's figure,
one would suppose that the eye-lines and their continuation beyond the
"ocelli" were superimposed upon the genal cæca without having any
definite connection with them, but as a matter of fact the radial
markings really diverge from the main trunks as in _Elyx_ and similar
forms.

_Summary._

As Reed has said, these lines are not mere ornamentation, but rather
represent traces of structures of some functional importance. They
probably can not be explained as traces of nerves and more likely
represent either traces of the gastric cæca or of the circulatory
system. While they are known chiefly in Cambrian and Lower Ordovician
trilobites, there is no evidence that the organs represented were not
present in later forms, even if the shell may not have been affected
by them. While they indicate very fine, thread-like canals, the
present evidence seems to be in favor of assigning to them the
function of lodging the glands which secreted the principal digestive
fluids.


HEART.

_Illænus._

Volborth (1863, pl. 1, fig. 12 = our fig. 26) has described the only
organ in a trilobite which suggests a heart. A Russian specimen of
_Illænus_ with the shell removed shows a somewhat flattened, tubular,
chambered organ extending from under the posterior end of the cephalon
to the anterior end of the pygidium. The posterior nine chambers were
each 1.5 mm. long and 1.5 mm. wide, while the two anterior chambers
were respectively 2.5 mm. and 3 mm. wide. These were all under the
thorax, and at least two more chambers are shown under the cephalon,
but rather obscurely. The species of the _Illænus_ is not stated, but
since no _Illænus_ has more than ten segments in the thorax, and
this tube has at least thirteen chambers, it is evident that its
constrictions are inherent in it, and are not due to the segmentation
of the thorax. Beecher has made a passing allusion to this organ as an
alimentary canal. This was the original opinion of Volborth. Pander,
however, suggested to him that it might be a heart. The alimentary
canal of _Cryptolithus_ does not show any constrictions, while the
heart of _Apus_ (see fig. 27) and other branchiopods does show them.
It should be noted, further, that while this heart enlarges toward the
front, it is everywhere very small as compared with the width of the
axial lobe, and much narrower than sections of _Ceraurus_ and
_Calymene_ would lead one to expect the alimentary canal of _Illænus_
to be. Where the heart is 1.5 mm. to 3 mm. wide, the axial lobe is 11
mm. wide.

[Illustration: Fig. 26. Copy of Volborth's figure of the heart of
_Illænus_.]

[Illustration: Fig. 27. Heart of _Apus_. Copied from Gerstäcker.]

While this may be merely a cast of the alimentary canal it is
sufficiently like a heart to deserve consideration as such an organ.

_Ceraurus and Calymene._

Nothing suggesting a heart has been seen in the sections of _Ceraurus_
and _Calymene_. The mesenteron and its sheath crowd so closely against
the dorsal test in the anterior part of the thorax that there seems
to be no room for the heart, but it must have been located beneath the
sheath and above the alimentary canal. If the latter were filled with
mud, and the animals lay on their backs, as most of them did at death,
the canal would drop down into the axial lobe and the soft heart would
naturally disappear and leave 110 trace of its presence in the
fossils.

_The Median "Ocellus" or "Dorsal Organ."_

Many trilobites, otherwise smooth, bear on the glabella a median
pustule which is usually referred to as a simple eye or median
ocellus, but whose function can not be said to have been certainly
demonstrated. Ruedemann (1916, p. 127), who has recently made a
careful study of this problem, lists about thirty genera, members
of ten families, Agnostidæ, Eodiscidæ Trinucleidæ, Harpedidæ,
Remopleuridæ, Asaphidæ Illænidæ, Goldiidæ, Cheiruridæ, and Phacopidæ,
in which this tubercle is present, and had he wished he might have
cited more, for it is of almost universal occurrence in Ordovician
trilobites.

I have not especially searched the literature for references to this
median tubercle. It is often mentioned by writers in descriptions of
species, but apparently few have tried to explain it. Beyrich (1846,
p. 30) suggested that it indicated the beginning of the alimentary
canal. Barrande mentioned it, but if he gave any explanation, it has
escaped me. McCoy (Syn. Pal. Foss. 1856, p. 146) called it an ocular
(?) tubercle, and that seems to have been the interpretation which
most writers on trilobites have assigned to it, if they suggested any
function at all. Beecher (1895 B, p. 309) concurred in this opinion.

Bernard (1894, p. 422) ascribed to this tubercle, as well as to the
median tubercle on the nuchal segment, an excretory function,
comparing it with the "dorsal organ" in _Apus_.

Reed (1916, p. 174) states that it may be either the representative of
the "dorsal" organ of the branchiopods, or a median unpaired ocellus.

Ruedemann (1916) has made the only real investigation of the subject.
He came to the conclusion that it was a parietal eye, without a
crystalline lens, but corresponding to the "parietal eye of other
crustaceans, and especially of the phyllopods, which is a lens-shaped
or pear-shaped sac, usually filled with sea water." He found that
above the "ocellus" the test was usually thin or even absent, and in a
few cases a dark line beneath seemed to outline the original form of
the sac. His summary follows:

     It is claimed that most, if not all, trilobites possessed a median
     or parietal eye on the glabella. [In proof of this assertion the
     following facts are stated:]

     1. A great number of species, belonging to more than thirty genera,
     possess a distinct tubercle on the glabella. This tubercle occurs
     alone in many genera, otherwise smooth, as in the Asaphidæ, and is
     hence of functional importance.

     2. In certain cases, as in _Cryptolithus tessellatus_, distinct
     lenticular bodies [not lenses] were recognized; in others, as in
     _Asaphus expansus_, only a thinner, probably transparent test.
     Many other species show a distinct pit in interior casts of the
     tubercle, indicating a lens-like thickening of the top of the
     tubercle. The median eye therefore probably possessed all the
     different stages of development seen in other crustaceans.

     3. As in the parietal eyes of the crustaceans and the eurypterids,
     the tubercles are most prominent and distinct in the earlier
     growth-stages, notably so in _Isotelus gigas_.

     4. The tubercle is especially well developed in the so-called blind
     forms where the lateral eyes are abortive, as in _Cryptolithus_
     (_Trinucleus_), _Dionide_, _Ampyx_.

     5. The tubercles always appear on the apex on the highest part of
     the glabella, where their visual function would be most useful.

     6. The tubercle is generally situated between the lateral eyes,
     like the parietal eye in crustaceans and eurypterids, on account of
     its close connection with the brain.

     7. Frequently it forms the posterior termination of a short crest,
     also as in certain eurypterids (_Stylonurus_), indicating the
     direction of the nerve.

     8. The median eye is borne on a tubercle or mound in the Ordovician
     and Silurian trilobites, while the tubercle is rarely noticed in
     the Devonian and in few Cambrian forms. In the Devonian forms,
     similarly as in many crustaceans and in later growth-stages of some
     asaphids, the strong development of the lateral eyes may have led
     to a loss of the parietal eyes. In the Cambrian genera evidence is
     present to suggest that the parietal eyes consisted only of
     transparent spots or lens-like thickenings of the exoskeleton,
     hardly noticeable from the outside.

     9. It is _a priori_ to be inferred that the trilobites should, as
     primitive crustaceans, have possessed median or parietal eyes.

As a student, I accepted Professor Beecher's dictum that this tubercle
represented a median _ocellus_, but more recently a number of things
have led me to the view that it is the point of attachment of the
ligament by which the heart is supported.

The chief arguments against its interpretation as a parietal eye seem
to be that its structure is not absolute proof, being capable of other
explanation; its position is variable, in front, between, or back of
the eyes; it is exactly like other tubercles on the median line,
especially the nuchal spine or tubercle, and the similar ones along
the axial lobe of the thorax; and it is not present in the protaspis
or very young trilobites.

1. The structure disclosed by Ruedemann's sections, a sort of sac-like
cavity beneath a thinned test, can be explained as a gland, a
ligamentary attachment, or a vestigial spine, as well as an eye. In a
section of _Asaphus expansus_, which I made some years ago when trying
to get some light on this problem, there is a similar cavity under the
pustule, but a secondary layer of shell lay beneath it and apparently
cut it off from the glabellar region, thus indicating that it had
lost its function in the adult of this animal. Sections through the
tubercles of the glabella of _Ceraurus_ show all of them hollow, with
very thin upper covering or none at all, and their structure is not
unlike that of the tubercle of _Cryptolithus_. In fact, sections can
be seen in Doctor Walcott's slices which are practically identical
with the one Ruedemann obtained from _Cryptolithus_. Since it is
obvious that not all of the pustules of a _Ceraurus_ could have been
eyes, the evidence from structure is rather against than for the
interpretation of the median pustule as such an organ.

2. The position of the tubercle varies greatly in different genera.
Where furthest forward (_Tretaspis_, _Goldius_), it is just back of
the frontal lobe, while in some species of asaphids it is in the neck
furrow. In species with compound eyes it is frequently between the
eyes, but more often back of them. If its history be traced in a
single family, it is generally found farthest forward in the more
ancient species and moves backward in the more recent ones. The eyes
do this same thing, but the median tubercle goes back further than the
eyes. This can be seen, for example, in the American Asaphidæ, where
the pustule is up between the eyes of _Hemigyraspis_ and _Symphysurus_
of the Beekmantown and back of the eyes of the _Isotelus_ of the
Trenton. Turning now to the under side of the head, it appears that
the tubercle bears a rather definite relation to the hypostoma. If the
hypostoma is short, the tubercle is well forward. If long, it is far
back on the head. It seems in many cases to be just back of the
posterior tip of the hypostoma, or just behind the position of the
mouth, while in others it is not as far back as the tip of the
hypostoma.

The median tubercle is in many cases developed into a long spine.
This is usually in an ancient member of a tubercle-bearing family,
and suggests that in most cases the tubercle is a vestigial organ.
An example of this occurs in _Trinucleoides_, the most ancient of the
Trinucleidæ. _Trinucleoides reussi_ (Barrande) (Supplement, 1872, pl.
5, figs. 17, 18) has a very long slender spine in this position. It
could be explained as an elevated median eye, but it also very
strongly suggests the zoæal spine of modern brachyuran Crustacea.
Gurney (Quart. Jour. Mic. Sci., vol. 46, 1902, p. 462) supports
Weldon in the conclusion that the long spines of the zoæa are
directive, and states that the animal swims in the direction of the
long axis of the spine. He also suggests that, since the period of
their presence corresponds to the period before the development of the
"auditory" organs, the spines may perform the functions of balancing
and orientation. It is generally admitted that the spine of the zoæa
is also protective, and the obvious function, first pointed out by
Spence Bate in 1859, is that it contains a ligament which helps
suspend the heart, which lies beneath the spine. This latter function
may have been that of the median tubercle in the trilobite. Such an
explanation would account for the backward migration mentioned above,
for as the stomach enlarged and the mouth moved backward on the
ventral side, the heart may have been pushed backward on the upper
side.

There is also a curious parallelism between the ontogenetic history of
the zoæal spine and the phylogenetic history of the Trinucleidæ or
Cheiruridæ (Nieszkowskia is the ancient member of this family in which
the spine replaces the tubercle). When first hatched, the larval crab
shows no trace of the spine, but very quickly it evaginates, lying
dorsally on the median line, pointing forward (Faxon, Bull. Mus. Comp.
Zool., vol. 6, 1880, pl. 2). With the splitting of the original
envelope, the spine becomes erect, but persists only a short time, and
is reduced to a vestigial tubercle toward the end of the zoæal stages,
its disappearance being, as pointed out by Gurney, coincident with the
development of the balancing organs. This manner of suspension of the
heart by a long tendon certainly does suggest that Gurney is right in
his interpretation of the function. Briefly, the zoæal spine served
for a short time a function later taken over by other organs. It was
not present in the youngest stages, it became prominent at a very
early stage, was soon vestigial, and then lost.

Take now the trilobites. There is no trace of the median pustule in
the protaspis of any form, and in many primitive trilobites it is
absent. It appears first as a long spine in certain families, and
later becomes vestigial and disappears. Very few trilobites of
Silurian and later times show it at all.

In the particular case of the Trinucleidæ, which were burrowers, the
spine is present on only the oldest and most primitive of the group, a
form which has only a most rudimentary fringe. It is obvious from the
large size of the pygidium in the larval trinucleid that this family
is derived from a group of free swimmers. _Trinucleoides reussi_ was
perhaps in the transitional stage, just leaving the swimming mode of
life, and belonged to a group which had not developed any other
"statocyst" than the median spine. Among the later Trinucleidæ the
spine became a vestigial tubercle, and in some cases entirely
disappeared. A similar history can be traced in the Cheiruridæ,
starting from some such forms as the American Lower Ordovician
_Nieszkowskia_ (_N. perforator_ p. ex.).

Another example of a median spine instead of a tubercle is in Goldius
rhinoceros (Barrande). Since this species is not from the oldest
Goldius-bearing rocks, but from the Lower Devonian, it does not follow
what seems to be the general rule, but makes an interesting exception.
Goldius rhinoceros (Barrande) (Supplement, 1872, pl. 9, figs. 12, 13)
has the median tubercle elevated into a stubby, recurved spine very
suggestive of the horn of a rhinoceros. Since the eyes of this species
are very well developed, there seems no especial reason for the
elevation of a parietal eye, and the example certainly does not
support that interpretation.

3. This tubercle is essentially similar to other tubercles on the
median line of cephalon, thorax, and even pygidium. This has been
discussed sufficiently under section 1 above, but it may perhaps be
justifiable to point out that in some of the Agnostidæ there is a
median tubercle on both shields, and since it has not yet been
demonstrated beyond question which shield is the cephalon, to say
which one is a parietal eye and which one is a tubercle is impossible.
In other words, the parietal eye can not be differentiated from any
other tubercle except by its position.

4. One of the as yet unexplained features of the protaspis of
trilobites is the absence of the "nauplius eye." Beecher (1897 B, p.
40) explained this on the ground of the extremely small size of the
protaspis and the imperfection of the preservation. If the median
tubercle were really a median eye, it should be present in the
protaspis and the earlier stages of the ontogeny, even if not in the
adult, and should certainly appear before the compound eyes. (In
_Limulus_, however, the compound eyes appear first.) The median eye
has not so far been seen in any young trilobite in any stage previous
to that in which compound eyes are present. The full ontogeny is not
known of any species with compound eyes in which the median tubercle
is present in the adult, but theoretically the median eye should be
most prominent in the young of just those primitive trilobites about
whose development most is known.


NERVOUS SYSTEM.

There has been a rather general impression among students of
trilobites that the eye-lines, which should be differentiated from the
genal cæca, denote the course of the optic nerves, but no other
evidence of the nervous system has been found, save the so-called
nervures which have been discussed above. In _Apus_ the nerves leading
to the eyes come off from the anterior ganglion or "brain" and run
directly to the eyes. If conditions were similar in the trilobites,
the "brain" was beneath the anterior glabellar lobe, provided, of
course, that the eye-lines do indicate the course of the optic nerve.

The ontogenetic history of the eye-lines of trilobites with compound
eyes is instructive, and has already been discussed by Lindstroem
(1901, pp. 12-25), but he did not cite the case of _Ptychoparia_,
which is particularly interesting, because in this genus both
eye-lines and "nervures" are present. Beecher (1895 C, p. 171, pl. 8,
figs. 5-7) has shown that in _Ptychoparia kingi_ the eye-lines of a
specimen in the metaprotaspis stage run forward at a low angle with
the glabella, while in the adult their course is nearly at right
angles to it. They have therefore swung through an arc of at least 60
and in so doing have had ample opportunity to become coincident with
the primary trunks of the genal cæca. Once that was accomplished, it
is quite likely that the one fold in the shell would continue to house
both structures. In other trilobites, there is a similar backward
progression of the eye-lines.

As would be expected, the ventral ganglia and the longitudinal cords
left no trace in the test. Since each segment has appendages, there
was probably a continuous chain of ganglia back to the posterior end
of the pygidium.



VARIOUS GLANDS.

_Dermal glands._--The surface of many trilobites is "ornamented" with
pustules and spines which on sectioning are nearly always found to be
hollow, and in many cases have a fine opening at the tip. While it is
generally believed that the purpose of these spines was protective,
yet it is possible that many of them were merely outgrowths which
increased the area through which the respiratory function could be
carried on. It will be recalled that most of the smooth trilobites
are punctate, some of them very conspicuously so, and the spines and
pustules of ornamented trilobites may merely subserve the same
function as the pores of smooth ones.

If the spines were protective, it would not be surprising if some of
them, hollow and open at the top, were poisonous also, and had glands
at the base. These are, however, purely matters of speculation so far.

_Renal excretory organs._--Nothing has been seen of any such organs,
unless the genal cæca may possibly be of that nature. The main trunks
of these always lead to the sides of the anterior glabellar lobe,
which is not the point of attachment of either antennæ or biramous
limbs, so that there seems little chance that they will bear this
interpretation.

_Reproductive organs._--Nothing is yet positively known about the
reproductive organs or the position of their external openings. If the
"exites" of _Neolenus_ could be interpreted as brood-pouches, which
does not seem probable, then the genital openings were located near
the base of some pair of anterior thoracic appendages.

_The Panderian Organs: Internal Gills or Poison Glands?_

At a meeting of the Mineralogical Society at St. Petersburg, Volborth
(1857) announced that Doctor Pander had two years before discovered
certain organs on the lower side of the doublure of the pleural lobes
of the thorax of a specimen of _Asaphus expansus_. These organs were
oval openings in the doublure, one near the posterior margin of the
cephalon, and one on each thoracic segment of the half-specimen
figured by Volborth in 1863. They were explained by Volborth and by
Eichwald (1860, 1863) as the points of attachment of appendages.
Billings (1870) described and figured the "Panderian organs" of
"_Asaphus platycephalus_" and stated that he had seen them in
_Asaphus_ [_Ogygites_] _canadensis_ and _A. megistos_ [_Isotelus
maximus_] as well. He thought some sort of organ was attached to them,
but could not suggest its function. Woodward (1870) thought that the
openings were "only the fulcral points on which the pleuræ move."
Their position outside the fulcra shows that this explanation is
impossible.

So far as I am aware, the Panderian organs have been seen only in
the Asaphidæ. Barrande figured them in "_Ogygia_" [_Hemigyraspis_]
_desiderata_ (1872) and Schmidt in two species of _Pseudasaphus_. They
seem to occupy the same position in Bohemian, Russian, and American
specimens. There is always one pair of openings on each thoracic
segment, and one pair in line with them on the posterior margin of the
cephalon. They occur near the anterior margin of the segment, and near
the inner end of the doublure. In some cases they are surrounded by a
ventrally projecting rim, while in others they have a thin edge. There
seem to be no markings on the interior of the shell which are
connected with them.

While thinking over the trilobites in connection with the origin of
insects, it occurred to me that these hitherto unexplained Panderian
organs might possibly be openings to internal gills and that the
Asaphidæ might have been tending toward an amphibious existence.
On mentioning this to Doctor R. V. Chamberlin of the Museum of
Comparative Zoology, he called my attention to the possibility that
they might be openings similar to those of the repugnatorial glands of
Diplopoda. While no definite decision as to the function can be made,
the explanation offered by Doctor Chamberlain seems more plausible
than my own, and has suggested still a third, namely, that they might
be the openings of poison glands.

If one were to argue that these apertures are really connected with
respiration, it might be pointed out that they are ventral in
position, while the _foramina repugnatoria_ are always dorsal or
lateral, even in diplopods with broad lateral expansions. If offensive
secretions were poured out beneath a concave shell like that of a
trilobite, they would be so confined as to be but slightly effective
against an enemy. This would indicate that if these openings were the
outlets of glands, the substance secreted might be a poison used to
render prey helpless. On the other hand, openings to gills are
normally ventral in position, and if the pleural lobes were folded
down against the body, they would be brought very close to the bases
of the legs.

A further curious circumstance is that so far no traces of exopodites
have been found on _Isotelus_. The endopodites of both _Isotelus
latus_ and _I. maximus_ are fairly well preserved in the single known
specimen of each, yet no authentic traces of exopodites have been
found with them. Moreover, Walcott sliced specimens of _Isotelus_ from
Trenton Falls and found only endopodites. It may also be recalled that
the finding of the specimen of _Isotelus arenicola_ at Britannia and
the tracks which I attributed to it, suggested to me that it was a
shore-loving animal (1910). It offers a field for further inquiry,
whether the Asaphidæ may not have had internal gills, and whether some
primitive member of the family may not have given rise to tracheate
arthropods.

[Illustration: Fig. 28. Side view of a specimen of _Isotelus gigas_
Dekay, from which the test of the pleural lobes has been broken to
show the position of the Panderian organs. Natural size. Specimen in
the Museum of Comparative Zoology.]

The explanation of the Panderian organs as openings of poison glands
is less radical than the one just set forth, and so possibly lies
nearer the truth. One would expect poison glands to lie at the bases
of fangs, and so they do in specialized animals like chilopods and
scorpions, but the trilobites may have had the less effective method
of pouring out the poison from numerous glands. The purpose may have
been merely to paralyze the brachiopod or pelecypod which was
incautious enough to open its shell in proximity to the asaphid.


MUSCULATURE.

This is a field which is rather one for investigation than for
exposition. Very little has been done, though probably much could be.
The chief obstacle to a clearer understanding of the muscular system
lies in the difficulty of getting at the inner surface of the test
without obscuring the faint impressions in the process.

There exist in the literature a number of references to scars of
attachment of muscles, and any study of the subject should of course
begin by the collection of such data. I shall at this time refer to
only a few observations on the subject.

The structure and known habits of trilobites make it obvious that
strong flexor and extensor muscles must have been present, and some
trace of them and of their points of attachment should be found. It is
likely that their proximal ends were tough tendons. The muscles
holding up the heart and alimentary canal would be less likely to
reveal their presence by scars, but there must have been at least one
pair of strong muscles extending from the under side of the head
across to the hypostoma. Judging from the method of attachment, the
muscles moving the limbs were short ones, chiefly within the segments
of the legs themselves.

_Flexor Muscles._

Since the majority of trilobites had the power of enrollment, and seem
also to have used the pygidia in swimming, the flexors must have been
important muscles. Beecher (1902, p. 170) appears to have been the
only writer to point out any tangible evidence of their former
presence. Walcott (1881, p. 199) had shown that the ventral membrane
was reinforced in each segment by a slightly thickened transverse
arch. Beecher showed that on this thickened arch in _Triarthrus_,
_Isotelus_, _Ptychoparia_, and _Calymene_, there are low longitudinal
internal ridges or folds. One of these is central, and there is a pair
of diagonal ridges on either side. Beecher interpreted these ridges as
separating the strands of the flexor muscles, and believed that a line
of median ridges divided a pair of longitudinal muscles, while the
outer ridges showed the place of attachment of the pair of strands
which was set off to each segment. He did not discuss the question as
to where the anterior and posterior ends were attached. In trilobites
with short pygidia, the attachment would probably have been near the
posterior end, and it is possible that the two scars beneath the
doublure and back of the last appendifers in _Ceraurus_ may indicate
the point of attachment in that genus.

There is as yet no satisfactory evidence as to where the anterior ends
of the flexors were attached. In _Apus_ these muscles unite in an
entosternal sinewy mass above the mouth, but no evidence of any
similar mass has been found in the trilobites and it is likely that
the muscles were anchored somewhere on the test of the head.

_Extensor Muscles._

The exact position of these muscles has not been previously discussed.
The interior of the dorsal test shows no such apodemes as are found on
the mesosternites, but, as I have shown in the discussion of the
alimentary canal of _Calymene_ and _Ceraurus_, there is an opening
on either side of the axial lobe between the dorsal test and the
abdominal sheath, and it is entirely probable that an extensor muscle
passed through each of these. The abdominal sheath extends only along
the posterior region of the glabella and the anterior part of the
thorax, and probably served to protect the soft organs from the strain
of the heavy muscles. The extensors (see fig. 29) probably lay along
the top of the axial lobe on either side of the median line of the
thorax to the pygidium, where they appear to have been attached
chiefly on the under side of the anterior ring of the axial lobe,
although strands probably continued further back. This is above and
slightly in front of the fulcral points on the pleura, and meets the
mechanical requirements. _Ceraurus_ (Walcott, 1875, and 1881, p. 222,
pl. 4, fig. 5) shows a pair of very distinct scars on the under side
of the first ring of the pygidium, and in many other trilobites
(_Illænus_, _Goldius_, etc.) distinct traces of muscular attachment
can be seen in this region, even from the exterior. The anterior ends
were probably attached by numerous small strands to the top of the
glabella, and, principally, to the neck-ring.

On enrolling, the sternites of all segments are pulled forward and the
tergites backward. In straightening out, the reverse process takes
place. The areas available for muscular attachment are so disposed as
to indicate longitudinal flexor and extensor muscles rather than short
muscles extending from segment to segment. Indeed, the tenuity of the
ventral membrane is such as to preclude the possibility of enrollment
by the use of muscles of that sort, while powerful longitudinal
flexors could have been anchored to cephalon and pygidium. The
strongly marked character of the neck-ring of trilobites is probably
to be explained as due to the attachment of the extensor muscles,
rather than to its recent incorporation in the cephalon. The same is
true of the anterior ring on the pygidium.

[Illustration: Fig. 29. Restoration of a part of the internal organs
of _Ceraurus pleurexanthemus_ as seen from above. At the sides are the
extensor muscles, and in the middle, the heart overlying the
alimentary canal. Drawn by Doctor Elvira Wood, under the supervision
of the author.]

_Possible preservations of extensors and flexors in Ceraurus_.--Among
Doctor Walcott's sections are four slices which I should not like to
use in proving the presence of longitudinal muscles, but which may be
admitted as corroborative evidence. Two of these transverse sections
(Nos. 114 and 199) show a dorsal and a ventral pair of dark spots in
positions which suggest that they represent the location of the dorsal
and ventral muscles, while two others (Nos. 131 and 140) show only the
upper pair of spots. The chief objection to this interpretation is
that it is difficult to imagine how the muscles could be so replaced
that they happen to show in the section. Both the sections showing all
four spots are evidently from the anterior part of the thorax, as they
show traces of the abdominal sheath, which seems to be squeezed
against the inside of the axial lobe, with the muscles (?) forced out
to the sides. The ventral pair lie just inside the appendifers, but
even if they are sections of muscles, all four are probably somewhat
out of place.

_Hypostomial Muscles._

The hypostoma fits tightly against the epistoma, or the doublure when
the epistoma is absent, but in no trilobite has it ever been seen
ankylosed to the dorsal test, and its rather frail connection
therewith is evidenced by the relative rarity of specimens found with
it in position. That the hypostoma was movable seems very probable,
and that it was held in place by muscles, certain. The maculæ were
always believed to be muscle scars until Lindstroem (1901, p. 8)
announced the discovery by Liljevall of small granules on those of
_Goldius polyactin_ (Angelin). These were interpreted as lenses
of eyes by Lindstroem, who tried to show that the maculæ of all
trilobites were functional or degenerate eyes. Most palæontologists
have not accepted this explanation, and since the so-called eyes cover
only a part of the surface of the maculæ, it is still possible to
consider the latter as chiefly muscle-scars.

In Lindstroem's summary (1901, pp. 71, 72) it is admitted that the
globular lenses are found only in _Bronteus_ (_Goldius_) (three
Swedish species only) and _Cheirurus spinulosus_ Nieszkowski, while
the prismatic structure supposed to represent degenerate eyes was
found in eleven genera (Asaphidæ, Illænidæ, Lichadidæ). All of these
are forms with well developed eyes, and Lindstroem himself points out
that the appearance of actual lenses in the hypostoma was a late
development, long after the necessity for them would appear to have
passed.

The use of the hypostoma has been discussed by Bernard (1892, p. 240)
and extracts from his remarks are quoted:

     The earliest crustacean-annelids possessed large labra or prostomia
     projecting backward, still retained in the Apodidæ and trilobites.
     This labrum almost necessitated a very deliberate manner of
     browsing. The animal would creep along, and would have to run some
     way over its food before it could get it into its mouth, the whole
     process, it seems to us, necessitating a number of small movements
     backwards and forwards. Small living prey would very often escape,
     owing to the fact that the animal's mouth and jaws were not ready
     in position for them when first perceived. The labrum necessitates
     the animal passing forwards over its prey, then darting backward to
     follow it with its jaws. We here see how useful the gnathobases of
     _Apus_ must be in catching and holding prey which had been thus
     passed over. Indeed the whole arrangement of the limbs of _Apus_
     with the sensory endites forms an excellent trap to catch prey
     over which the labrum has passed.

In alcoholic specimens of _Apus_ the labrum is not in a horizontal
plane, as it is in most well preserved trilobites, but is tipped down
at an angle of from 30 to 45, and the big mandibles lie under it. It
has considerable freedom of motion and is held in place by muscles
which run forward and join the under side of the head near its
posterior margin. It seems entirely possible that the hypostoma of
the trilobite had as much mobility as the labrum of _Apus_, and that
byopening downward it brought the mouth lower and nearer the food. It
will be recalled that the hypostomata of practically all trilobites
are pointed at the posterior margin, there being either a central
point or a pair of prongs. By dropping down the hypostoma until
the point or prongs rested on or in the substratum, and sending food
forward to the mouth by means of the appendages, a trilobite could
make of itself a most excellent trap, and if the animal could dart
backward as well as forward, the hypostoma would be still more useful.
There is no reason to suppose that they could not move backward, and
the "pygidial antennæ" of _Neolenus_ indicate that animals of that genus
at least did so. This habit of dropping down the hypostoma would also
permit the use of those anterior gnathobases which seem too far ahead
of the mouth in the trilobites with a long hypostoma.

For actual evidence on this point, it is necessary to have recourse
once more to Doctor Walcott's exceedingly valuable slices. From such
sections of _Ceraurus_ as his Nos. 100, 106, 108, 170, and 173, it is
evident that the hypostoma of that form could be dropped considerably
without disrupting the ventral membrane (fig. 30). Sections of
_Calymene_ already published (Walcott 1881, pl. 5, figs. 1, 2) show
the hypostoma turned somewhat downward, and the slices themselves show
sections of the anterior pair of gnathobases beneath the hypostoma.
When the hypostoma was horizontal, these gnathobases were crowded out
at the sides.

[Illustration: Fig. 30.--Longitudinal section of cephalon of _Ceraurus
pleurexanthemus_, to show position of the mouth and folds of the
ventral membrane between the glabella and the hypostoma. The test is
in solid black and the part within the ventral membrane dotted. From a
photographic enlargement. Specimen 169. × 3.9.]

[Illustration: Fig. 31.--A copy of Doctor Moberg's figure of _Nileus
armadillo_, showing the position of the muscle scars.]

If the hypostoma were used in the manner indicated, the muscles must
have been more efficient than those of the labrum of _Apus_, and it is
probable that they crossed to the dorsal test. Just where they were
attached is an unsolved problem. Barrande (1852, pl. 1, fig. 1) has
indicated an anterior pair of scars and a single median one on the
frontal lobe of _Dalmanites_ that may be considered in this connection,
and also three pairs of scars on the last two lobes of the glabella of
_Proëtus_ (1852, pl. 1, fig. 7). Moberg (1902, p. 295, pl. 3, figs. 2,
3, text fig. 1) has described in some detail the muscle-scars of a
rather remarkable specimen of _Nileus armadillo_ Dalman. While, as I
shall point out, I do not agree wholly with Professor Moberg's
interpretation, I give here a translation (made for Professor Beecher)
of his description, with a copy of his text figure:

     The well preserved surface of the shell permits one to note not
     only the tubercle (t) but a number of symmetrically arranged
     glabellar impressions. And because of their resemblance to the
     muscular insertions of recent crustaceans, I must interpret them as
     such. They appear partly as rounded hollows (k and i), also as
     elongate straight or curved areas (a, b, c, e, g, h) made up of
     shallow impressions or furrows about 1 mm. long, sub-parallel, and
     standing at an angle to the trend of the areas. Impression e is
     especially well marked, inasmuch as the perpendicular furrows are
     arranged in a shallow crescentic depression; and impression d shows
     besides the obscure furrows a number of irregularly rounded
     depressions. Larger similar ones occur at f, and in part extend
     over toward g.

     The meaning of these impressions, or their myologic significance,
     could be discussed, although such discussion might rather be termed
     guessing.

     Inner organs, such as the heart and stomach, might have been
     attached to the shell along impressions a and b. Also along or
     behind c and h, which both continue into the free cheeks, ligaments
     or muscular fibers may have been inserted. From d, e, f, and g,
     muscles have very likely gone out to the cephalic appendages.
     Against this it may be urged that impression d is too far forward
     to have belonged to the first pair of feet. Again, the impression h
     may in reality represent two confluent muscular insertions, from
     the first of which, in that case, arose the muscles of the fourth
     pair of cephalic feet. Were this the case, the muscles of the first
     pair of cheek feet should be attached at e. And d in turn may be
     explained as the attachment of the muscles of the antennæ, k those
     of the hypostoma, and from i possibly those of the epistoma. That k
     is here named as the starting point of the hypostomial muscles and
     not those of the antennæ, depends partly on the analogous position
     of i and partly on the fact that the hypostoma of _Nileus
     armadillo_ (text figure, in which the outline of the hypostoma is
     dotted), by reason of it? wing-like border, could not have
     permitted the antennæ to reach forward, but rather only outward or
     backward.

My own explanation would be that impressions e, f, and g correspond to
the glabellar furrows, h the neck furrow, and all four show the places
of attachment of the appendifers. Those at d may possibly be connected
with the antennæ, although I should expect those organs to be attached
under the dorsal furrows at the sides of the hypostoma. It will be
noted that either b, k, or i correspond well with the maculæ of the
hypostoma and some or all of them may be the points of attachment of
hypostomial muscles. They correspond also with the anterior scars of
_Dalmanites_.


EYES.

While I have nothing to add to what has been written about the eyes of
trilobites, this sketch of the anatomy would be incomplete without
some reference to the little which has been done on the structure of
these organs.

Quenstedt (1837, p. 339) appears to have been the first to compare the
eyes of trilobites with those of other Crustacea. Johannes Müller had
pointed out in 1829 (Meckel's Archiv) that two kinds of eyes were
found in the latter group, compound eyes with a smooth cornea, and
compound eyes with a facetted coat. Quenstedt cited _Trilobites
esmarkii_ Schlotheim (=_Illænus crassicauda_ Dalman) as an example of
the first group, and _Calymene macrophthalma_ Brongniart (=_Phacops
latifrons_ Bronn) for the second. Misreading the somewhat careless
style of Quenstedt, Barrande (1852, p. 133) reverses these, one of the
few slips to be found in the voluminous writings of that remarkable
savant.

Burmeister (1843; 1846, p. 19) considered the two kinds of eyes as
essentially the same, and accounted for the conspicuous lenses of
Phacops on the supposition that the cornea was thinner in that genus
than in the trilobites with smooth eyes.

Barrande (1852, p. 135) recognized three types of eyes in trilobites,
adding to Quenstedt's smooth and facetted compound eyes the groups of
simple eyes found in Harpes. In his sections of 1852, pl. 3, figs.
15-25, which are evidently diagrammatic, he shows separated biconvex
lenses in both types of compound eyes, _Phacops_ and _Dalmanites_ on
one hand, and _Asaphus_, _Goldius_, _Acidaspis_, and _Cyclopyge_
on the other. Clarke ( 1888), Exner ( 1891 ) and especially
Lindstroem (1901) have since published much more accurate figures and
descriptions. The first person to study the eye in thin section seems
to have been Packard (1880), who published some very sketchy figures
of specimens loaned him by Walcott. He studied the eyes of _Isotelus
gigas_, _Bathyurus longispinus_, _Calymene_, and _Phacops_, and
decided that the two types of eyes were fundamentally the same.
He also compared them with the eyes of _Limulus_.

Clarke (1888), in a careful study of the eye of _Phacops rana_, found
that the lenses were unequally biconvex, the curvature greater on the
inner surface. The lens had a circular opening on the inner side,
leading into a small pear-shaped cavity. The individual lenses were
quite distinct from one another, and separated by a continuation of
the test of the cheek.

Exner (1891, p. 34), in a comparison of the eyes of Phacops and
_Limulus_, came to the opinion that they were very unlike, and that
the former were really aggregates of simple eyes.

Lindstroem (1901, pp. 27-31) came to the conclusion that besides the
blind trilobites there were trilobites with two kinds of compound
eyes, trilobites with aggregate eyes, and trilobites with stemmata and
ocelli. His views may be briefly summarized.

     I. Compound eyes.

     1. Eyes with prismatic, plano-convex lenses.

     "A pellucid, smooth and glossy integument, a direct continuation of
     the common test of the body, covers the corneal lenses, quite as is
     the case in so many of the recent Crustacea. The lenses are closely
     packed, minute, usually hexagonal in outline, flat on the outer and
     convex on the inner surface. Such eyes are best developed in
     _Asaphus_, _Illænus_, _Nileus_, _Bumastus_, _Proëtus_, etc."

     2. Eyes with biconvex lenses.

     The surface of the eye is a mass of contiguous lenses, covered by a
     thin membrane which is frequently absent from the specimens, due to
     poor preservation. The lenses are biconvex, and being in contact
     with one another, are usually hexagonal, although in some cases
     they nearly retain their globular shape. Such eyes are found in
     Bury care, _Peltura_, _Sphæropthalmus_, _Ctenopyge_, _Goldius_,
     _Cheirurus_, and probably others.

     II. Aggregate eyes.

     The individual lenses are comparatively large, distinct from one
     another, each lying in its own socket. There is, however, a thin
     membrane, which covers all those in any one aggregate, and is a
     continuation of the general integument of the body. This membrane
     is continued as a thickened infolding which forms the sockets of
     the lenses.

     Such eyes are known in the Phacopidæ only.

     III. Stemmata and ocelli.

     The stemmata are present only in _Harpes_, where there may be on
     the summit of the cheek two or three ocelli lying near one another.
     Each, viewed from above, is nearly circular in outline, almost
     hemispheric, glossy and shining. In section they prove to be convex
     above and flat or slightly concave beneath. The test covers and
     separates them, as in the case of the aggregate eyes.

     The ocelli of the Trinucleidæ and _Eoharpes_ are smaller, and the
     detailed structure not yet investigated.

     Lindstroem concludes that so far as its facets or lenses are
     concerned, the eye of the trilobite shows the greatest analogy with
     the Isopoda, and the least with _Limulus_.


SUMMARY.

The simplest eyes found among the Trilobita are the ocelli. These
consist of a Simple thickening of the test to form a convex surface
capable of concentrating light. The similarity in position of the
paired ocelli of trilobites and the simple eyes of copepods has
perhaps a significance.

The schizochroal eyes may well be compared with the aggregate eyes of
the chilopods and scorpions. The mere presence of a common external
covering is not sufficient to prove this a true compound eye,
especially as the covering is merely a continuation of the general
test.

The holochroal eyes are of two kinds, one with plano-convex and one
with biconvex lenses. The latter would seem to be mechanically the
more perfect of the two, and it is worthy of note that the trilobites
possessing the biconvex lenses have, in general, much smaller eyes
than those with the other type.

If, as some investigators claim, the parietal eye of Crustacea
originates by the fusion of two lateral ocelli, trilobites show a
primitive condition in lacking this eye, which may have originated
through the migration toward the median line of ocelli like those of
the Trinucleidæ.


SEX.

That the sexes were separate in the Trilobita there can be very little
doubt, but the study of the appendages has as yet revealed nothing in
the way of sexual differences. One of the most important points still
to be determined is the location of the genital openings.

In many modern Crustacea, the antennæ or antennules are modified as
claspers, and it is barely possible that the curious double curvature
of the antennules of Triarthrus indicates a function of this sort. The
antennules of many specimens have the rather formal double curvature,
turning inward at the outer ends, and retain this position of the
frontal appendages, no matter what may be the condition of those on
the body. Other specimens have the antennules variously displaced,
indicating that they are quite flexible. It is conceivable that the
individuals with rigid antennules are males, the others females.

It is interesting to note that the antennules of _Ptychoparia
permulta_ Walcott (1918, pl. 21, fig. 1) have the same recurved form.
All the specimens of Neolenus, however, show very flexible antennas.

Barrande and Salter laid great stress upon the "forme longue" and
"forme large" as indicating male and female. This was based upon the
supposition that the female of any animal would probably have a
broader test than the male, a hypothesis which seems to be very little
supported by fact. In practical application it was found that the
apparent difference was so often due to the state of preservation or
the confusion of two or more species, that for many years little
reference has been made to this supposed sex difference.


EGGS.

In his classic work on the trilobites of Bohemia, Barrande described
three kinds of spherical and one of capsule-shaped bodies which he
considered to be the eggs of trilobites. After a review of the
literature and a study of specimens in the collections of the Museum
of Comparative Zoology, it can be said that none of these fossils has
proved to be a trilobite egg, but that they may be plants. A full
account of them will be published elsewhere.

Walcott (1881) and Billings (1870) have described similar bodies
within the tests of _Calymene_ and _Ceraurus_, but without showing
positive evidence as to their nature.


Methods Of Life.

This is a subject upon which much can be inferred, but little proved.
Without trying to cover all possibilities, it may be profitable to
see what can be deduced from what is known of the structure of the
external test, the internal anatomy, and the appendages. This can, to
a certain extent, be controlled by what is inferred from the strata
in which the specimens are found, the state of preservation, and the
associated animals. (For other details, see the discussion of
"Function of the Appendages" in Part I.)


HABITS OF LOCOMOTION.

The methods of locomotion may be deduced with some safety from a study
of the appendages, and, as has repeatedly been pointed out, all
trilobites could probably swim by their use. This swimming was
evidently done with the head directed forward, and could probably be
accomplished indifferently well with either the dorsal (gastronectic,
Dollo) or the ventral (notonectic) side up. If food were sought on the
bottom by means of sight, the animal would probably swim dorsal side
up, for by canting from side to side it could see the bottom just as
easily as though it were ventral side up, and at the same time it
would be in position to drop quickly on the prey. In collecting food
at the surface, it might swim ventral side up.

All trilobites could probably crawl by the use of the appendages, and,
as has already been pointed out, there are great differences in the
adjustment of the appendages to different methods of crawling. Some
crawled on their "toes," some by means of the entire endopodites, and
some apparently used the coxopodites to push themselves along. That
the normal direction of crawling was forward is indicated by the
position of the eyes and sensory antennules. There is no evidence that
their mechanism was irreversible, however, and the position of the
mouth and the shape of the hypostoma indicate that they usually backed
into feeding position. The caudal rami of Neolenus were evidently
sensory, and the animal was prepared to go in either direction.

The use of the pygidium as a swimming organ, suggested by Spencer
(1903, p. 492) on theoretical grounds, developed by Staff and Reck
(1911, p. 141) from a mechanical standpoint, and elaborated in
the present paper by evidence from the ontogeny, phylogeny, and
musculature, provided the animal with a swifter means of locomotion.
By a sudden flap of this large fin, a backward darting motion could be
obtained, which would be invaluable as a means of escape from enemies.
Staff and Reck seem to think that in this movement the two shields
were clapped together, and that the animal was projected along
with the hinge-like thorax forward. This might be a very plausible
explanation in the case of the bivalve-like Agnostidæ, and it is one
I had suggested tentatively for that family before I read Staff and
Reck's paper. In the case of the large trilobites with more segments,
however, it would be more natural to think of a mode of progression in
which there was an undulatory movement of the body and the pygidium,
up-and-down strokes being produced by alternately contracting the
dorsal and ventral muscles. Bending the pygidium down would tend to
pull the animal backward, while bringing it back into position would
push it forward. It follows, therefore, that one of these movements
must have been accomplished very quickly, the other slowly. If the
muscle scars have been interpreted properly, the ventral muscles were
probably the more powerful, an indication that the animal swam
backward, using the cephalon and antennules as rudders.

The chief objection to the theory of swimming by clapping the valves
together is that where the thorax consists of several segments it no
longer acts like the hinge of a bivalve, and a sudden downward flap of
the pygidium would impart a rotary motion to the animal. Take, for
example, such nearly spherical animals as the Illænidæ, and it will
readily be seen that there is nothing to give direction to the motion
if the pygidium be brought suddenly against the lower surface of the
cephalon. A lobster, it is true, progresses very well by this method,
but it depends upon its great claws and long antennæ to direct its
motions. The whole shape of the trilobite is of course awkward for a
rapidly swimming animal. It could keep afloat with the minimum of
effort and paddle itself about with ease, but it was not built on the
correct lines for speed.

Dollo (1910, p. 406), and quickly following his lead, Staff and Reck
(1911, p. 130), have published extremely suggestive papers, showing
that by the use of the principle of correlation of parts, much can be
inferred about the mode of life of the trilobites merely from the
structure of the test.

Dollo studied the connection between the shape of the pygidium and the
position and character of the eyes. As applied by him, and later by
Clarke and Ruedemann, to the eurypterids, this method seems most
satisfactory. He pointed out that in Eurypterida like _Stylonurus_ and
_Eurypterus_, where there is a long spine-like telson, the eyes are
back from the margin, so that a _Limulus_-like habit of pushing the
head into the sand by means of the limbs and telson was possible.
_Erettopterus_ and _Pterygotus_, on the other hand, have the eyes on
the margin, so that the head could not be pushed into the mud without
damage, and have a fin-like telson, suggesting a swimming mode of
life.

In carrying this principle over to the trilobites, Dollo was quite
successful, but Staff and Reck have pointed out some modifications
of his results. The conclusions reached in both these papers are
suggestive rather than final, for not all possible factors have been
considered. The following are given as examples of interesting
speculations along this line.

_Homalonotus delphinocephalus_, according to Dollo, was a crawling
animal adapted to benthonic life in the euphotic region, and an
occasional burrower in mud. This is shown by well developed eyes in
the middle of the cephalon, a pointed pygidium, and the plow-like
profile of the head. This was as far as Dollo went. From the very
broad axial lobe of _Homalonotus_ it is fair to infer that, like
_Isotelus_, it had very long, strong coxopodites which it probably
vised in locomotion, and also very well-developed longitudinal
muscles, to be used in swimming. From the phylogeny of the group, it
is known that the oldest homalonotids had broad unpointed pygidia of
the swimming type, and that the later species of the genus (Devonian)
are almost all found in sandstone and shale, and all have wider axial
lobes than the Ordovician forms. It is also known that the epistoma
is narrower and more firmly fused into the doublure in later than in
earlier species. These lines of evidence tend to confirm Dollo's
conclusion, but also indicate that the animals retained the ability to
swim well.

On the same grounds, _Olenellus thompsoni_ and _Dalmanites limulurus_
were assigned the same habitat and habits. Both were considered to
have used the terminal spine as does _Limulus_.

_Olenellus thompsoni_ is generally considered to be unique among
trilobites in having a _Limulus_-like telson in place of a pygidium.
This "telson" has exactly the position and characteristics of the
spine on the fifteenth segment of _Mesonacis_, and so long ago as
1896, Marr (Brit. Assoc. Adv. Sci., Rept. 66th Meeting, page 764)
wrote:

     The posterior segments of the remarkable trilobite _Mesonacis
     vermontana_ are of a much more delicate character than the anterior
     ones, and the resemblance of the spine on the fifteenth "body
     segment" of this species to the terminal spine of _Olenellus_
     proper, suggests that in the latter subgenus posterior segments of
     a purely membranous character may have existed devoid of hard
     parts.

This prophecy was fulfilled by the discovery of the specimens which
Walcott described as _Pædeumias transitans_, a species which is said
by its author to be a "form otherwise identical with _O. thompsoni_,
[but] has rudimentary thoracic segments and a _Holmia_-like pygidium
posterior to the fifteenth spine-bearing segment of the thorax." A
good specimen of this form was found at Georgia, Vermont, associated
with the ordinary specimens of _Olenellus thompsoni_, and I believe
that it is merely a complete specimen of that species. _Olenellus
gilberti_, which was formerly supposed to have a limuloid telson, has
now been shown by Walcott (Smithson. Misc. Coll., vol. 64, 1916, p.
406, pl. 45, fig. 3) to be a _Mesonacis_ and to have seven or eight
thoracic segments and a small plate-like pygidium back of the
spine-bearing fifteenth segment. All indications are that the spine
was not in any sense a pygidium. Walcott states that _Olenellus_
resulted from the resorption of the rudimentary segments of forms such
as _Mesonacis_ and _Pædeumias_, leaving the spine to function as a
pygidium. This would mean the cutting off of the anus and the
posterior part of the alimentary canal, and developing a new anal
opening on the spine of one of the thoracic segments!

If the spine of the fifteenth segment is not a pygidium, could it be
used, as Dollo postulates, as a pushing organ? Presumably not, for
though in entire specimens of _Olenellus_ (_Pædeumias_) it extends
back beyond the pygidium, it probably was borne erect, like the
similar spines in _Elliptocephala_, and not in the horizontal plane in
which it is found in crushed specimens.

While this removes some of the force of Dollo's argument, his
conclusion that _Olenellus_ was a crawling, burrowing animal living
in well lighted shallow waters was very likely correct. The long,
annelid-like body indicates numerous crawling legs, there is no
swimming pygidium, and the fusion of the cheeks in the head makes a
stiff cephalon well adapted for burrowing.

Staff and Reck have pointed out that _Dalmanites limulurus_ was not
entirely a crawler, but, as shown by the large pygidium, a swimmer
as well. This kind of trilobite probably represents the normal
development of the group in Ordovician and later times. The Phacopidæ,
Proëtidæ, Calymenidæ, and other trilobites of their structure could
probably crawl or swim equally well, and could escape enemies by
darting away or by "digging themselves in."

_Cryptolithus tessellatus_ (_Trinucleus concentricus_) is cited by
Dollo as an example of an adaptation to life in the aphotic benthos,
permanently buried in the mud. In this case he appealed to Beecher's
interpretation of the appendages, and pointed out that while the adult
is blind, the young have simple eyes and probably passed part of their
life in the lighted zone. It needs only a glance at the very young to
convince one that the embryos had swimming habits, so that in this
form one sees the adaptation of the individual during its history to
all modes of life open to a trilobite. The habits of the Harpedidæ may
have been similar to those of the Trinucleidæ, but the members of
this family are supplied with broad flat genal spines. It has been
suggested that these served like pontoons, runners, or snow-shoes, to
enable the animal to progress over soft mud without sinking into it.
Some such explanation might also be applied to the similar development
in the wholly unrelated Bathyuridæ. The absence of compound eyes and
the poor development of ocelli in the Harpedidæ suggest that they were
burrowers, and from these two families, Trinucleidæ and Harpedidæ, it
becomes evident that a pygidial point or spine is not a necessary part
of the equipment of a burrowing trilobite. In fact, from the habits of
_Limulus_ it is known that the appendages are relied upon for digging,
and that the telson is a useful but not indispensable pushing organ.

_Deiphon_ is an interesting trilobite from many points of view. Its
pleural lobes are reduced to a series of spines on either side of the
body, and its pygidium is a mere spinose vestige. Dollo considered
this animal a swimmer in the euphotic zone, because its eyes are on
the anterior margin, its body depressed, its glabella globose, and its
pygidium flat and spinose. That such a method of life was secondary
in a cheirurid was indicated to him by the fact that the more
primitive members of the family seemed adapted for crawling. Staff and
Reck have gone further and shown that the pygidium is only the vestige
of a swimming pygidium, and that the great development of spines
suggests a floating rather than a swimming mode of life. They
therefore argue for a planktonic habitat. A similar explanation is
suggested for _Acidaspis_ and other highly spinose species.

The Aeglinidæ, or Cyclopygidæ as they are more properly called,
present the most remarkable development of eyes among the trilobites.
In this, Dollo saw, as indeed earlier writers have, an adaptation
to a region of scanty light. The cephalon is not at all adapted to
burrowing, but the pygidium is a good swimming organ, and one is apt
to agree that this animal was normally an inhabitant of the ill
lighted dysphotic region, but also a nocturnal prowler, making trips
to the surface at night. Similar habits and habitat are certainly
indicated for _Telephus_ and the Remopleuridæ, but whether _Nileus_
and the large-eyed _Bumastus_ are capable of the same explanation is
doubtful.

Finch (1904, p. 181) makes the suggestion that "_Nileus_" (_Vogdesia_)
_vigilans_, an abundant trilobite in the calcareous shale of the
Maquoketa, was in the habit of burying itself, posterior end first. He
found a slab containing fifteen entire specimens, all of which had the
cephalon extended horizontally near the surface of the stratum, and
the thorax and pygidium projecting downward. The rock showed no
evidence that they were in burrows, and the fact that all were in the
same position indicates that the attitude was voluntarily assumed.
They appear to have entrenched themselves by the use of the pygidia,
which are incurved plates readily adapted for such use, and, buried up
to the eyes, awaited the coming of prey, but were, apparently,
smothered by a sudden influx of mud. The form of the eye in _Vogdesia
vigilans_ bears out this supposition of Finch's. Not only are the eyes
unusually tall, but the palpebral lobe is much reduced, so that many
of the lenses look upward and inward, as well as outward, forward and
backward. The particular food required by _V. vigilans_ must have been
very plentiful in the Maquoketa seas of Illinois and Iowa, for the
species was very abundant, but that its habits were self-destructive
is also shown by the great number of complete enrolled specimens of
all ages now found there. The soft mud was apparently fatal to the
species before the end of the Maquoketa, for specimens are seen but
very rarely in the higher beds.

_Vogdesia vigilans_ is shaped much like _Bumastus_, _Illænus_,
_Asaphus_, _Onchometopus_, and _Brachyaspis_, and it may be that these
trilobites with incurved pygidia had all adopted the habit of digging
in backward. As noted above, their pygidia are not very well adapted
for swimming, and most of them have large or tall eyes.

Dollo's comparison of the Cyclopygidæ to the huge-eyed modern amphipod
_Cystosoma_ is instructive. This latter crustacean, which has the
greater part of the dorsal surface of the carapace transformed into
eyes, is said to live in the dysphotic zone, at depths of from 40 to
100 fathoms, and to come to the surface at night. It swims ventral
side down.

The kinds of sediments in which trilobites are entombed have so far
afforded little evidence as to their habitat. Frech (Lethæa
palæozoica, 1897-1902, p. 67 _et seq._) who has collected such
evidence as is available on this subject, places as deeper water
Ordovician deposits the "Trinucleus-Schiefer" of the upper Ordovician
of northern Europe and Bohemia, the "Triarthrus-Schiefer" of America,
the "Asaphus-Schiefer" of Scandinavia, Bohemia, Portugal, and France,
and the Dalmania quartzite of Bohemia. .

_Cryptolithus_ and _Triarthrus_, although not confined to such
deposits, are apt to occur chiefly in very fine-grained shales, in
company with graptolites. These latter are distributed by currents
over great distances within short periods. It is somewhat curious that
the nearly blind burrowing Trinucleidæ, the dysphotic, large-eyed
Remopleuridæ and Telephus, the blind nektonic Agnostidæ and Dionide,
and the planktonic graptolites should go together and make up almost
the entire fauna of certain formations. Yet, when the life history of
each type is studied, a logical explanation is readily at hand, for
all have free-swimming larvæ.

A list of the methods of life noted above is given by way of summary,
with examples.

          {Planktonic  {Primarily    Earliest protaspis of all trilobites
          {            {Secondarily  _Deiphon_, _Odontopleura_, etc.
          {
  Pelagic {            {Primarily    Later protaspis of all trilobites.
          {            {               _Naraoia_
          {            {
          {            {            {Probably many thin-shelled
          {            {            {  trilobites with large pygidia
          {            {            {  (only partially nektonic)
          {Nektonic    {Secondarily {Cyclopygidæ    }
                                    {Remopleuridæ   } (nektonic dysphotic)

            {Crawlers and
            {  slow swimmers   Most trilobites with small pygidia.
            {                     _Triarthrus_, _Paradoxides_, etc.
  Benthonic {Crawlers and      Most trilobites with large pygidia.
            {  active swimmers   _Isotelus_, _Dalmanites_, etc.
            {
            {Crawlers, slow
            {  swimmers, and   Trinucleidæ, Harpedidæ,
            {  burrowers         some Mesonacidæ, etc.


FOOD AND FEEDING METHODS.

This subject has been less discussed than the methods of locomotion.
The study of the appendages has shown that while the mouth parts were
not especially powerful, they were at least numerous, and sufficiently
armed with spines to shred up such animal and vegetable substances as
they were liable to encounter. It having been ascertained that the
shape of the glabella and axial lobe furnishes an indication of the
degree of development of the alimentary canal it is possible to infer
something of the kind of food used by various trilobites.

The narrow glabellæ and axial lobes of the oldest trilobites would
seem to indicate a carnivorous habit, while the swollen glabellæ and
wider lobes of later ones probably denote an adaptation to a mixed or
even a vegetable diet. This can not be relied upon too strictly, of
course, for the swollen glabellæ of such genera as Deiphon or
Sphærexochus may be due merely to the shortening up of the cephalon.

Walcott (1918, p. 125) suggests that the trilobites lived largely upon
worms and conceives of them as working down into the mud and prowling
around in it in search of such prey. While there can be no doubt that
many trilobites had the power of burying themselves in loose sand or
mud, a common habit with modern crustaceans, most of them were of a
very awkward shape for habitual burrowers, and how an annelid could be
successfully pursued through such a medium by an animal of this sort
is difficult to understand. In fact, the presence of the large
hypostoma and the position of the mouth were the great handicaps of
the trilobite as a procurer of live animal food, and coupled with the
relatively slow means of locomotion, almost compel the conclusion that
errant animals of any size were fairly safe from it. This restricts
the range of animal food to small inactive creatures and the remains
of such larger forms as died from natural causes. The modern Crustacea
are effective scavengers, and it is probable that their early
Palæozoic ancestors were equally so. It is a common saying that in the
present stressful stage of the world's history, very few wild animals
die a natural death. In Cambrian times, competition for animal food
was less keen, and with the exception of a few cephalopods, a few
large annelids, and a few Crustacea like _Sidneyia_, there seem to
have been no aggressive carnivores. In consequence, millions of
animals must have daily died a natural death, and had there been no
way of disposing of their remains, the sea bottom would soon have
become so foul that no life could have existed. For the work of
removal of this decaying matter, the carnivorous annelids and the
Crustacea, mostly trilobites, were the only organisms, and it is
probable that the latter did their full share. After prowling about
and locating a carcass, the trilobite established himself over it, the
cephalon and hypostoma on one end and the pygidium on the other
enclosing and protecting the prey, which was shredded off and passed
to the mouth at leisure by means of the spinose endobases.

Even in Middle Cambrian times some trilobites (e. g., _Paradoxides_)
seem to have enlarged the capacity of the stomach and taken vegetable
matter, but later, in the Upper Cambrian and Ordovician, when the
development of cephalopods and fishes caused great competition for all
animal food, dead or alive, most trilobites seem to have become
omnivorous. This is of course shown by the swollen glabella, with
reduced lateral furrows, and, in the case of a few species
(_Calymene_, _Ceraurus_), the known enlargement of the stomach.

_Cryptolithus_ is the only trilobite which has furnished any actual
evidence as to its food. From the fact that the alimentary tract is
found stuffed from end to end with fine mud, and because it is known
to have been a burrower, it has been suggested by several that it was
a mud feeder, passing the mud through the digestive tract for the sake
of what organic matter it contained. Spencer (1903, p. 491) has
suggested a modification of this view:

     The phyllopods appear to feed by turning over whilst swimming and
     seizing with their more posterior appendages a little mud which
     swarms with infusoria, etc. This mud is then pushed along the
     ventral groove to the mouth. Casts, of the intestine of trilobites
     are still found filled with the mud.

_Ceraurus_ and _Calymene_ also must have occasionally swallowed mud in
quantity, otherwise the form of the alimentary canal could not have
been preserved as it is in a few of Doctor Walcott's specimens.


TRACKS AND TRAILS OF TRILOBITES.

Tracks and trails of various sorts have been ascribed by authors to
trilobites since these problematic markings first began to attract
attention, but as the appendages were until recently quite unknown,
all the earlier references were purely speculative. The subject is a
difficult one, and proof that any particular track or trail could have
been made in only one way is not easily obtained. Since the appendages
have actually been described, comparatively little has been done,
Walcott's work on _Protichnites_ (1912 B, p. 275) being the most
important. Since the first description of _Protichnites_ by Owen
(Quart. Jour. Geol. Soc., London, 1852, vol. 8, p. 247), it has been
thought that these trails were made by crustaceans, and the only known
contemporaneous crustaceans being trilobites, these animals were
naturally suggested. Dawson (Canadian Nat. Geol., vol. 7, 1862, p.
276) ascribed them, with reserve, to _Paradoxides_, and Billings
(1870, p. 484) suggested _Dikelocephalus_ or _Aglaspis_. Walcott
secured well preserved specimens which showed trifid tracks, and these
were readily explained when he found the legs of _Neolenus_, which
terminated with three large spines. Similar trifid terminations had
already been described by Beecher, and clearly pictured in his
restoration of _Triarthrus_, but the spines and the tracks had
somehow not previously been connected in the mind of any observer.
Walcott concluded that the tracks had been made by a species of
_Dikelocephalus_, possibly by _D. hartti_, which occurs both north
and south of the Adirondacks. In a recent paper, Burling (Amer. Jour.
Sci., ser. 4, vol. 44, 1917, p. 387) has argued that Protichnites was
not the trail of a trilobite, but of a "short, low-lying, more or less
heavy set, approximately 12-legged, crab-like animal," which had an
oval shape, toed in, and was either extremely flexible or else short
and more or less flexible in outline. This seems to describe a
trilobite.

_Climactichnites_, the most discussed single trail of all, has also
been ascribed to trilobites,--by Dana (Manual of Geology, 1863, p.
185), Billings (1870, p. 485), and Packard (Proc. Amer. Acad. Arts and
Sci., vol. 36, 1900, p. 64),--though less frequently than to other
animals. The latest opinion (see paper by Burling cited above) seems
to be against this theory.

Miller (1880, p. 217) described under the generic name
_Asaphoidichnus_ two kinds of tracks which were such as he supposed
might be made by an _Asaphus_ (_Isotelus_). In referring to the second
of the species, he says: "Some of the toe-tracks are more or less
fringed, which I attribute to the action of water, though Mr. Dyer is
impressed with the idea that it may indicate hairy or spinous feet."
The type of this species, _A. dyeri_, is in the Museum of Comparative
Zoology, and while it may be the trail of a trilobite, it would be
difficult to explain how it was produced.

Ringueberg (1886, p. 228) has described very briefly tracks found in
the upper part of the Medina at Lockport, New York. These consisted of
a regularly succeeding series of ten paired divergent indentations
arranged in two diverging rows, with the trail of the pygidium showing
between each series. The ten pairs of indentations he considered could
have been made by ten pairs of legs like those shown by the specimen
of Isotelus described by Mickleborough, and the intermittent
appearance of the impression of the pygidium suggested to him that the
trilobite proceeded by a series of leaps.

Walcott (1918, pp. 174-175, pl. 37-42) has recently figured a number
of interesting trails as those of trilobites, and has pointed out that
a large field remains open to anyone who has the patience to develop
this side of the subject.




PART III.




RELATIONSHIP OF THE TRILOBITES TO OTHER ARTHROPODA.


It can not be said that the new discoveries of appendagiferous
trilobites have added greatly to previous knowledge of the systematic
position of the group. Probably none will now deny that trilobites are
Crustacea, and more primitive and generalized than any other group in
that class. The chief interest at present lies in their relation to
the most nearly allied groups, and to the crustacean ancestor.

Trilobites have been most often compared with Branchiopoda, Isopoda,
and Merostomata, the present concensus of opinion inclining toward the
notostracan branchiopods (Apodidæ in particular) as the most closely
allied forms. It seems hardly worth while to burden these pages
with a history of opinion on this subject, since it was not until the
appendages were fully made out, from 1881 to 1895, that zoologists and
palæontologists were in a position to give an intelligent judgment.
The present status is due chiefly to Bernard (1894), Beecher (1897,
1900, et seq.), and Walcott (1912, et seq.).

The chief primitive characteristics of trilobites are: direct
development from a protaspis common to the subclass; variability in
the number of segments, position of the mouth, and type of eyes; and
serially similar biramous appendages.

The recent study has modified the last statement slightly, since it
appears that in some trilobites there was a modification of the
appendages about the mouth, suggesting the initiation of a set of
tagmata.

In comparing the trilobites with other Crustacea, the condition of the
appendages must be especially borne in mind, for while these organs
are those most intimately in contact with the environment, and most
subject to modification and change, yet they have proved of greatest
service in classification.

Appendages have been found on trilobites from only the Middle Cambrian
and Middle and Upper Ordovician, but as the Ordovician was the time of
maximum development of the group, it is probable that trilobites of
later ages would show degradational rather than progressive changes.
All the genera which are known show appendages of the same plan, and
although new discoveries will doubtless reveal many modifications of
that plan, general inferences may be drawn now with some assurance.

The chief characteristics of the appendages are: first, simple
antennules, a primitive feature in all Crustacea, as shown by
ontogeny; second, paired biramous appendages, similar to each other
all along the body, the youngest and simplest in front of the anal
segment, the oldest and most modified on the cephalon. The endobases
are retained on all the coxopodites, except possibly, in some species,
the anterior ones, and these gnathobases are modified in some genera
as mouth-parts, while in others they are similar throughout the
series. With these few fundamentals in mind, other Crustacea may be
examined for likenesses. The differences are obvious.




Crustacea.


BRANCHIOPODA.

The early idea that the trilobites were closely related to the
Branchiopoda was rejuvenated by the work of Bernard on the Apodidæ
(1892) and has since received the support of most writers on the
subject. Fundamentally, a great deal of the argument seems to be that
_Apus_ lies the nearest of any modern representative of the class to the
theoretical crustacean ancestor, and as the trilobites are the oldest
Crustacea, they must be closely related. Most writers state that the
trilobites could not be derived from the Branchiopoda (see, however,
Walcott 1912 A), nor the latter from any known trilobite, but both
subclasses are believed to be close to the parent stem.

Viewed from the dorsal side, there is very little similarity between
any of the branchiopods and the trilobites, and it is only in the
Notostraca, with their sessile eyes and depressed form, that any
comparison can be made. The chief way in which modern Branchiopoda and
Trilobita agree is that both have a variable number of segments in
the body, that number becoming very large in _Apus_ on the one hand and
_Mesonacis_ and _Pædeumias_ on the other. In neither are the appendages,
except those about the mouth, grouped in tagmata. Other likenesses
are: the Branchiopoda are the only Crustacea, other than Trilobita, in
which gnathobases are found on limbs far removed from the mouth; the
trunk limbs are essentially leaf-like in both, though the limb of the
branchiopod is not so primitive as that of the trilobite; caudal cerci
occur in both groups.

If the appendages be compared in a little more detail, the differences
prove more striking than the likenesses.

In the Branchiopoda, the antennules are either not segmented or only
obscurely so. In trilobites they are richly segmented.

In Branchiopoda, the antennæ are variable. In the Notostraca they are
vestigial, while in the males of the Anostraca they are powerful and
often complexly developed claspers. Either condition might develop
from the generalized biramous antennas of Trilobita, but the present
evidence indicates a tendency toward obsolescence. Claus' observations
indicate that the antennæ of the Anostraca are developments of the
exopodites, rather than of the endopodites.

The mandibles and maxillæ of the Branchiopoda are greatly reduced, and
grouped closely about the mouth. Only the coxopodites of the Trilobita
are modified as oral appendages.

The trunk limbs of _Apus_ are supposed to be the most primitive among
the Branchiopoda, and comparison will be made with them. Each
appendage consists of a flattened axial portion, from the inner margin
of which spring six endites, and from the outer, two large flat exites
(see fig. 34). This limb is not articulated with the ventral membrane,
but attached to it, and, if Lankester's interpretation of the origin
of schizopodal limbs be correct, then the limb of _Apus_ bears very
little relation to that of the Trilobita. In _Apus_ there is no
distinct coxopodite and the endobases which so greatly resemble the
similar organs in the Trilobita are not really homologous with them,
but are developments of the first endite. Beecher's comparison of the
posterior thoracic and pygidial limbs of _Triarthrus_ with those of
_Apus_ can not be sustained. Neither _Triarthrus_ nor any other
trilobite shows any trace of phyllopodan limbs. Beecher figured (1894
B, pl. 7, figs. 3, 4) a series of endopodites from the pygidium of a
young _Triarthrus_ beside a series of limbs from a larval _Apus_.
Superficially, they are strikingly alike, but while the endopodites of
_Triarthrus_ are segmented, the limbs of _Apus_ are not, and the parts
which appear to be similar are really not homologous. The similarity
of the thoracic limbs in the two groups is therefore a case of
parallelism and does not denote relationship.

Geologically, the Branchiopoda are as old as the Trilobita, and while
they did not have the development in the past that the trilobite
had, they were apparently differentiated fully as early. Anostraca,
Notostraca and Conchostraca, three of the four orders, are represented
in the Cambrian by forms which are, except in their appendages, as
highly organized as the existing species. Brief notes on the principal
Middle Cambrian Branchiopoda follow:


=Burgessia bella= Walcott.

     Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 177,
     pl. 27, figs. 1-3; pl. 30, figs. 3, 4.

This is the most strikingly like the modern Branchiopoda of any
species described by Walcott from the Middle Cambrian, and invites
comparison with _Apus_. The carapace is long, loosely attached to the
body, and extends over the greater part of the thorax. The eyes are
small, sessile, and close to the anterior margin.

The appendages of the head consist of two pairs of antennæ, and three
pairs of slender, jointed legs. Both pairs of antennæ are slender and
many-jointed, the antennules somewhat smaller than the antennæ. The
exact structure of the limbs about the mouth has not yet been made
out, but they are slender, tapering, endopodite-like legs, with at
least three or four segments in each, and probably more.

There are eight pairs of thoracic appendages, each limb having the
form of the endopodite of a trilobite and consisting of seven segments
and a terminal spine. The proximal three segments of each appendage
are larger than the outer ones, and have a flattened triangular
expansion on the inner side. Walcott also states that "One specimen
shows on seven pairs of legs, small, elongate, oval bodies attached
near the first joint to the outer side of the leg. These bodies left
but slight impression on the rock and are rarely seen. They appear to
represent the gills." They are not figured, but taken in connection
with the endopodite-like appearance of the segmented limbs, one would
expect them to be vestigial exopodites.

A small hypostoma is present on the ventral side, and several of the
specimens show wonderfully well the form of the alimentary canal and
the hepatic cæca. The main branches of the latter enter the mesenteron
just behind the fifth pair of cephalic appendages.

Behind the thorax the abdomen is long, limbless, and tapers to a
point. It is said to consist of at least thirty segments.

Compared with _Apus_, _Burgessia_ appears both more primitive and more
specialized. The carapace and limbless abdomen are _Apus_-like, but
there are very few appendagiferous segments, and the appendages are
not at all phyllopodan, but directly comparable with those of
trilobites, except, of course, for the uniramous character of the
cephalic limbs. A closer comparison may be made with _Marrella_.


=Waptia fieldensis= Walcott.

     Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 181,
     pl. 27, figs. 4, 5.

The carapace is short, covering the head and the anterior part of the
thorax. The latter consists of eight short segments with appendages,
while the six abdominal segments, which are similar to those of the
thorax, are without limbs except for the last, which bears a pair
of broad swimmerets. The eyes are marginal and pedunculate. The
antennules are imperfectly known, but apparently short, while the
antennas are long and slender, with relatively few, long, segments.
The mandibles appear to be like endopodites of trilobites and show
at least six segments. As so often happens in these specimens from
British Columbia, the preservation of the other appendages is
unsatisfactory. As illustrated (Walcott, 1912 A, pl. 27, fig. 5), both
endopodites and exopodites appear to be present, and the shaft of the
exopodite seems to be segmented as in _Triarthrus_.

Walcott considers _Waptia_ as a transitional form between the
Branchiopoda and the Malacostraca.


=Yohoia tenuis= Walcott.

     Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 172,
     pl. 29, figs. 7-13.

This species, though incompletely known, has several interesting
characteristics. The head shows, quite plainly in some specimens, the
five segments of which it is composed. The eyes are small, situated in
a niche between the first and second segments, and are described as
being pedunculate. The eight segments of the thorax all show short
triangular pleural extensions, somewhat like those of _Remopleurides_
or _Robergia_. The abdomen consists of four cylindrical segments, the
last with a pair of expanded caudal rami.

The antennules appear to be short, while the antennas are large, with
several segments, ending in three spines, and apparently adapted for
serving as claspers in the male. The third, fourth, and fifth pairs of
cephalic appendages are short, tapering, endopodite-like legs similar
to those of _Burgessia_.

The appendages of the thorax are not well preserved, and there seem to
be none on the abdomen.

This species is referred by Walcott to the Anostraca.


=Opabina regalis= Walcott.

     Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 167,
     pl. 27, fig. 6; pl. 28, fig. 1.

This most remarkably specialized anostracan is not well enough known
to allow comparison to be made with other contemporaneous Crustacea,
but it is worthy of mention.

There is no carapace, the eyes are pedunculated, thorax and abdomen
are not differentiated, and the telson is a broad, elongate, spatulate
plate. There seem to be sexual differences in the form of the anterior
cephalic and caudal appendages, but this is not fully established. The
most remarkable feature is the long, large, median cephalic appendage
which is so suggestive of the proboscis of the recent _Thamnocephalus
platyurus_ Packard. The appendages are not well enough preserved to
permit a determination as to whether they are schizopodal or
phyllopodan.

_Summary._

Walcott referred _Burgessia_ and _Waptia_ to new families under the
Notostraca, while _Yohoia_ and _Opabina_ were placed with the
Anostraca. Except for the development of the carapace, there is a
striking similarity between _Waptia_ and _Yohoia_, serving to connect
the two groups.

The Branchiopoda were very highly specialized as early as Middle
Cambrian time, the carapace of the Notostraca being fully developed
and the abdomen limbless. Some (_Burgessia_) had numerous segments,
but most had relatively few. The most striking point about them,
however, is that so far as is known none of them had phyllopodan
limbs. While the preservation is in most cases unsatisfactory, such
limbs as are preserved are trilobite-like, and in the case of
_Burgessia_ there can be no possible doubt of the structure. Another
interesting feature is the retention by _Yohoia_ of vestiges of
pleural lobes. The Middle Cambrian Branchiopoda are more closely
allied to the Trilobita than are the modern ones, but still the
subclass is not so closely related to that group as has been thought.
Modern _Apus_ is certainly much less like a trilobite than has been
supposed, and very far from being primitive. The Branchiopoda of the
Middle Cambrian could have been derived from the trilobites by the
loss of the pleural lobes, the development of the posterior margin of
the cephalon to form a carapace, and the loss of the appendages from
the abdominal segments. Modern branchiopods can be derived from those
of the Middle Cambrian by the modification of the appendages through
the reduction of the endopodite and exopodite and the growth of the
endites and exites from the proximal segments.

Carpenter (1903, p. 334), from his study of recent crustaceans, has
already come to the conclusion that the Branchiopoda are not the most
primitive subclass, and this opinion is strengthened by evidence
derived from the Trilobita and from the Branchiopoda of the Middle
Cambrian.


COPEPODA.

The non-parasitic Eucopepoda are in many ways much nearer to the
trilobites than any other Crustacea. These little animals lack the
carapace, and the body is short, with typically ten free segments and
a telson bearing caudal furcæ. The head is composed of five segments
(if the first thoracic segment is really the fused first and second),
is often flattened, and lacks compound eyes. Pleural lobes are well
developed, but instead of being flattened as in the trilobite, they
are turned down at the sides or even incurved. A labrum is present.

The antennules, antennæ, and mandibles are quite like those of
trilobites. The antennules are very long and made up of numerous
segments. The antennæ are biramous, the junction between the
coxopodite and basipodite is well marked, and the endopodite consists
of only two segments.

The mandibles are said to "retain more completely than in any other
Crustacea the form of biramous swimming limbs which they possess in
the nauplius." The coxopodites form jaws, while both the reduced
endopodite and exopodite are furnished with long setæ. The maxillulæ
are also biramous, but very different in form from those of the
trilobite, and the maxillæ are phyllopodan.

The first thoracic limb is uniramous and similar to the maxillæ, but
the five following pairs are biramous swimming legs with coxopodite,
basipodite, exopodite, and endopodite. Both the exopodite and
endopodite are shorter than in the trilobites, but bear setæ and
spines.

The last pair of thoracic limbs are usually modified in the male into
copulatory organs. In some females they are enlarged to form plates
for the protection of the eggs, in others they are unmodified. In
still others they are much reduced or disappear. The abdomen is
without appendages.

The development in Copepoda is direct, by the addition posteriorly to
the larval form (nauplius) of segments, and the appendages remain
nearly unmodified in the adult.

Altogether, the primitive Copepoda seem much more closely allied to
the Trilobita than any other modern Crustacea, but unfortunately no
fossil representative of the subclass has been found. This is not so
surprising when one considers the habits and the habitat of most of
the existing species. Many are parasitic, many pelagic in both fresh
and marine waters, and many of those living on the bottom belong to
the deep sea or fresh water. Most free-living forms are minute, and
all have thin tests.

The eyes of copepods are of interest, in that they suggest the paired
ocelli of the Harpedidæ and Trinucleidæ. In the Copepoda there are, in
the simplest and typical form of these organs, three ocelli, each
supplied with its own nerve from the brain. Two of these are dorsal
and look upward, while the third is ventral. In some forms the dorsal
ocelli are doubled, so that five in all are present (cf. some species
of Harpes with three ocelli on each mound). In some, the cuticle over
the dorsal eyes is thickened so as to form a lens, as appears to be
the case in the trilobites. These peculiar eyes may be a direct
inheritance from the Hypoparia.


ARCHICOPEPODA.

Professor Schuchert has called my attention to the exceedingly curious
little crustacean which Handlirsch (1914) has described from the
Triassic of the Vosges. Handlirsch erected a new species, genus,
family, and order for this animal, which he considered most closely
allied to the copepods, hence the ordinal name. _Euthycarcinus
kessleri_, the species in question, was found in a clayey lens in the
Voltzia sandstone (Upper Bunter). Associated with the new crustacean
were specimens of _Estheria_ only, but in the Voltzia sandstone itself
land plants, fresh and brackish water animals, and occasionally,
marine animals are found. The clayey lens seems to have been of fresh
or brackish water origin.

All of the specimens (three were found) are small, about 35 mm. long
without including the caudal rami, crushed flat, and not very well
preserved. The head is short, not so wide as the succeeding segments,
and apparently has large compound eyes at the posterior lateral
angles. The thorax consists of six segments which are broader than the
head or abdomen. The abdomen, which is not quite complete in any one
specimen, is interpreted by Handlirsch as having four segments in the
female and five in the male. Least satisfactory of all are traces of
what are interpreted by the describer as a pair of long stiff
unsegmented cerci or stylets on the last segment.

The ventral side of one head shield shows faint traces of several
appendages which must have presented great difficulty in their
interpretation. A pair of antennules appear to spring from near the
front of the lower surface, and the remainder of the organs are
grouped about the mouth, which is on the median line back of the
center. Handlirsch sees in these somewhat obscure appendages four
pairs of biramous limbs, antennæ, mandibles, maxillulæ, and maxillæ,
both branches of each consisting of short similar segments,
endopodites and exopodites being alike pediform.

Each segment of the thorax has a pair of appendages, and those on
the first two are clearly biramous. The endopodites are walking legs
made up of numerous short segments (twelve or thirteen according to
Handlirsch's drawing), while the exopodite is a long breathing and
rowing limb, evidently of great flexibility and curiously like the
antennules of the same animal. The individual segments are narrow at
the proximal end, expand greatly at the sides, and have a concave
distal profile. A limb reminds one of a stipe of _Diplograptus_.
Both branches are spiniferous.

No appendages are actually present on the abdomen, but each segment
has a pair of scars showing the points of attachment. From the small
size of these, it is inferred that the limbs were poorly developed.

This species is described in so much detail because, if it is a
primitive copepod, it has a very important bearing on the ancestry of
that group and is the only related form that has been found fossil.

The non-parasitic copepods have typically ten (eleven) free segments,
including the telson, and the four abdominal segments are much more
slender than the six in front of them. In this respect the agreement
is striking, and the presence of five pairs of appendages in the head
and six free segments in the thorax is a more primitive condition than
in modern forms where the first two thoracic segments are apparently
fused (Calman, 1909, p. 73).

The large compound eyes of this animal are of course not present in
the copepods, but as vestiges of eyes have been found in the young of
_Calanus_, it is possible that the ancestral forms had eyes.

The greatest difficulty is in finding a satisfactory explanation of
the appendages. The general condition is somewhat more primitive than
in the copepods, for all the appendages are biramous, while in the
modern forms the maxillipeds are uniramous and the sixth pair of
thoracic appendages are usually modified in the male as copulatory
organs. In the copepods the modification is in the direction of
reduction, both endopodites and exopodites usually possessing fewer
segments than the corresponding branches in the trilobites. The
endopodite of _Euthycarcinus_, on the contrary, possesses, if
Handlirsch's interpretation is correct, twice as many segments as the
endopodite of a trilobite. If the Copepoda are descended from the
trilobites, as everything tends to indicate, then _Euthycarcinus_ is
certainly not a connecting link. The only truly copepodan
characteristic of this genus is the agreement in number and
disposition of free segments. The division into three regions instead
of two, the compound eyes, and the structure of the appendages are all
foreign to that group.

With the Limulava fresh in mind, one is tempted to compare
_Euthycarcinus_ with that ancient type. The short head and large
marginal eyes recall _Sidneyia_, and the grouping of the appendages
about the mouth also suggests that genus and _Emeraldella_. In the
Limulava likewise there is a contraction of the posterior segments,
although it is behind the ninth instead of the sixth. There is no
likeness in detail between the appendages of the Limulava and those of
_Euthycarcinus_, but the composite claws of _Sidneyia_ show that in
this group there was a tendency toward the formation of extra
segments.

If this fossil had been found in the Cambrian instead of the Triassic,
it would probably have been referred to the Limulava, and is not
at all impossible that it is a descendant from that group. As a
connecting link between the Trilobita and Copepoda it is, however,
quite unsatisfactory.


OSTRACODA.

The bivalved shell of the Ostracoda gives to this group of animals an
external appearance very different from that of the trilobites, but
the few appendages, though highly modified, are directly comparable.
The development, although modified by the early appearance of the
bivalved shell within which the nauplius lies, is direct. Imperfect
compound eyes are present in one family.

The antennules are short and much modified by functioning as swimming,
creeping, or digging organs. They consist of eight or less segments.
The antennas are also locomotor organs, and in most orders are
biramous. The mandibles are biramous and usually with, but sometimes
without, a gnathobase. The maxillulæ are likewise biramous but much
modified.

The homology of the third post-oral limb is in question, some
considering it a maxilla and others a maxilliped. It has various forms
in different genera. It is always much modified, but exopodite and
endopodite are generally represented at least by rudiments. The fourth
post-oral limb is a lobed plate, usually not distinctly segmented, and
the fifth a uniramous pediform leg. The sixth, if present at all, is
vestigial.

Very little comparison can be made between the Ostracoda and
Trilobita, other than in the ground-plan of the limbs, but the
presence of biramous antennæ is a primitive characteristic.


CIRRIPEDIA.

Like the ostracod, the adult cirriped bears little external
resemblance to the trilobite. The form of the nauplius is somewhat
peculiar, but it has the typical three pairs of appendages, to which
are added in the later metanauplius stages the maxillæ and six pairs
of thoracic appendages. In the adult, the antennules, which serve for
attachment of the larva, usually persist in a functionless condition,
while the antennas disappear. The mandibles, maxillulæ, and maxillæ
are simple and much modified to form mouth parts, and the six pairs of
thoracic appendages are developed into long, multisegmented, biramous
appendages bearing numerous setæ which serve for catching prey. Paired
eyes are present in later metanauplius stages, but lost early in the
development. The relationship to the trilobite evidently is not close.


MALACOSTRACA.

_1. Phyllocarida._

The oldest malacostracans whose appendages are known are species of
_Hymenocaris_. One, described as long ago as 1866 by Salter, has what
seem to be a pair of antennæ and a pair of jaw-like mouth-parts.
Another more completely known species has recently been reported by
Walcott (1912 A, p. 183, pl. 31, figs. 1-6). This latter form is
described as having five pairs of cephalic appendages: a pair of
minute antennules beside the small pedunculated eyes, a pair of large
uniramous antennæ, slender mandibles and maxillulæ, and large maxillæ
composed of short stout segments. There are eight pairs of biramous
thoracic limbs, the exopodites setiferous, the endopodites composed of
short wide segments and ending in terminal claw-like spines. These
appendages are like those of trilobites.

_Hymcnocaris_ belongs to the great group of extinct ceratocarid
Crustacea which are admitted to the lowest of the malacostracan
orders, Phyllocarida, because of their resemblance to _Nebalia_,
_Paranebalia_, _Nebaliopsis_, and _Nebaliella_, the four genera which
are at present living. The general form of the recent and fossil
representatives of the order is strikingly similar. The chief outward
difference is that in many of the fossils the telson is accompanied by
two furcal rami, while in the modern genera it is simple. It now
becomes possible to make some comparison between the appendages of
_Hymcnocaris_ of the Middle Cambrian and the Nebaliidæ of modern seas.

In both there are five pairs of cephalic and eight of thoracic
appendages, while those of the abdomen of Hymenocaris are not known.

In both, the antennules are less developed than the antennæ. In the
Nebaliidæ the antennules show evidence of having been originally
double (they are obviously so in the embryo), while they are single in
_Hymcnocaris_. In both, the antennæ are simple. The remaining cephalic
organs are too little shown by the specimen from the Middle Cambrian
to allow detailed comparison. The mandibles, maxillulæ, and maxillæ of
_Nebalia_ are, however, of types which could be derived from the
trilobite.

In three of the genera of the Nebaliidæ, the eight pairs of thoracic
limbs are all similar to one another, though those of the genera
differ. All are biramous. The limbs of _Hymcnocaris_ can apparently be
most closely correlated with those of _Nebalia antarctica_, in which
the endopodite consists of short flattened segments, and the exopodite
is a long setiferous plate. Epipodites are present in both _Nebalia_
and _Hymcnocaris_.

So far as the appendages of _Hymenocaris_ are known, they agree very
well with those of the Nebaliidæ, and since they are of the trilobite
type, it may safely be stated that the Trilobita and Malacostraca are
closely related.

_2. Syncarida._

Walcott (1918, p. 170) has compared the limbs of _Neolenus_ with those
of the syncarid genera _Anaspides_ and _Koonunga_. These are primitive
Malacostraca without a carapace, but as they have a compressed test
and _Anaspides_ has stalked eyes, their gross anatomy does not suggest
the trilobite. The thoracic appendages are very trilobite-like, since
the endopodite has six segments (in _Anaspides_) and a multisegmented
setiferous exopodite. The coxopodites, except of the first thoracic
segment, do not, however, show endobases, and those which are
present are peculiar articulated ones. The cephalic appendages are
specialized, and the antennules double as in most of the Malacostraca.
External epipodites are very numerous on the anterior limbs.

This group extends back as far as the Pennsylvanian and had then
probably already become adapted to fresh-water life. It may be
significant that the Palæozoic syncarids appear to have lacked
epipodites. While differing very considerably from the Trilobita, the
Syncarida could have been derived from them.

_3. Isopoda._

Since the earliest times there has been a constant temptation to
compare the depressed shields of the trilobites with the similar ones
of isopods. Indeed, when _Scrolls_ with its Lichadian body was first
discovered about a hundred years ago, it was thought that living
trilobites had been found at last. The trilobate body, cephalic
shield, sessile eyes, abdominal shield, and pleural extensions make a
wonderful parallel. This similarity is, however, somewhat superficial.
The appendages are very definitely segregated in groups on the various
regions of the body, and while the pleopods are biramous, the thoracic
legs are without exopodites (except in very early stages of
development of one genus). The Isopoda arose just at the time of the
disappearance of the Trilobita, and there seems a possibility of a
direct derivation of the one group from the other. It should be
pointed out that while the differences of Isopoda from Trilobita are
important, they are all of a kind which could have been produced by
the development from a trilobite-like stock. For example:

Isopoda have a definite number of segments. There is less variation in
the number of segments among the later than the earlier trilobites.

Isopoda have no facial suture. In at least three genera of trilobites
the cheeks become fused to the cranidium and the sutures obliterated.

Isopoda have one or two segments of the thorax annexed to the head.
While this is not known to occur in trilobites, it is possible that it
did.

Most Isopoda have a fairly stiff ventral test. The ventral membrane of
trilobites would probably have become stiffened by impregnation of
lime if the habit of enrollment had been given up.

In Isopoda the antennæ are practically uniramous sensory organs. The
second cephalic appendages of trilobites are capable of such
development through reduction of the exopodite.

In the Isopoda the coxopodites are usually fused with the body,
remaining as free, movably articulated segments only in a part of the
thoracic legs of one suborder, the Asellota. Endobases are entirely
absent. This is of course entirely unlike the condition in Trilobita,
but a probable modification.

In Isopoda there is a distinct grouping of the appendages, with
specialization of function. The trilobites show a beginning of
tagmata, and such development would be expected if evolution were
progressive.

In both groups, development from the embryo is direct. Rudiments of
exopodites of thoracic legs have been seen in the young of one genus.

The oldest known isopod is _Oxyuropoda ligioides_ Carpenter and
Swain (Proc. Royal Irish Acad., vol. 27, sect. B, 1908, p. 63,
fig. 1), found in the Upper Devonian of County Kilkenny, Ireland. The
appendages are not known, but the test is in some ways like that of a
trilobite. The thorax, abdomen, and pygidium are especially like those
of certain trilobites, and there is no greater differentiation between
thorax and abdomen than there is between the regions before and behind
the fifteenth segment of a _Pædeumias_ or _Mesonacis_. The anal
segment is directly comparable to the pygidium of a _Ceraurus_, the
stiff unsegmented uropods being like the great lateral spines of that
genus.

The interpretation of the head offered by Carpenter and Swain is very
difficult to understand, as their description and figure do not seem
to agree. What they consider the first thoracic segment (fused with
the head) seems to me to be the posterior part of the cephalon and it
shows at the back a narrow transverse area which is at least analogous
to the nuchal segment of the trilobite. If this interpretation can be
sustained, _Oxyuropoda_ would be a very primitive isopod in which the
first thoracic segment (second of Carpenter and Swain) is still free.
According to the interpretation of the original authors, the species
is more specialized than recent Isopoda, as they claim that two
thoracic segments are fused in the head. The second interpretation was
perhaps made on the basis of the number of segments (nineteen) in a
recent isopod.


=Marrella splendens= WALCOTT.

     Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 192,
     pls. 25, 26.

Among the most wonderful of the specimens described by Doctor
Walcott is the "lace crab." While the systematic position was not
satisfactorily determined by the describer, it has been aptly compared
to a trilobite. The great nuchal and genal spines and the large
marginal sessile eyes, coupled with the almost total lack of thoracic
and abdominal test, give it a bizarre appearance which may obscure its
real relationships.

The cephalon appears to bear five pairs of appendages, antennules, and
antennæ, both tactile organs with numerous short segments, mandibles,
and first and second maxillæ. The last three pairs are elongate, very
spinose limbs, of peculiar appearance. They seem to have seven
segments, but are not well preserved. These organs are attached near
the posterior end of the labrum.

There are twenty-four pairs of biramous thoracic appendages, which
lack endobases. The endopodites are long and slender, with numerous
spines; the exopodites have narrow, thin shafts, with long, forward
pointed setæ. The anal segment consists of a single plate.

Further information about this fossil will be eagerly awaited. None of
the illustrations so far published shows biramous appendages on the
cephalon. This, coupled with the presence of tactile antennæ, makes
its reference to the Trilobita impossible, but the present
interpretation indicates that it was closely allied to them.

[Illustration: Fig. 32. _Marrella splendens_ Walcott. Restoration of
the ventral surface, based upon the photographs and descriptions
published by Walcott. Although all the limbs of the trunk appear to be
biramous, only endopodites are placed on one side and exopodites on
the other, for the sake of greater clearness in the illustration.
Drawn by Doctor Elvira Wood, under the supervision of the writer.
× about 6.]



_Restoration of Marrella._

(Text fig. 32.)

The accompanying restoration of the ventral surface of _Marrella_ is a
tentative one, based on Doctor Walcott's description and figures. The
outline is taken from his plate 26, figure 1; the appendages of the
head from plate 26, figures 1-3, 5, and plate 25, figures 2, 3; the
endopodites, shown on the left side only, from figures 3 and 6, plate
25. I have not studied actual specimens, and the original description
is very incomplete. The restoration is therefore subject to revision
as the species becomes better known.




Arachnida.


No attempt will be made to pass in review all of the subclasses of the
arachnids. Some of the Merostomata are so obviously trilobite-like
that it would seem that their relationship could easily be proved. The
task has not yet been satisfactorily accomplished, however, and new
information seems only to add to the difficulties.

So far as I know, the Araneæ have not previously been compared
directly with trilobites, although such treatment consists merely in
calling attention to their crustacean affinities, as has often been
done.

Carpenter's excellent summary (1903, p. 347) of the relationship of
the Arachnida to the trilobites may well be quoted at this point:

     The discussion in a former section of this essay on the
     relationship between the various orders of Arachnida led to the
     conclusion that the primitive arachnids were aquatic animals,
     breathing by means of appendicular gills. Naturally, therefore, we
     compare the arachnids with the Crustacea rather than with the
     Insecta. The immediate progenitors of the Arachnida appear to have
     possessed a head with four pairs of limbs, a thorax with three
     segments, and an abdomen with thirteen segments and' a telson, only
     six of which can be clearly shown by comparative morphology to have
     carried appendicular gills. But embryological evidence enables us
     to postulate with confidence still more remote ancestors in which
     the head carried well developed compound eyes and five pairs of
     appendages, while it may be supposed that all the abdominal
     segments, except the anal, bore limbs. In these very ancient
     arthropods, all the limbs, except the feelers, had ambulatory and
     branchial branches; and one important feature in the evolution of
     the Arachnida must have been the division of labour between the
     anterior and posterior limbs, the former becoming specialized for
     locomotion, the latter for breathing. Another was the loss of
     feelers and the degeneration of the compound eyes. Thus we are led
     to trace the Arachnida (including the Merostomata and Xiphosura)
     back to ancestors which can not be regarded as arachnids, but which
     were identical with the primitive trilobites, and near the
     ancestral stock of the whole crustacean class.


TRILOBITES NOT ARACHNIDA.

While no one having any real knowledge of the Trilobita has adopted
Lankester's scheme of the inclusion of the group as the primitive
grade in the Arachnida, reference to it may not be amiss. This theory
is best set forth in the Encyclopædia Britannica, Eleventh Edition,
under the article on Arachnida. It is there pointed out that the
primitive arachnid, like the primitive crustacean, should be an animal
without a fixed number of somites, and without definitely grouped
tagmata. As Lankester words it, they should be anomomeristic and
anomotagmatic. The trilobites are such animals, and he considers them
Arachnida and not Crustacea for the following reasons:

Firstly and chiefly, because they have only one pair (apart from the
eyes) of pre-oral appendages. "This fact renders their association
with the Crustacea impossible, if classification is to be the
expression of genetic affinity inferred from structural coincidence."

Secondly, the lateral eyes resemble no known eyes so closely as the
lateral eyes of _Limulus_.

Thirdly, the trilobation of the head and body, due to the expansion
and flattening of the sides or pleura, is like that of _Limulus_, but
"no crustacean exhibits this trilobite form."

Fourthly, there is a tendency to form a pygidial or telsonic shield,
"a fusion of the posterior somites of the body, which is precisely
identical in character with the metasomatic carapace of _Limulus_." No
crustacean shows metasomatic fusion of segments.

Fifthly, a large post-anal spine is developed "in some trilobites" (he
refers to a figure of _Dalmanites_).

Sixthly, there are frequently lateral spines on the pleura as in
_Limulus_. No crustacean has lateral pleural spines.

These points may be taken up in order.

1. If trilobites have one appendage-bearing segment in front of the
mouth, they are Arachnida; if two, Crustacea. This is based on the
idea that in the course of evolution of the Arthropoda, the mouth has
shifted backward from a terminal position, and that as a pair of
appendages is passed, they lose their function as mouth-parts and
eventually become simple tactile organs. Thus arise the cheliceræ of
most arachnids, and the two pairs of tactile antennæ of most
Crustacea. This theory is excellent, and the rule holds well for
modern forms, but as shown by the varying length of the hypostoma in
different trilobites, the position of the mouth had not become fixed
in that group. In some trilobites, like _Triarthrus_, the gnathobases
of the second pair of appendages still function, but in all, so far as
known, the mouth was back of the points of attachment of at least two
pairs of appendages, and in some at least, back of the points of
attachment of four pairs. As pointed out in the case of _Calymene_ and
_Ceraurus_, the trilobites show a tendency toward the degeneration of
the first and second pairs of biramous appendages, particularly of the
gnathobases. They are in just that stage of the backward movement of
the mouth when the function of the antennæ as mandibles has not yet
been lost. If the presence of functional gnathobases back of the
mouth, rather than the points of attachment in front of the mouth, is
to be the guide, then Triarthrus might be classed as an arachnid and
_Calymene_ and _Isotelus_ as crustaceans. In other words, the rule
breaks down in this primitive group.

2. Superficially, the eyes of some trilobites do look like those of
_Limulus_, but how close the similarity really was it is impossible to
say. The schizochroal eyes were certainly very different, and Watase
and Exner both found the structure of the eye of the trilobite unlike
that of _Limulus_.

3. The importance of the trilobate form of the trilobite is very much
overestimated. It and the pygidium are due solely to functional
requirements. The axial lobe contained practically all the vital
organs and the side lobes were mechanical in origin and secondarily
protective. That the crustacean is not trilobate is frequently
asserted by zoologists, yet every text-book contains a picture of a
segment of a lobster with its axial and pleural lobes. It is a
fundamental structure among the Crustacea, obscured because most of
them are compressed rather than depressed.

4. The pygidium of trilobites is compared with the metasomatic shield
of _Limulus_. No homology, if homology is intended, could be more
erroneous. The metasomatic shield of _Limulus_ is, as shown by
ontogeny and phylogeny, formed by the fusion of segments formerly
free, and includes the segments between the cephalic and anal shields,
or what would be known as the thorax of a trilobite. No trilobite
has a metasomatic shield. The pygidium of a trilobite, as shown by
ontogeny, is built up by growth in front of the anal region, and since
the segments were never free, it can not strictly be said to be
composed of fused segments. Some Crustacea do form a pygidial shield,
as in certain orders of the Isopoda.

5. The post-anal spine of Dalmanites and some other trilobites is
similar to that of _Limulus_, but this seems a point of no especial
significance. That a similar spine has not been developed in the
Crustacea is probably due to the fact that they do not have the broad
depressed shape which makes it so difficult for a _Limulus_ to right
itself when once turned on its back. Relatively few trilobites have
it, and it is probably correlated with some special adaptation.

6. There is nothing among the trilobites comparable to the movable
lateral spines of the metasoma of _Limulus_.

While, as classifications are made up, the Trilobita must be placed in
the Crustacea rather than the Arachnida, there is no reason why both
the modern Crustacea and the Arachnida should not be derived from the
trilobites.



MEROSTOMATA.

It has been a custom of long standing to compare the trilobite with
_Limulus_. Packard (1872) gave great vitality to the theory of
the close affinity of the two when he described the so-called
trilobite-stage in the development of _Limulus polyphemus_. His
influence on Walcott's ideas (1881) is obvious. Lankester has gone
still further, and associated the Trilobita with the Merostomata in
the Arachnida.

The absence of antennules at any stage in development allies _Limulus_
so closely with the Arachnida and separates it so far from the
Trilobita that in recent years there has been a tendency to give up
the attempt to prove a relationship between the merostomes and
trilobites, especially since Clarke and Ruedemann, in their extensive
study of the Eurypterida, found nothing to indicate the crustacean
nature of that group. A new point of view is, however, presented by
the curious _Sidneyia inexpectans_ and _Emeraldella brocki_ described
by Walcott from the Middle Cambrian.


=Sidneyia inexpectans= Walcott.

     Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1911, p. 21,
     pl. 2, fig. 1 (not figs. 2, 3); pls. 3-5; pl. 6, fig. 3; pl. 7,
     fig. 1.

The body of this animal is elongate, somewhat eurypterid-like, but
with a broad telson supplied with lateral swimmerets. The cephalon is
short, with lateral compound eyes. The trunk consists of eleven
segments, the anterior nine of which are conspicuously wider than the
two behind them, and the telson consists of a single elongate plate.

On the ventral side of the head there is a large hypostoma and five,
pairs of appendages. The first pair are multisegmented antennules. The
second pair have not been adequately described. The third are large,
complex claws, and the fourth and fifth suggest broad, stocky
endopodites. Broad gnathobases are attached to the coxopodites of the
third to fifth pairs of appendages and form very strong jaws.

The first nine segments of the thorax have one pair each of broad
filiform branchial appendages, suggestive of the exopodites of
trilobites, but no endopodites have been seen. The tenth and eleventh
segments seem to lack appendages entirely.


=Emeraldella brocki= Walcott.

     Illustrated: _Sidneyia inexpectans_ Walcott _partim_, Smithson.
     Misc. Coll., vol. 57, 1911, pl. 2, figs. 2, 3 (not fig. 1);--Ibid.,
     1912, p. 206, text fig. 10.

     _Emeraldella brocki_ Walcott, Ibid., 1912, p. 203, pl. 30, fig. 2;
     text fig. 8;--Ibid., vol. 67, 1918, p. 118 (correction).

_Emeraldella_ has much the same shape as _Sidneyia_ and the same
number of segments, but instead of a broad flat telson, it has a long
_Limulus_-like spine. The cephalon is about as wide as long, and eyes
have not yet been seen. The body consists of eleven segments and a
telson (Walcott says twelve and a telson but shows only eleven in the
figures). Nine of the segments, as in _Sidneyia_, are broad, the next
two narrow.

The ventral side of the cephalon has a long hypostoma, and five pairs
of appendages. The first pair are very long multi segmented antennules
and the next four pairs seem to be rather slender, spiniferous,
jointed endopodites. Whether or not gnathobases were present is not
shown by the figures, but owing to the long hypostoma the appendages
are grouped about the mouth. All the segments of the body, unless it
were the telson, seem to have borne appendages. On the anterior end,
they were clearly biramous (1912, p. 206, text fig. 10), and that they
were present along the body is shown by figure 2, plate 30, 1912.

The present state of knowledge of both these peculiar animals leaves
much to be desired. The indications are that the cephalic appendages
are not biramous, and that only one pair of antennæ, the first, are
developed as tactile organs. The thoracic appendages of _Emeraldella_
are biramous, and also possibly those of _Sidneyia_. In the latter,
the last two abdominal segments seem to have been without appendages,
while in _Emeraldella_ at least one branch of each appendage, and
possibly both, is retained.

These animals, which may be looked upon as the last survivors of an
order of pre-Cambrian arthropods, have the appearance of an
eurypterid, but their dominant characteristics are crustacean. The
features which suggest the Eurypterida are: elongate, obovate,
non-trilobate, tapering body; telson-like posterior segment; marginal,
compound, sessile eyes; claw-like third cephalic appendages; and, more
particularly, the general resemblance of the test to that of an
eurypterid like _Strabops_. In form, _Sidneyia_ agrees with the
theoretical prototype of the Eurypterida reconstructed by Clarke and
Ruedemann (Mem. 14, N. Y. State Mus., vol. 1, 1912, p. 124) in its
short wide head with marginal eyes, and its undifferentiated body.
There is, moreover, no differentiation of the postcephalic appendages.

The crustacean characteristics are seen in the presence of five,
instead of six, pairs of appendages on the head, the first of which
are multisegmented antennules, and in the biramous appendages on the
body of _Emeraldella_. It should be noted that these latter are
typically trilobitic, each consisting of an endopodite with six
segments and a setiferous exopodite.

Clarke and Ruedemann (1912, p. 406) have discussed _Sidneyia_ briefly,
and conclude:

     It seems to us probable that the Limulava [_Sidneyia_ and
     _Amiella_] as described are not eurypterids but constitute a
     primitive order, though exhibiting some remarkable adaptive
     features. This order possibly belongs to the Merostomata, but is
     distinctly allied to the crustaceans in such important characters
     as the structure of the legs and telson, and is therefore much
     generalized.

The specialization of _Sidneyia_ consists in the remarkable
development of a highly complex claw on each of the third cephalic
appendages, and in the compound tail-fin, built up of the last segment
and one or more pairs of swimmerets. These two characteristics seem to
preclude the possibility of deriving the eurypterids from _Sidneyia_
itself, but it seems entirely within reason that they may have been
derived from another slightly less specialized member of the same
order.

That _Sidneyia_ is descended from any known trilobite seems highly
improbable, but that it was descended from the same ancestral stock as
the trilobites is, I believe, indicated by the presence of five pairs
of appendages on the cephalon and trilobitic legs on the abdomen.

=Molaria= and =Habelia.=

Other so-called Merostomata found by Walcott in the Middle Cambrian
are the genera _Molaria_ and _Habelia_, both referred to the Cambrian
family Aglaspidæ. These genera seem to conform with _Aglaspis_ of the
Upper Cambrian in having a trilobite-like cephalon without facial
sutures, a trilobite-like thorax of a small but variable (7-12) number
of segments, and a _Limulus_-like telson. Neither of them has yet been
fully described or figured, but (Walcott 1912 A, p. 202) _Habelia_
appears to have five pairs of cephalic appendages, the first two pairs
of which are multisegmented antennæ. The thoracic appendages are
likewise none too well known, but they appear to have been biramous.
The endopodites are better preserved than the exopodites, but in at
least one specimen of _Molaria_ the exopodites are conspicuous.

If these genera are properly described and figured, their appendages
are typically crustacean, and fundamentally in agreement with those of
_Marrella_. The relation to the Trilobita is evidently close, the
principal differences being the absence of facial sutures and the
presence of true antennæ. I am therefore transferring the Aglaspidæ
from the Merostomata to a new subclass under the Crustacea.


ARANEÆ.

The spiders have the head and thorax fused, the abdomen unsegmented
except in the most primitive suborder, and so appear even less
trilobite-like than the insects. The appendages likewise are highly
specialized. The cephalothorax bears six pairs of appendages, the
first of which are the pre-oral cheliceræ, while behind the mouth are
the pedipalpi and four pairs of ambulatory legs. The posterior pairs
of walking legs belong to the thorax, but the anterior ones are to be
homologized with the maxillæ of Crustacea, so that the spiders are
like the trilobites in having functional walking legs on the head.

The chief likenesses are, however, seen in the very young. On the germ
band there appear a pair of buds in front of the rudiments of the
cheliceræ which later unite to form the rostrum of the adult. At the
time these buds appear, the cheliceræ are post-oral, but afterward
move forward so that both rostrum and cheliceræ are in front of the
mouth. The rostrum is therefore the product of the union of the
antennules, and the cheliceræ are to be homologized with the antennæ.
There seems to be some doubt about the homology of the pedipalps with
the mandibles, as at least one investigator claims to have found
rudiments of a segment between the one bearing the cheliceræ and that
with the pedipalps.

Jaworowski (Zool. Anzeiger, 1891, p. 173, fig. 4) has figured the
pedipalp from the germ band of _Trochosa singoriensis_, and called
attention to the fact that it consists of a coxopodite and two
segmented branches which may be interpreted as exopodite and
endopodite. He designated as exopodite the longer branch which
persists in the adult, but since the ambulatory legs of Crustacea are
endopodites, that would seem a more likely interpretation. As the
figure is drawn, the so-called endopodite would appear to spring from
the proximal segment of the "exopodite." If the two terms were
interchanged, the homology with the limb of the trilobite or other
crustacean would be quite perfect.

In the young, the abdomen is segmented and the anterior segments
develop limb-buds, the first pair of which become the lung books and
the last two pairs the spinnerets of the adult. There seems to be some
question about the number of segments. Montgomery (Jour. Morphology,
vol. 20, 1909, p. 337). reviewing the literature, finds that from
eight to twelve have been seen in front of the anal segment. The
number seem to vary with the species studied. This of course suggests
connection with the anomomeristic trilobites.

The oldest true spiders are found in the Pennsylvanian, and several
genera are now known. The head and thorax are fused completely, but
the abdomen is distinctly segmented. Some of the Anthracomarti
resemble the trilobites more closely than do the Araneæ, as they lack
the constriction between the cephalothorax and abdomen. The spiders of
the Pennsylvanian have this constriction less perfectly developed than
do modern Araneæ, and occupy an intermediate position in this respect.
In the Anthracomarti, the pedipalpi are simple, pediform, and all the
appendages have very much the appearance of the coxopodites and
endopodites of trilobites. Cheliceræ are not known, and pleural lobes
are well developed in this group. Anthracomarti have not yet been
found in strata older than the Pennsylvanian, but they seem to be to a
certain extent intermediate between true spiders and the marine
arachnid.




Insecta.


Handlirsch (in several papers, most of which are collected in "Die
Fossilen Insekten," 1908) has attempted to show that all the
Arthropoda can be derived from the Trilobita, and has advocated the
view that the Insecta sprang directly from that group, without the
intervention of other tracheate stock. At first sight, this
transformation seems almost an impossibility, and the view does not
seem to have gained any great headway among entomologists in the
fourteen years since it was first promulgated. If an adult trilobite
be compared with an adult modern insect, few likenesses will be seen,
but when the trilobite is stripped of its specializations and compared
with the germ-band of a primitive insect, the theory begins to seem
more possible.

Handlirsch really presented very little specific evidence in favor of
his theory. In fact, one gets the impression that he has insisted on
only two points. Firstly, that the most ancient known insects, the
Palæodictyoptera, were amphibious, and their larvæ, which lived in
water, were very like the adult. Secondly, that the wings of the
Palæodictyoptera probably worked vertically only, and the two main
wings were homologous with rudimentary wing-like outgrowths on each
segment of the body. These outgrowths have the appearance of, and
might have been derived from, the pleural lobes of trilobites.

He figured (1908, p. 1305, fig. 7) a reconstructed larva of a
palæodictyopterid as having biramous limbs on each segment, but so far
as I can find, this figure is purely schematic, for there seems to be
no illustration or description of any such larva in the body of his
work.

That the insects arose directly from aquatic animals is of course
possible, and Handlirsch's first argument has considerable force. It
may, however, be purely a chance that the oldest insects now known to
us happen to be an amphibious tribe. The Palæodictyoptera are not yet
known to antedate the Pennsylvanian, but there can be no doubt that,
insects existed long before that time, and the fact that their remains
have not been found is good evidence that the pre-Pennsylvanian
insects were not aquatic. Comstock, who has recently investigated the
matter, does not believe that the Palæodictyoptera were amphibious
(The Wings of Insects, Ithaca, N. Y., 1918, p. 91).

The second argument, that wings arose from the pleural lobes of
trilobites, is exceedingly weak. Where most fully set forth (1907, p.
157), he suggests that trilobites may occasionally have left the
water, climbed a steep bank or a plant, and then glided back into
their native element, taking advantage of the broad flat shape to make
a comfortable and gentle descent! This sport apparently became so
engaging that the animal tried experiments with flexible wing tips,
eventually got the whole of the pleural lobes in a flexible condition,
and selected those of the second and third thoracic segments for
preservation, while discarding the remainder. The pleural lobes of
trilobites are not only too firmly joined to the axial portion of the
test to be easily transformed into movable organs, but they are
structurally too unlike the veined wings of insects to make the
suggestion of this derivation even worthy of consideration.

Tothill (1916) has recently reinvestigated the possible connection
between insects, chilopods, and trilobites, and, from the early
appearance of the spiracles in the young, came to the conclusion that
the insects were derived from terrestrial animals. He suggested that
they may have come through the chilopods from the trilobites. The
hypothetical ancestor of the insects, as restored by Tothill from the
evidence of embryology and comparative anatomy, is an animal more
easily derived from the Chilopoda than from the Trilobita. Five pairs
of appendages are present on the head, and the trunk is made up of
fourteen similar segments, each with a pair of walking limbs and a
pair of spiracles.

Only the maxillæ and maxillulæ are represented as biramous. If the
ancestor of the Insecta was, as seems possible, tracheate, this fact
alone would rule out the trilobites. Among tracheates, the Chilopoda
are certainly more closely allied to the Insecta than are any other
wingless forms. If the ancestors of the insects were not actually
chilopods, they may have been chilopod-like, and there can be little
doubt that both groups trace to the same stock.

As to the ancestry of the Chilopoda, it is probable that they had the
same origin as the other Arthropoda. Tothill has pointed out that in
the embryo of some chilopods there are rudiments of two pairs of
antennæ and that the two pairs of maxillæ and the maxillipeds are
biramous. This would point rather to the Haplopoda than directly to
the trilobites as possible ancestors, and may explain why the former
vanish so suddenly from the geological record after their brief
appearance in the Middle Cambrian. They may have gone on to the land.

There seem to be no insuperable obstacles to prevent the derivation,
indirectly, of the insects from some trilobite with numerous free
segments, and small pygidium. The antennules and pleural lobes must be
lost, the antennas and trunk limbs modified by loss of exopodites.
Wings and tracheæ must be acquired.

Handlirsch places the date of origin of the Insecta rather late, just
at the end of the Devonian and during the "Carboniferous." By that
time most families of trilobites had died out, so that the
possibilities of origin of new stocks were much diminished. If the
haplopod-chilopod-insect line is a better approximation to the truth,
then the divergence began in the Cambrian.




Chilopoda.


The adult chilopod lacks the antennules, and all of the other
appendages, with the exception of the maxillulæ, are uniramous. The
walking legs are similar to the endopodites of trilobites, and usually
have six or seven segments. The appendages are therefore such as could
be derived by modification of those of trilobites by the almost
complete loss of the exopodites and shortening of the endopodites of
the head. The position of the post-oral appendages, the posterior ones
outside those closest the mouth, is perhaps foreshadowed in the
arrangement of those of Triarthrus.

The Chilopoda differ from the Hexapoda in developing the antennæ
instead of the antennules as tactile organs, but this can not be used
with any great effect as an argument that the latter did not arise
from the ancestors of the former, since it is entirely possible that
in early Palæozoic times the pre-Chilopoda possessed two pairs of
antennæ. The first pair are still recognizable in the embryo of
certain species.

The oldest chilopods are species described by Scudder (Mem. Boston
Soc. Nat. Hist., vol. 4, 1890, p. 417, pl. 38) from the Pennsylvania!!
at Mazon Creek, Grundy County, Illinois. Only one of these, _Latzelia
primordialis_ Scudder (pl. 38 fig. 3), is at all well preserved. This
little animal, less than an inch long, had a depressed body, with a
median carina, exceedingly long slender legs, and about nineteen
segments. The head is very nearly obliterated.




Diplopoda.


The diplopods, especially the polydesmids with their lateral
outgrowths, often have a general appearance somewhat like that of a
trilobite, but on closer examination few likenesses are seen. The most
striking single feature of the group, the possession by each segment
of two pairs of appendages, is not in any way foreshadowed in the
trilobites, none of which shows any tendency toward a fusion of pairs
of adjacent segments. The antennules are short, antennæ absent,
mandibles and maxillulæ much modified, the latter possibly biramous,
and the maxillæ absent. The trunk appendages are very similar to those
of chilopods, and could readily be derived from the endopodites of
trilobites.

The oldest diplopods are found in the Silurian (Ludlow) and Devonian
(Lower Old Red) of Scotland, and three species belonging to two genera
are known. The oldest is _Archidesmus loganensis_ Peach (1889, p. 123,
pl. 4, fig. 4), and the Devonian species are _Archidesmus macnicoli_
Peach and _Kampecaris forfarensis_ Page (Peach 1882, p. 182, pl. 2,
fig. 2, 2a, and p. 179, pl. 2, figs. 1-1g). All of these species show
lateral expansions like the recent Polydesmidæ, and these of course
suggest the pleural lobes of trilobites. All three of the species are
simpler than any modern diplopod, for there is only a single pair of
appendages on each segment. No _foramina repugnatoria_ were observed,
and the eyes of _Kampecaris forfarensis_ as described are singularly
like those of a phacopid.

Peach says: "The eye itself is made up of numerous facets which are
arranged in oblique rows, the posterior end of each row being inclined
downwards and outwards, the facets being so numerous and so close
together that the eye simulates a compound one." There is also a
protecting ridge which somewhat resembles a palpebral lobe (1882, pl.
7, fig. la). Peach comments on the strength of the test, and from his
description it appears that it must have been preserved in the same
manner as the test of trilobites. It was punctate, and granules and
spines were also present. The presence of the lateral outgrowths in
these ancient specimens would seem to indicate that they are primitive
features, and may have been inherited. While possibly not homologous
with the pleural extensions of trilobites, they may be vestiges of
these structures.

The limbs are made up of seven segments which are circular in section
and expand at the distal end. The distal one bears one or two minute
spines. They are most readily compared with the endopodites of
_Isotelus_. The resemblance is, in fact, rather close. The sternal
plates are wider and the limbs of opposite sides further apart than in
modern diplopods. Except for one pair of antennæ, no cephalic
appendages are preserved.

While these specimens do not serve to connect the Diplopoda with the
Trilobita, they do show that most of the specializations of the former
originated since Lower Devonian times, and lead one to suspect that
the derivation from marine ancestors took place very early, perhaps in
the Cambrian. If no very close connection with the trilobites is
indicated, there is also nothing to show that the diplopods could not
have been derived from that group.




Primitive Characteristics of Trilobites.


TRILOBITES THE MOST PRIMITIVE ARTHROPODS.

The Arthropoda, to make the simplest possible definition, are
invertebrate animals with segmented body and appendages. The most
primitive arthropod would appear to be one composed of exactly similar
segments bearing exactly similar appendages, the segments of the
appendages themselves all similar to one another. It is highly
improbable that this most primitive arthropod imaginable will ever be
found, but after a survey of the whole phylum, it appears that the
simpler trilobites approximate it most closely.

That the trilobites are primitive is evidenced by the facts that they
have been placed at the bottom of the Crustacea by all authors and
claimed as the ancestors of that group by some; that Lankester derived
the Arachnida from them; and that Handlirsch has considered them the
progenitors of the whole arthropodan phylum.

Specializations among the Arthropoda, even among the free-living
forms, are so numerous that it would be difficult to make a complete
list of them. In discussing the principal groups, I have tried to show
that the essential structures can be explained as inherited from the
Trilobita, changed in form by explainable modifications, and that new
structures, not' present in the Trilobita, are of such a nature that
they might be acquired independently in even unrelated groups.

The chief objections to the derivation of the remainder of the
Crustacea from the trilobites have been: first, that the trilobites
had broad pleural extensions; second, that they had a large pygidium;
and lastly, that they had only one pair of tactile antennæ.

It has now been pointed out that many modern Crustacea have pleural
extensions, but that they usually bend down at the sides of the body,
and also that in the trilobites and more especially in _Marrella_,
there was a tendency toward the degeneration of the pleural lobes. A
glance at the Mesonacidæ or Paradoxidæ should be convincing proof that
in some trilobites the pygidium is reduced to a very small plate.

In regard to the second antennæ standard text-books contain statements
which are actually surprising. A compilation shows that the antennæ
are entirely uniramous in but a very few suborders, chiefly among the
Malacostraca; that they are biramous with both exopodite and
endopodite well developed in most Copepoda, Ostracoda, and
Branchiopoda; and that the exopodite, although reduced in size, still
has a function in some suborders of the Malacostraca. The Crustacea
could not possibly be derived from an ancestor with two pairs of
uniramous antennæ.

Although I have defended the trilobites, perhaps with some warmth,
from the imputation that they were Arachnida, my argument does not
apply in the opposite direction, and I believe Lankester was right in
deriving the Arachnida from them. If the number of appendages in front
of the mouth is fundamental, then the trilobites were generalized,
primitive, and capable of giving rise to both' Crustacea and
Arachnida. As shown on a previous page (p. 119), the "connecting
links" so far found tend to disprove rather than to prove the thesis,
but the present finds should be looked upon as only the harbingers of
the greater ones which are sure to come.


LIMBS OF TRILOBITES PRIMITIVE.

The general presence, in an adult or larva, of some sort of biramous
limbs throughout the whole class Crustacea has led most zoologists to
expect such a limb in the most primitive crustaceans, and apparently
the appendage of the trilobite satisfies the expectation. It is well,
perhaps, as a test, to consider whether by modification this limb
could produce the various types of limbs seen in other members of the
class. In the first place, it is necessary to have clearly in mind the
peculiarities of the appendage to be discussed.

It should first of all be remembered that the limb is articulated with
the dorsal skeleton in a manner which is very peculiar for a
crustacean. The coxopodite swings on a sort of ball-and-socket joint,
and at the outer end both the exopodite and the basipodite articulate
with it. Since the exopodite articulates with the basipodite as well
as with the coxopodite, the two branches are closely connected with
one another and there is little individual freedom of movement. This
is, of course, a necessary consequence of their articulation with a
segment which is itself too freely movable to provide a solid base for
attachment of muscles. The relation of the appendifer, coxopodite, and
two rami is here shown diagrammatically (fig. 33), the exopodite
branching off from the proximal end of the basipodite at the junction
with the coxopodite.

In all trilobites the endopodite consists of six segments, and the
coxopodite of a single segment the inner end of which is prolonged as
an endobase. There does not seem to be any variation from this plan in
the subclass, although individual segments are variously modified. The
exopodites are more variable, but all consist of a flattened shaft
with setæ on one margin. No other organs such as accessory gills,
swimming plates, or brood pouches have yet been found attached to the
appendages, the evidence for the existence of the various epipodites
and exites described by Walcott being unsatisfactory (see p. 23).

[Illustration: Fig. 33.--Diagrammatic representation of an appendage
of the anterior end of the thorax of _Triarthrus becki_ Green, to show
relation of exopodite and endopodite to each other and to the
coxopodite. Much enlarged.]

In the Ostracoda the appendages are highly variable, but it is easily
seen that they are modifications of a limb which is fundamentally
biramous. In most species, both exopodite and endopodite suffer
reduction. The exopodite springs from the basipodite and that segment
is closely joined to the coxopodite, producing a protopodite. In some
cases the original segments of the endopodites fuse to form a stiff
rod. While highly diversified, these appendages are very
trilobite-like, and some Ostracoda even have biramous antennæ.

The non-parasitic Copepoda have limbs exceedingly like those of
trilobites. Many of them are biramous, the endopodites sometimes
retaining the primitive six segments. Coxopodite and basipodite are
generally united, and endopodite and exopodite variously modified.
Like some of the Ostracoda, the more primitive Copepoda have biramous
antennæ.

As would be expected, the appendages of the Cirripedia are much
modified, although those of the nauplius are typical. The thoracic
appendages of many are biramous, but both branches are multisegmented.

In the modern Malacostraca the ground plan of the appendages is
biramous, but in most orders they are much modified. In many, however,
the appendages of some part of the body are biramous, and in many the
endopodites show the typical six segments. From the coxopodites arise
epipodites, some of which assist in swimming, and some in respiration.
Because of the many instances in which such extra growths arise, and
because of the form of the appendages of the Branchiopoda, it has
been suggested that the primitive crustacean leg must have been more
complex than that of the trilobite. In looking over the Malacostraca,
however, one is struck by the fact that epipodites generally arise
where the exopodites have become aborted or are poorly developed, and
seem largely to replace them. The coxopodite and basipodite are
usually fused to form a protopodite, and a third segment is sometimes
present in the proximal part of the appendage.

In the Branchiopoda are found the most complex crustacean limbs, and
the ones most difficult to homologize with those of trilobites. In
recent years, Lankester's homologies of the parts of the limbs of
_Apus_ with those of the Malacostraca have been quite generally
accepted, and the appendages of the former considered primitive.
Now that it is known that the Branchiopoda of the Middle Cambrian
(_Burgessia_ _et at._) had simple trilobite-like appendages, it
becomes necessary to exactly reverse the opinion in this matter. The
same homologies stand, but the thoracic limbs of _Apus_ must be looked
upon as highly specialized instead of primitive.

[Illustration: Fig. 34.--One of the appendages of the anterior part of
the trunk of _Apus_, showing the endites (beneath) and exites (above).
The proximal endite forms a gnathobase which is not homologous with
the gnathobase (or endobase) of the trilobite. Copied from Lankester.
Much enlarged.]

Lankester (Jour. Micros. Sci., vol. 21, 1881) pointed out that the
axial part of the thoracic limb of _Apus_ (fig. 34) is homologous with
the protopodite in the higher Crustacea, that the two terminal endites
corresponded to the exopodite and endopodite, and that the other
endites and exites were outgrowths from the protopodite analogous
to the epipodites of Malacostraca. There seems to be no objection
to retaining this interpretation, but with the meaning that both
endopodite and exopodite are much reduced, and their functions
transferred to numerous outgrowths of the protopodite. One of the
endites grows inward to form an endobase, the whole limb showing an
attempt to return to the ancestral condition of the trilobite. The
limbs of some other branchiopods are not so easy to understand, but
students of the Crustacea seem to have worked out a fairly
satisfactory comparison between them and _Apus_.

The discovery that the ancestral Branchiopoda had simple biramous
appendages instead of the rather complex phyllopodan type is another
case in which the theory of "recapitulation" has proved to hold. It
had already been observed that in ontogeny the biramous limb preceded
the phyllopodan, but so strong has been the belief in the primitive
character of the Apodidæ that the obvious suggestion has been ignored.
Even in such highly specialized Malacostraca as the hermit crabs the
development of certain of the limbs illustrates the change from the
schizopodal to the phyllopodan type, and Thompson (Proc. Boston Soc.
Nat. Hist., vol. 31, 1903, pl. 5, fig. 12) has published an especially
good series of drawings showing the first maxilliped. In the first to
fourth zoeæ the limb is biramous but in the glaucothoe a pair of broad
processes grow out from the protopodite, while the exopodite and
particularly the endopodite become greatly reduced. In the adult the
endopodite is a mere vestige, while the flat outgrowths from the
protopodite have become very large and bear setæ.

_Summary._

The limbs of most Crustacea are readily explained as modifications of
a simple biramous type. These modifications usually take the form of
reduction by the loss or fusion of segments and quite generally either
the entire endopodite or exopodite is lacking. Modification by
addition frequently occurs in the growth of epipodites, "endites," and
"exites" from the coxopodite, basipodite, or both. A protopodite is
generally formed by the fusion of coxopodite and basipodite,
accompanied by a transference of the proximal end of the exopodite to
the distal end of the basipodite. A new segment, not known in the
trilobites (precoxal), is sometimes added at the inner end.

Among modern Crustacea, the anterior cephalic appendages and thoracic
appendages of the Copepoda and the thoracic appendages of certain
Malacostraca, Syncarida especially, are most nearly like those of the
trilobite. The exact homology, segment for segment, between the
walking legs of the trilobite and those of many of the Malacostraca,
even the Decapoda, is a striking instance of retention of primitive
characteristics in a specialized group, comparable to the retention of
primitive appendages in man.


NUMBER OF SEGMENTS IN THE TRUNK.

Various attempts have been made to show that despite the great
variability, trilobites do show a tendency toward a definite number of
segments in the body.

Emmrich (1839), noting that those trilobites which had a long thorax
usually had a short pygidium, and that the reverse also held true,
formulated the law that the number of segments in the trunk was
constant (20 + 1) Very numerous exceptions to this law were, however,
soon discovered, and while the condition of those with less than
twenty-one segments was easily explained, the increasing number of
those with more than twenty-one soon brought the idea into total
disrepute.

Quenstedt (1837) had considered the number of segments of at least
specific importance, and both he and Burmeister (1843) considered that
the number of segments in the thorax must be the same for all members
of a genus. As first shown by Barrande (1852. p. 191 et seq.), there
are very many genera in which there is considerable variation in the
number of thoracic segments, and a few examples can be cited in which
there is variation within a species, or at least in very closely
related species.

Carpenter (1903, p. 333) has tabulated the number of trunk segments of
such trilobites as were listed by Zittel in 1887 and finds a steady
increase throughout the Palæozoic. His table, which follows, is,
however, based upon very few genera.

  Period         No. of Genera    Average No. of
                                   body-segments
  ===============================================
  Cambrian            12             17.66
  Ordovician          23             18.58
  Silurian            16             19.34
  Devonian            10             20.70
  Carboniferous        2             20.75

Due chiefly to the efforts of Walcott, an increasingly large number of
Cambrian genera are now represented by entire specimens, and since
these most ancient genera are of greatest importance, a few comments
on them may be offered.

The total number of segments can be fairly accurately determined in at
least nineteen genera of trilobites from the Lower Cambrian. These
include eight genera of the Mesonacidæ (_Olenellus_ was excluded)
and _Eodiscus_, _Goniodiscus_, _Protypus_, _Bathynotus_, _Atops_,
_Olenopsis_, _Crepicephalus_, _Vanuxemella_, _Corynexochus_,
_Bathyuriscus_, and _Poliella_. The extremes of range in total
segments of the trunk is seen in _Eodiscus_ (9) and _Pædeumias_ (45+),
and these same genera show the extremes in the number of thoracic
segments, there being 3 in the one and 44+ in the other. _Pædeumias_
probably shows the greatest variation of any one genus of trilobites,
various species showing from 19 to 44+ thoracic segments. The average
for the nineteen genera is 13.9 segments in the thorax, 3.7 segments
in the pygidium, or a total average of 17.6 segments in the trunk.
_Crepicephalus_ with 12-14 segments in the thorax and 4-6 in the
pygidium, and _Protypus_, with 13 in the thorax and 4-6 in the
pygidium, are the only genera which approach the average. All of the
Mesonacidæ, except one, _Olenelloides_, have far more thoracic and
fewer pygidial segments than the average, while the reverse is true of
the Eodiscidæ, _Vanuxemella_, _Corynexochus_, _Bathyuriscus_, and
Poliella.

The eight genera of the Mesonacidæ, _Nevadia_, _Mesonacis_,
_Elliptocephala_, _Callavia_, _Holmia_, _Wanneria_, _Pædeumias_, and
_Olenelloides_, have an average of 20.25 segments in the thorax and
1.5 in the pygidium, a total of 21.75. If, however, the curious little
_Olenelloides_ be omitted, the average for the thorax rises to 22.14
and the total to 23.84. _Olenelloides_ is, in fact, very probably the
young of an _Olenellus_. Specimens are only 4.5 to 11 mm. long, and
occur in the same strata with _Olenellus_ (see Beecher 1897 A, p.
191).

Thirty-three genera from the Middle Cambrian afford data as to the
number of segments, the Agnostidæ being excluded. The extreme of
variation there is smaller than in the Lower Cambrian. The number of
thoracic segments varies from 2 in Pagetia to 25 in _Acrocephalites_,
and these same genera show the greatest range in total number of trunk
segments, 8 and 29 respectively.

The average of thoracic segments for the entire thirty-three genera is
10.5, of pygidial segments 5.9, a total average of 16.4. It will be
noted that the thorax shows on the average less and the pygidium more
segments than in the Lower Cambrian. If the Agnostidæ could be
included, this result would doubtless be still more striking. Of the
genera considered, _Asaphiscus_ with 7-11 thoracic and 5-8 pygidial
segments, _Blainia_ with 9 thoracic and 6-11 pygidial, _Zacanthoides_
with 9 thoracic and 5 pygidial, and _Anomocare_ with 11 thoracic
and 7-8 pygidial segments came nearest to the average. Only a few
departed widely from it. The genera tabulated were _Acrocephalites_,
_Alokistocare_, _Crepicephalus_, _Karlia_, _Hamburgia_,
_Corynexochus_, _Bathyuriscus_, Poliella, _Agraulos_,
_Dolichometopus_, _Ogygopsis_, _Orria_, _Asaphiscus_, _Neolenus_,
_Burlingia_, _Blainia_, _Blountia_, _Marjumia_, _Pagetia_, _Eodiscus_,
_Goniodiscus_, _Albertella_, _Oryctocara_, _Zacanthoides_,
_Anomocare_, _Anomocarella_, _Coosia_, _Conocoryphe_, _Ctenocephalus_,
_Paradoxides_, _Ptychoparia_, _Sao_, and _Ellipsocephalus_.

Enough genera of Upper Cambrian trilobites are not known from entire
specimens to furnish satisfactory data. Excluding from the list the
Proparia recently described by Walcott, the average total trunk
segments in ten genera is 18, but as most of the genera are Olenidæ or
olenid-like, not much weight can be attached to these figures.

For the Cambrian as a whole, the average for sixty-two genera is
between 17 and 18 trunk segments, which is surprisingly like the
result obtained by Carpenter from only twelve genera, and tends to
indicate that it must be somewhere near the real average. If the 5 or
6 segments of the head be added, it appears that the "average" number
of segments is very close to the malacostracan number 21. Genera with
16 to 18 trunk segments are Callavia, _Protypus_, _Bathynotus_,
_Crepicephalus_, _Bathyuriscus_, _Ogygopsis_, _Burlingia_, _Orria_,
_Asaphiscus_, _Blainia_, _Zacanthoides_, _Neolenus_, _Anomocare_,
_Conocoryphe_, _Saukia_, _Olenus_, and _Eurycare_.

The order Proparia originated in the Cambrian, and Walcott has
described four genera, one from the Middle, and three from the Upper.
The number of segments in these genera is of interest. _Burlingia_,
the oldest, has 14 segments in the thorax and 1 in the pygidium. Of
the three genera in the Upper Cambrian, _Norwoodia_ has 8-9 segments
in the thorax and 3-4 in the pygidium; _Millardia_ 23 in thorax and
3-4 in pygidium; and _Menomonia_ 42 in thorax and 3-4 in pygidium. It
is of considerable interest and importance to note that the very
elongate ones are not from the Middle but from the Upper Cambrian.

Forty genera of Ordovician trilobites known from entire specimens were
tabulated, and it was found that the range in the number of segments
in the thorax and pygidium was surprisingly large. _Agnostus_, which
was not included in the table, has the fewest, and _Eoharpes_, with
29, the most. While the range in number of segments in the thorax is 2
to 29, the range of the number in the pygidium, 2 to 26, is almost as
great. A species of _Dionide_ has 26 in the pygidium, while
_Remopleurides_ and _Glaphurus_ have evidence of only 2. The average
number of segments in the thorax for the forty genera was 10.15, in
the pygidium 8.81, and the average number for the trunk 19.

Genera with just 19 segments in the trunk appear to be rare in the
Ordovician, a species of _Ampyx_ being the only one I have happened to
notice. _Calymene_, _Tretaspis_, _Triarthrus_, _Asaphus_, _Ogygites_,
and _Goldius_ come with the range of 18 to 20. _Goldius_, with 10
segments in the thorax and (apparently) 8 in the pygidium, comes
nearest to the averages for these two parts of the trunk. _Goldius_,
_Amphilichas_, _Bumastus_, _Acidaspis_, _Actinopeltis_, and
_Sphærexochus_ are among the genera having 10 segments in the thorax,
and there are many genera which have only one or two segments more or
less than 10.

In most Ordovician genera, thirty-five out of the forty tabulated, the
number of segments in the thorax is fixed, and the variation is in any
case small. In four of the five genera where it was not fixed, there
was a variation of only one segment, and the greatest variation was in
_Pliomerops_, where the number is from 15 to 19. This of course
indicates that the number of segments in the thorax tends to become
fixed in Ordovician time. The variation in the number of segments in
the pygidium is, however, considerable. It is difficult in many cases
to tell how many segments are actually present in this shield, as it
is more or less smooth in a considerable number of genera. Extreme
cases of variation within a genus are found in _Encrinurus_, species
of which have from 7 to 22 segments in the pygidium, _Cybeloides_ with
10 to 20, and _Dionide_ with 10 to 26. As the number in the thorax
became settled, the number in the pygidium became more unstable, so
that not even in the Ordovician can the total number of segments in
the trunk be said to show any tendency to become fixed.

The genera used in this tabulation were: _Eoharpes_, _Cryptolithus_,
_Tretaspis_, _Trinucleus_, _Dionide_, _Raphiophorus_, _Ampyx_,
_Endymionia_, _Anisonotus_, _Triarthrus_, _Remopleurides_,
_Bathyurus_, _Bathyurellus_, _Ogygiocaris_, _Asaphus_, _Ogygites_,
_Isotelus_, _Goldius_, _Cyclopyge_, _Amphilichas_, _Odontopleura_,
_Acidaspis_, _Glaphurus_, _Encrinurus_, _Cybele_, _Cybeloides_,
_Ectenonotus_, _Calymene_, _Ceraurus_, _Pliomera_, _Pliomerops_,
_Pterygometopus_, _Chasmops_, _Eccoptochile_, _Actinopeltis_,
_Sphærexochus_, _Placoparia_, _Pilekia_, _Selenopeltis_, and
_Calocalymene_.

Only sixteen genera of Devonian trilobites were available for
tabulation, and it is not always possible to ascertain the exact
number of segments in the pygidium, although genera with smooth caudal
shields had nearly all disappeared. The number of segments in the
thorax had become pretty well fixed by the beginning of the Devonian,
_Cyphaspis_ with a range of from 10 to 17 furnishing the only notable
exception. The range for the sixteen genera is from 8 to 17, the
average 11, the number exhibited by the Phacopidæ which form so large
a part of the trilobites of the Devonian. The greater part of the
species have large pygidia, and while the range is from 3 to 23, the
average is 11.2. _Probolium_, with 11 in the thorax and 11-13 in the
pygidium, and _Phacops_, with 11 in the thorax and 9-12 in the
pygidium, approach very closely to the "average" trilobite, and
various species of other genera of the Phacopidæ have the same number
of segments as the norm. In every genus, however, the number of
segments in the pygidium is variable, the greatest variation being in
_Dalmanites_, with a range of from 9 to 23. The number of segments in
the pygidium was therefore not fixed and was on the average higher
than in earlier periods.

The genera used in the tabulation were: _Calymene_, _Dipleura_,
_Goldius_, _Proëtus_, _Cyphaspis_, _Acidaspis_, _Phacops_,
_Hausmania_, _Coronura_, _Odontochile_, _Pleuracanthus_, _Calmonia_,
_Pennaia_, _Dalmanites_, _Probolium_, and _Cordania_.

The trilobites of the late Palæozoic (Mississippian to Permian)
belong, with two possible exceptions, to the Pröetidæ, and only three
genera, _Proëtus_, _Phillipsia_, and _Griffithides_, appear to be
known from all the parts. I am, however, assuming that both
_Brachymetopus_ and _Anisopyge_ have 9 segments in the thorax, and so
have tabulated five genera. The range in the number of segments in the
pygidium is large, from 10 in some species of _Proëtus_ to 30 in
_Anisopyge_, and the average, 17.3, is high, as is the average for
total number in the trunk, 26.3. _Anisopyge_, a late Permian trilobite
described by Girty from Texas, is perhaps the last survivor of the
group. It seems to have had 39 segments in the trunk, making it, next
to the Cambrian _Pædeumias_ and _Menomonia_, the most numerously
segmented of all the trilobites.

The above data may be summarized in the following table:

  Period          No. of  Av. No. of   Av. No. of   Av. No.
                  genera  segments in  segments in  of trunk
                          thorax       pygidium     segments
  ==========================================================
  Lower Cambrian    19       13.9         3.7        17.6
  Middle Cambrian   33       10.5         5.9        16.4
  Entire Cambrian   62        ...         ...        17-19
  Ordovician        40       10.15        8.81       18.96
  Devonian          16       11          11.2        22.2
  Late Palæozoic     5        9          17.3        26.3


This table confirms that made up by Carpenter, and shows even more
strikingly the progressive increase in the average number of segments
in the trunk throughout the Palæozoic.

While the two trilobites with the greatest number of segments are
Cambrian, yet on the average, the last of the trilobites had the more
numerously segmented bodies. The multisegmented trilobites are:

  Period            Genus       Av. No. of   Av. No. of   Av. No.
                                segments in  segments in  of trunk
                                thorax       pygidium     segments
  ================================================================
  Lower Cambrian  _Pædeumias_       44+          1           45+
  Upper Cambrian  _Menomonia_       42           4           46
                  _Ectenonotus_     12          22           34
  Ordovician      _Encrinurus_      11          22           33
                  _Dionide_          6          26           32
  Silurian        _Harpes_          29           3           32
  Devonian        _Coronura_        11          23           34
                  _Dalmanites_      11          23           34
  Permian         _Anisopyge_        7+(9?)     30           39?


_Anisopyge_, the last of the trilobites, stands third on the list of
those having great numbers of segments, and in each period there are a
few which have considerably more than the average number. It may be of
some significance that of these nine genera only _Pædeumias_ and
_Anisopyge_ belong to the Opisthoparia, the great central group, and
that five are members of the Proparia, the latest and most specialized
order.


FORM OF THE SIMPLEST PROTASPIS.

It would naturally be expected that the young of the Cambrian
trilobites should be more primitive than the young of species from
later formations, and Beecher (1895 C) has shown that this is the
case. He had reference, however, chiefly to the eyes, free cheeks, and
spines, and by comparison of ontogeny and phylogeny, demonstrated the
greater simplicity of the protaspis which lacked these organs. It
remains to inquire which among the other characteristics are most
fundamental.

Among the trilobites of the Lower Cambrian, no very young have been
seen except of Mesonacidæ. Of these, the ontogeny of _Elliptocephala
asaphoides_ Emmons is best known, thanks to Ford, Walcott, and
Beecher, but, as the last-named has pointed out, the actual protaspis
or earliest shield has not yet been found. The youngest specimen is
the one roughly figured by Beecher (1895 C, p. 175, fig. 6). It lacks
the pygidium, but if completed by a line which is the counterpart of
the outline of the cephalon, it would have been 0.766 mm. long. The
pygidium would have been 0.183 mm. long, or 23 per cent of the whole
length. The axial lobe was narrow, of uniform width along the
cephalon, showed a neck-ring and four indistinct annulations, but did
not reach quite to the anterior end, there being a margin in front of
the glabella about 0.1 mm. wide. The greatest width of the cephalon
was 0.66 mm., and of the glabella 0.233 mm., or practically 35 per
cent of the total width. Other young _Elliptocephala_ up to a length
of 1 mm., and young _Pædeumias_, _Mesonacis_, and _Holmia_ (see Kiær,
Videnskaps, Skrifter, 1 Mat.-Naturv. Klasse, 1917, No. 10) show about
the same characteristics, but all these have large compound eyes on
the dorsal surface and specimens in still younger stages are expected.
It may be pointed out, however, that in these specimens the pygidium
is proportionately larger than in the adult. Walcott cites one adult
126 mm. long in which the pygidium is 6 mm. long, or between 4 and 5
per cent of the total length, while in the incomplete specimen
described above, it was apparently 23 per cent. In a specimen 1 mm.
long figured by Walcott, the pygidium is 0.15 mm. long, or 15 per cent
of the whole length.

The development of several species of trilobites from the Middle
Cambrian is known. Barrande (1852) described the protaspis of _Sao
hirsuta_, _Peronopsis integer_, _Phalacroma bibullatum_, _P. nudum_,
and _Condylopyge rex_. Broegger figured that of a _Liostracus_ (Geol.
For. Förhandl., 1875, pl. 25, figs. 1-3) and Lindstroem (1901, p. 21)
has reproduced the same. Matthew (Trans. Roy. Soc. Canada, vol. 5,
1888, pl. 4, pls. 1, 2) has described the protaspis of a _Liostracus_,
_Ptychoparia linnarssoni_ Broegger, and _Solenopleura robbi_ Hartt.
Beecher (1895 C, pl. 8) has figured the protaspis of _Ptychoparia
kingi_ Meek, and the writer that of a Paradoxides (Bull. Mus. Comp.
Zool., vol. 58, No. 4, 1914, pl. i).

_Sao_, _Liostracus_, _Ptychoparia_, and _Solenopleura_ all have the
same sort of protaspis. In all, the axial lobe reaches the anterior
margin and is somewhat expanded at that end; in all, the glabella
shows but slight trace of segmentation; and in all, the pygidium
occupies from one fifth to one fourth the total length. There is
considerable variation in the width of the axial lobe. It is narrowest
in _Ptychoparia_, where in the middle it is only 14 per cent of the
whole width, and widest in _Solenopleura_, where it is 28 per cent. In
_Ptychoparia_ the pygidium of the protaspis occupies from 18 to 22 per
cent of the whole length. In the adult it occupies 10 to 12 per cent.
In _Solenopleura_ it makes up about 26 per cent of the protaspis, and
in the adult about 8 per cent.

In the youngest stages of all these trilobites, the pygidium is
incompletely separated from the cephalon. The first sign of
segmentation is a transverse crack which begins to separate the
cephalon and pygidium, and by the time this has extended across the
full width the neck segment has become rather well defined. In this
stage the animal is prepared to swim by means of the pygidium, and
first becomes active. The coincident development of the free pygidium
and the neck-ring strongly suggests that the dorsal longitudinal
muscles are attached beneath the neck-fur row.

The single protaspis of _Paradoxides_ now known, while only 1 mm.
long, is not in the youngest stage of development. It is like the
protaspis of _Olenellus_ in having large eyes on the dorsal surface
and a narrow brim in front of the glabella. The glabella is narrower
than in the adult.

The initial test of no agnostid has probably as yet been seen, as
all the young now known show the cephalon and pygidium distinctly
separated. _Phalacroma bibullatum_ and _P. nudum_ are both practically
smooth and isopygous when 1.5 mm. long. _P. bibullatum_ shows no axial
lobe at this stage, but a wide glabella and median tubercle develop
later, and when the glabella first appears, it extends to the anterior
margin. In _Peronopsis integer_ and _Condylopyge rex_, the axial lobe
is outlined on each of the equal shields in specimens about 1 mm.
long, but is without furrows and reaches neither anterior nor
posterior margin.

From the foregoing brief description it appears that the pygidium of
the protaspis varies in different groups from as little as 15 per cent
of the total length in the Mesonacidæ to as much as 50 per cent in the
Agnostidæ; that the axial lobe varies from as little as 14 per cent of
the total width in one _Ptychoparia_ to as much as 50 per cent in
_Phalacroma nudum_; that the glabella reaches the anterior margin in
the Olenidæ, Solenopleuridæ, and _Phalacroma bibullatum_, while there
is a brim in front of it in the Olenellidæ, Paradoxidæ, and three of
the species of the Agnostidæ. The decision as to which of these
conditions are primitive may be settled quite satisfactorily by study
of the ontogeny of the various species.


ORIGIN OF THE PYGIDIUM.

Taking first the pygidium, it has already been pointed out that in
each case the pygidium of the adult is proportionally considerably
smaller than the pygidium of the protaspis. The stages in the growth
of the pygidium are better known in Sao hirsuta than in any other
trilobite, and a review of Barrande's description will be
advantageous.

Barrande recognized twenty stages in the development of this species,
but there was evidently a still simpler protaspis in his hands than
the smallest he figured, for he says, after describing the specimen in
the first stage: "We possess one specimen on which the head extends
from one border to the other of the disk, but as this individual is
unique we have not thought it sufficient to establish a separate
stage." This specimen is important as indicating a stage in which
there was not even a suggestion of division between cephalon and
pygidium.

In the first stage described by Barrande, the form is circular, the
length is about 0.66 mm., and the glabella is narrow with parallel
sides and no indications of lateral furrows. The neck segment is
indicated by a slight prominence on the axial lobe, and back of it a
constriction divides the axial lobe of the pygidium into two nodes,
but does not cross the pleural lobes. The position of the nuchal
segment permits a measurement of the part which is to form the
pygidium, and shows that that shield made up 30 per cent of the entire
length.

In the second stage, when the test is 0.75 mm. long, the cephalon and
pygidium become distinctly separated, and the latter shield shows
three annulations on the axial and two pairs of ribs on the pleural
lobes. It now occupies 33-1/3 per cent of the total length.

In the third stage, when the total length is about 1 mm., the pygidium
has continued to grow. It now shows five annulations on the axial
lobe, and is 46 per cent of the total length.

In the fourth stage, two segments of the axial lobe have been set free
from the front of the pygidium. The length is now 1.5 mm. and the
pygidium makes up 32 per cent of the whole. From this time the
pygidium continues to decrease in size in proportion to the total
length, as shown in the following table.

  Stage  Length in  Percentage   Segments in  Segments in
          mm.        of pygidium  thorax       pygidium
  ========================================================
     1       0.66        30            0            2
     2       0.75        33-1/3        0            3
     3       1.00        46            0            5
     4       1.50        32            2            5-6
     5       1.50        25            3            4
     6       1.75        23            4            4
     7       1.80        21            5            3
     8       2.00        17            6            3
     9       2.50        13            7            3
    10       3.00        12            8            3
    11       3.50        11            9            3-4
    12       4.00        11           10            3-4
    13       5.00        10           11            3
    14       5.50         9           12            2-4
    15       6.00         8           13            3-4
    16       6.50         8           14            3
    17       7.00         7           15            3
    18       7.50         7           16            3
    19       7.50         6           17            2
    20      10.25         6           17            2

This table shows the rapid increase in the length of the pygidium till
the time when the thorax began to be freed, the very rapid decrease
during the earlier part of its formation until six segments had been
set free, and then a more gradual decrease until the entire seventeen
segments had been acquired, after which time the relative length
remained constant. From an initial proportion of 30 per cent, it rose
to nearly one half the whole length, and then dwindled to a mere 6 per
cent, showing conclusively that the thorax grew at the expense of the
pygidium.

If this conclusion can be sustained by other trilobites, it indicates
that the large pygidium is a more primitive characteristic of a
protaspis than is a small one. I have already shown that the pygidium
is proportionately larger in the protaspis in the Mesonacidæ,
Solenopleuridæ, and Olenidæ, and a glance at Barrande's figures of
_"Hydrocephalus" carens_ and _"H." saturnoides_, both young of
_Paradoxides_ will show that the same process of development goes
on in that genus as in _Sao_. There is first an enlargement of the
pygidium to a maximum, a rise from 20 per cent to 33 per cent in
the case of _H. carens_ and then, with the introduction of thoracic
segments, a very rapid falling off. All of these are, however,
trilobites with small pygidia, and it has been a sort of axiom among
palæontologists that large pygidia were made up of a number of
coalesced segments. While not definitely so stated, it has generally
been taken to mean the joining together of segments once free. The
asaphid, for instance, has been thought of as descended from some
trilobite with rich segmentation, and a body-form like that of a
_Mesonacis_ or _Paradoxides_.

The appeal to the ontogeny does not give as full an answer to this
question as could be wished, for the complete life-history of no
trilobite with a large pygidium is yet known. While the answer is not
complete, enough can be gained from the study of the ontogeny of
_Dalmanites_ and _Cyclopyge_ to show that in these genera also the
thorax grows by the breaking down of the pygidium and that no segment
is ever added from the thorax to the pygidium. The case of _Dalmanites
socialis_ as described by Barrande (1852, p. 552, pl. 26) will be
taken up first, as the more complete. The youngest specimen of this
species yet found is 0.75 mm. long, the pygidium is distinctly
separated from the cephalon, and makes up 25 per cent of the length.
This is probably not the form of the shell as it leaves the egg. At
this stage there are two segments in the pygidium, but they increase
to four when the test is 1 mm. long. The cephalon has also increased
in length, however, so that the proportional length is the same. The
subjoined table, which is that compiled by Barrande with the
proportional length of the pygidium added, is not as complete as could
be desired, but affords a very interesting history of the growth of
the caudal shield. The maximum proportional length is reached before
the introduction of thoracic segments, and during the appearance of
the first five segments the size of the pygidium drops from 25 to 15
per cent. Several stages are missing at the critical time between
stages 8 and 9 when the pygidium had added three segments to itself
and has supplied only one to the thorax. This would appear to have
been a sort of resting or recuperative stage for the pygidium, for it
increased its own length to 20 per cent, but from this stage up to
stage 12 it continued to give up segments to the thorax and lose in
length itself. After stage 12, when the specimens were 8 mm. long, no
more thoracic segments were added, but new ones were introduced into
the pygidium, until it reached a size equal to one fifth the entire
length, as compared with one fourth in the protaspis.

  Stage  Length  Percentage   Segments in  Segments in
         in mm.  of pygidium  thorax       pygidium
  ====================================================
   1      0.75        25         0            2
   2      0.75        25         0            3
   3      1.00        25         0            4
   4      1.00        22         1            3
   5      1.25        20         2            3
   6      1.25        18         3            3
   7      1.60        15         4            3
   8      1.60        15         5            3
   9      3.00        20         6            6
  10      3.50        20         7            6
  11      8.00        18         9            7
  12      8.00        16        11            5
  13     12.00        16        11            7
  14     19.00        18        11            9
  15     95.00        20        11           11


Since the above was written, Troedsson (1918, p. 57) has described the
development of _Dalmanites eucentrus_, a species found in the
Brachiopod shales (Upper Ordovician) of southern Sweden. This species
follows a course similar to that of _D. socialis_, so that the full
series of stages need not be described. The pygidium is, however, of
especial interest, for there is a stage in which it shows two more
segments than in the adult. Troedsson figures a pygidium 1.28 mm. long
which has eight pairs of pleural ribs, while the adult has only six
pairs. The ends of all these ribs are free spines, and were the
development not known one would say that this was a case of incipient
fusion, while as a matter of fact, it is incipient freedom.

A further interest attaches to this case, because of the close
relationship between _D. eucentrus_ and _D. mucronatus_. The latter
species appears first in the _Staurocephalus_ beds which underlie the
Brachiopod shales, so that in its first appearance it is somewhat the
older. The pygidium of the adult _D. mucronatus_ is larger than that
of _D. eucentrus_, having eight pairs of pleural ribs, the same number
as in the young of the latter. In short, _D. eucentrus_ is probably
descended from _D. mucronatus_, and in its youth passes through a
stage in which it has a large pygidium like that species. Once more it
appears that the small pygidium is more specialized than the large
one.

The full ontogeny of _Cyclopyge_ is not known, but young specimens
show conclusively that segments are not transferred from the thorax to
the pygidium, but that the opposite occurs. As shown by Barrande
(1852) and corroborated by specimens in the Museum of Comparative
Zoology, the process is as follows: The third segment of the adult of
this species, that is, the fourth from the pygidium, bears a pair of
conspicuous cavities on the axial portion. In a young specimen, 7 mm.
long, the second segment bears these cavities, but as the thorax has
only four segments, this segment is also the second instead of the
fourth ahead of the pygidium. The pygidium itself, instead of being
entirely smooth, as in the adult state, is smooth on the posterior
half, but on the anterior portion has two well formed but still
connected segments, the anterior one being more perfect than the
other. These are evidently the two missing segments of the thorax, and
instead of being in the process of being incorporated in the pygidium,
they are in fact about to be cast off from it to become free thoracic
segments. In other words, the thorax grows through the degeneration
of the pygidium. That the thorax grows at actual expense to the
pygidium is shown by the proportions of this specimen. In an adult of
this species the pygidium, thorax, and cephalon are to each other as
9:11:13. In the young specimen they are as 10:6:12, the pygidium being
longer in proportion both to the thorax and to the cephalon than it
would be in the adult.

This conception of the breaking down of the pygidium to form the
thorax will be very helpful in explaining many things which have
hitherto seemed anomalous. For instance, it indicates that the
Agnostidæ, whose subequal shields in early stages have been a puzzle,
are really primitive forms whose pygidia do not degenerate; likewise
the Eodiscidæ, which, however, show within the family a tendency to
free some of the segments. The annelidan Mesonacidæ may not be so
primitive after all, and their specialized cephala may be more truly
indicative of their status than has previously been supposed.

The facts of ontogeny of trilobites with both small and large pygidia
do show that there is a reduction of the relative size of the caudal
shield during the growth-stages, and therefore that the large pygidium
in the protaspis is probably primitive. The same study also shows that
the large pygidium is made up of "coalesced segments" only to the
extent that they are potentially free, and not in the sense of fused
segments.


WIDTH OF THE AXIAL LOBE.

That the narrow type of axial lobe is more primitive than the wide one
has already been demonstrated by the ontogeny of various species, and
space need not be taken here to discuss the question. Most Cambrian
trilobites have narrow axial lobes even in the adult so that their
development does not bring this out very strikingly, though it can be
seen in Sao, Ptychoparia, etc., but in Ordovician trilobites such as
Triarthrus and especially Isotelus, it is a conspicuous feature.


PRESENCE OR ABSENCE OF A "BRIM."

That the extension of the glabella to the front of the cephalon is a
primitive feature is well shown by the development of Sao (Barrande,
1852, pl. 7), Ptychoparia (Beecher, 1895 C, pl. 8), and Paradoxides
(Raymond, Bull. Mus. Comp. Zool., vol. 57, 1914), although in the last
genus the protaspis has a very narrow brim, the larva during the
stages of introduction of new segments a fairly wide one, and most
adults a narrow one.

The brim of Sao seems to be formed partly by new growth and partly at
the expense of the frontal lobe, for that lobe is proportionately
shorter in the adult than in the protaspis. In _Cryptolithus_ and
probably in _Harpes_, _Harpides_, etc., the brim is quite obviously new
growth and has nothing to do with the vital organs. Its presence or
absence may not have any great significance, but when the glabella
extends to the frontal margin, it certainly suggests a more anterior
position of certain organs. In _Sao_, the only trilobite in which
anything is known of the position of the hypostoma in the young, the
posterior end is considerably further forward in a specimen a. 5 mm.
long than in one 4 mm. long, thus indicating a backward movement of
the mouth during growth, comparable to the backward movement of the
eyes.


SEGMENTATION OF THE GLABELLA.

The very smallest specimens of _Sao_ show a simple, unsegmented axial
lobe, and the same simplicity has been noted in the young of other
genera. Beecher considered this as due to imperfect preservation of
the exceedingly small shells, which practically always occur as moulds
or casts in soft shale. There is, however, a very general increase in
the strength of glabellar segmentation in the early part of the
ontogeny of all trilobites whose life history is known, and in some
genera, like the Agnostidæ, there is no question of the comparatively
late acquisition of glabellar furrows. Even in _Paradoxides_, the
furrows appear late in the ontogeny.

_Summary._

If absence of eyes on the dorsal surface be primitive, as Beecher
has shown, and if the large pygidium, narrow axial lobe, and long
unsegmented glabella be primitive, then the known protaspis of the
Mesonacidæ and Paradoxidæ is not primitive, that of the Olenidæ is
very primitive, and that of the Agnostidæ is primitive except that in
one group the axial lobe, when it appears, is rather wide, and in the
other a brim is present.

[Illustration: Fig. 35.--A specimen of _Weymouthia nobilis_ (Ford),
collected by Mr. Thomas H. Clark at North Weymouth, Mass. Note the
broad smooth shields of this Lower Cambrian eodiscid. × 6.]

Subsequent development from the simple unsegmented protaspis would
appear to show, first, an adaptation to swimming by the use of the
pygidium; next, the invagination of the appendifers as shown in the
segmentation of the axial lobe indicates the functioning of the
appendages as swimming legs; then with the introduction of thoracic
segments the assumption of a bottom-crawling habit is indicated. Some
trilobites were fully adapted for bottom life, and the pygidium became
reduced to a mere vestige in the production of a worm-like body. Other
trilobites retained their swimming habits, coupled with the crawling
mode of life, and kept or even increased (_Isotelus_) the large
pygidium.




The Simplest Trilobite.


In the discussion above I have placed great emphasis on the large size
of the primitive pygidium, because, although there is nothing new in
the idea, its significance seems to have been overlooked.

If the large pygidium is primitive, then multisegmentation in
trilobites can not be primitive but is the result of adaptation to a
crawling life. It is annelid-like, but is not in itself to be relied
upon as showing relationship to the Chætopoda. Simple trilobites with
few segments, like the Agnostidæ, Eodiscidæ etc., were, therefore,
properly placed by Beecher at the base of his classification, and
there is now less chance than ever that they can be called degenerate
animals.

From the phylogeny of certain groups, such as the Asaphidæ, it is
learned that the geologically older members of the family have more
strongly segmented anterior and posterior shields than the later ones.
That there has been a "smoothing out" is demonstrated by a study of
the ontogeny of the later forms. From such examples it has come to
be thought that all smooth trilobites are specialized and occupy a
terminal position in their genealogical line. This has caused some
wonder that smooth agnostids like _Phalacroma bibullatum_ and _P.
nudum_ should be found in strata so old as the Middle Cambrian, and
was a source of great perplexity to me in the case of _Weymouthia_
(Ottawa Nat., vol. 27, 1913) (fig. 35). This is a smooth member of the
Eodiscidæ, and, in fact, one of the simplest trilobites known, for
while it has three thoracic segments, it shows almost no trace of
dorsal furrows or segmentation on cephalon or pygidium, and, of
course, no eyes. Following the general rule, I took this to be a
smooth-out eodiscid, and was surprised that it should come from the
Lower Cambrian, where it is associated with _Elliptocephala_ at Troy,
New York, and with _Callavia_ at North Weymouth, Massachusetts, and
where it has lately been found by Kiær associated with _Holmia_ and
_Kjerulfia_ at Tømten, Norway. It now appears it is really in its
proper zone, and instead of being the most specialized, is the
simplest of the Eodiscidæ.

What appears to be a still simpler trilobite is the form described by
Walcott as Naraoia.


=Naraoia compacta= Walcott.

(Text fig. 36.)

     Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 175,
     pl. 28, figs. 3, 4.--Cleland, Geology, Physical and Historical, New
     York, 1916, p. 412, fig. 382 F (somewhat restored).

This very imperfectly known form is referred by Walcott to the
Notostraca on what appear to be wholly inadequate grounds, and while I
do not insist on my interpretation, I can not refrain from calling
attention to the fact that it _can_ be explained as the most primitive
of all trilobites. It consists of two subequal shields, the anterior
of which shows slight, and the posterior considerable evidence of
segmentation. It has no eyes, no glabella, and no thorax, and is
directly comparable to a very young _Phalacroma bibullatum_ (see
Barrande 1852, pl. 49, figs. a, b). Walcott states that there is
nothing to show how many segments there are in the cephalic shield,
but that on one specimen fourteen were faintly indicated on the
abdominal covering. The appendages are imperfectly unknown, as no
specimen showing the ventral side has yet been described. The possible
presence of antennas and three other appendages belonging to the
cephalic shield is mentioned, and there are tips of fourteen legs
projecting from beneath the side of one specimen. As figured, some of
the appendages have the form of exopodites, others of endopodites,
indicating that they were biramous.

_Naraoia_ is, so far as now known, possessed of no characteristics
which would prevent its reference to the Trilobita, while the
presence of a large abdominal as well as a cephalic shield would make
it difficult to place in even so highly variable a group as the
Branchiopoda. On the other hand, its only exceptional feature as a
trilobite is the lack of thorax, and all study of the ontogeny of the
group has led us to expect just that sort of a trilobite to be found
some day in the most ancient fossiliferous rocks. _Naraoia_ can, I
think, be best explained as a trilobite which grew to the adult state
without losing its protaspian form. It was found in the Middle
Cambrian of British Columbia.

Even if _Naraoia_ should eventually prove to possess characteristics
which preclude the possibility of its being a primitive trilobite, it
at least represents what I should expect a pre-Cambrian trilobite to
look like. What the ancestry of the nektonic primitive trilobite may
have been is not yet clear, but all the evidence from the morphology
of cephalon, pygidium, and appendages indicates that it was a
descendant of a swimming and not a crawling organism.

Since the above was written, the Museum of Comparative Zoology has
purchased a specimen of this species obtained from the original
locality. The shields are subequal, the posterior one slightly the
larger, and the axial lobes are definitely outlined on both. The
glabella is about one third the total width, nearly parallel-sided,
somewhat pointed at the front. There are no traces of glabellar
furrows. The axial lobe of the pygidium is also about one third the
total width, extends nearly to the posterior margin, and has a rounded
posterior end. The measurements are as follows: Length, 33 mm.; length
of cephalon, 16 mm., width, 15 mm.; length of glabella, 11.5 mm.,
width, 5.5 mm.; length of pygidium, 17 mm., width, 15 mm.; length of
axial lobe, 14 mm., width, 5.5 mm.

The species is decidedly _Agnostus_-like in both cephalon and
pygidium, and were it not so large, might be taken for the young of
such a trilobite. The pointed glabella is comparable to the axial
lobes of the so-called pygidia of the young of _Condylopyge rex_ and
_Peronopsis integer_ (Barrande, Syst. Sil., vol. 1, pl. 49).




The Ancestor of the Trilobites, and the Descent of the Arthropoda.


The "annelid" theory of the origin of the Crustacea and therefore of
the trilobites, originating with Hatschek (1877) and so ably
championed by Bernard (1892), has now been a fundamental working
hypothesis for some years, and has had a profound influence in
shaping thought about trilobites. This hypothesis has, however,
its weak points, the principal one being its total inhibition of
the workings of that great talisman of the palæontologist, the law of
recapitulation. Its acceptance has forced the zoologist to look upon
the nauplius as a specially adapted larva, and has caused more than
one forced explanation of the protaspis of the trilobite. When so keen
a student as Calman says that the nauplius must point in some way to
the ancestor of the Crustacea (1909, p. 26), it is time to reëxamine
some of the fundamentals. This has been done in the preceding pages
and evidence adduced to show that the primitive features of a
trilobite indicate a swimming animal, and that the adaptations are
those which enabled it to assume a crawling mode of existence. It has
also been pointed out that in Naraoia there is preserved down to
Middle Cambrian times an animal like that to which ontogeny points as
a possible ancestor of the trilobites. _Naraoia_ is not the simplest
conceivable animal of its own type, however, for it has built up a
pygidium of fourteen or fifteen somites. One would expect to find in
Proterozoic sediments remains of similar animals with pygidia composed
of only one or two somites, with five pairs of appendages on the
cephalon, one or two pairs on the pygidium, a ventral mouth, and a
short hypostoma. Anything simpler than this could not, in my opinion,
be classed as a trilobite.

What the ancestor of this animal was is mere surmise. It probably had
no test, and it may be noted in this connection that _Naraoia_ had a
very thin shell, as shown by its state of preservation, and was in
that respect intermediate between the trilobite and the theoretical
ancestor. Every analysis of the cephalon of the trilobite shows that
it is made up of several segments, certainly five, probably six,
possibly seven. Every study of the trilobite, whether of adult, young,
or protaspis, indicates the primitiveness of the lateral extensions or
pleural lobes. The same studies indicate as clearly the location of
the vital organs along the median lobe. These suggestions all point to
a soft-bodied, depressed animal composed of few segments, probably
with simple marginal eyes, a mouth beneath the anterior margin,
tactile organs at one or both ends, with an oval shape, and a straight
narrow gut running from anterior mouth to terminal anus. The broad
flat shape gives great buoyancy and is frequently developed in the
plankton. Inherited by the trilobites, it proved of great use to the
swimmers among them.

The known animal which most nearly approaches the form which I should
expect the remote ancestor of the trilobites to have had is _Amiskwia
sagittiformis_ Walcott (Smithson. Misc. Coll., vol. 57, 1911, p. 112,
pl. 22, figs. 3, 4). This "worm" from the Middle Cambrian is similar
in outline to the recent _Spadella_, and is referred by Walcott to the
Chætognatha. It has a pair of lateral expansions and a flattened
caudal fin, a narrow median alimentary canal, and a pair of rather
long simple tentacles. With the exception of a thin septum back of the
head, no traces of segmentation are shown.

Some time in the late pre-Cambrian, the pre-trilobite, which probably
swam by rhythmic undulations of the body, began to come into
occasional contact with a substratum, and two things happened:
symmetrically placed, i. e., paired, appendages began to develop on
the contact surface, and a test on the dorsal side. The first use of
the appendages may have been in pushing food forward to the mouth,
and for the greater convenience in catching such material, a fold
in front of the mouth may have elongated to form the prototype of the
hypostoma. At this time the substratum may not have been the ocean
bottom at all, but the animals, still free swimmers, may have alighted
at feeding time on floating algæ from the surface of which they
collected their food. While the dorsal test was originally jointed at
every segment, the undulatory mode of swimming seems to have given way
to the method of sculling by means of the posterior end only, or by
the use of the appendages, and the anterior segments early became
fused together.

The result of the hardening of the dorsal test was of course to reduce
to that extent the area available for respiration, and this function
was now transferred in part to the limbs, which bifurcated, one branch
continuing the food-gathering process and the other becoming a gill.
The next step may have been the "discovery" of the ocean bottom and
the tapping of an hitherto unexploited supply of food. Upon this,
there set in those adaptations to a crawling mode of existence which
are so well shown in the trilobite. The crawling legs became
lengthened and took on a hardened test, the hypostoma was greatly
elongated, pushing the mouth backward, and new segments were added to
produce a long worm-like form which could adapt itself to the
inequalities of the bottom. That the test of the appendages became
hardened later than that of the body is shown by the specimens of
Neolenus, in which the dorsal shell as preserved in the shale is thick
and solid, while the test of the appendages is a mere film.

The late Proterozoic or very earliest Cambrian was probably the time
of the great splitting up into groups. The first development seems to
have been among the trilobites themselves, the Hypoparia giving rise
to two groups with compound eyes, first the Opisthoparia and later the
Proparia. About this same time the Copepoda may have split off from
the Hypoparia, continuing in the pelagic habitat. At first, most of
the trilobites seem to have led a crawling existence, but about Middle
Cambrian time they began to go back partially to the ancestral
swimming habits, and retained some of the trunk segments to form a
larger pygidium. The functional importance of the pygidium explains
why it can not be used successfully in making major divisions in
classification. Nearly related trilobites may be adapted to diverse
methods of life.


EVOLUTION WITHIN THE CRUSTACEA.

The question naturally arises as to whether the higher Crustacea were
derived from some one trilobite, or whether the different groups have
been developed independently from different stocks. The opinion that
all other crustaceans could have been derived from an _Apus_-like form
has been rather generally held in recent years, but Carpenter (1903,
p. 334) has shown that the leptostracan, _Nebalia_, is really a more
primitive animal than _Apus_. He has pointed out that in Leptostraca
the thorax bears eight pairs of simple limbs with lamelliform
exopodites and segmented endopodites, while the abdomen of eight
segments has six pairs of pleopods and a pair of furcal processes,
so that only one segment is limbless. Contrasted with this are the
crowded and complicated limbs of the anterior part of the trunk of
_Apus_, and the appendage-less condition of the hinder portion.
Further, a comparison between the appendages of the head of _Nebalia_
and those of _Apus_ shows that the former are the more primitive. The
antennules of Nebalia are elongate, those of _Apus_ greatly reduced;
the mandible of _Nebalia_ has a long endopodite, and Carpenter points
out that from it either the malacostracan mandible with a reduced
endopodite or the branchiopodan mandible with none could be derived,
but that the former could not have arisen from the latter. The maxillæ
of _Apus_ are also much the more specialized and reduced.

_Nebalia_ being in all else more primitive than _Apus_, it follows
that the numerous abdominal segments of the latter may well have
arisen by the multiplication of an originally moderate number, and the
last trace of primitiveness disappears.

It is now possible to add to the results obtained from comparative
morphology the testimony of palæontology, already outlined above, and
since the two are in agreement, it must be admitted that the modern
Branchiopoda are really highly specialized.

As has already been pointed out, _Hymenocaris_, the leptostracan of
the Middle Cambrian, has very much the same sort of appendages as the
Branchiopoda of the same age, both being of the trilobite type. Which
is the more primitive, and was one derived from the other?

The Branchiopoda were much more abundant and much more highly
diversified in Cambrian times than were the Leptostraca, and,
therefore, are probably older. Some of the Cambrian branchiopods were
without a carapace, and some were sessile-eyed. These were more
trilobite-like than Hymenocaris. Many of the Cambrian branchiopods had
developed a bivalved carapace, though not so large a one as that of
the primitive Leptostraca. The present indications are, therefore,
that the Branchiopoda are really older than the Leptostraca, and also
that the latter were derived from them. It seems very generally agreed
that the Malacostraca are descended from the Leptostraca, and the
fossils of the Pennsylvanian supply a number of links in the chain of
descent. Thus, _Pygocephalus cooperi_, with its brood pouches, is
believed by Calman (1909, p. 181) to stand at the base of the
Peracaridan series of orders, and _Uronectes_, _Palæocaris_, and
the like are Palæozoic representatives of the Syncarida. Others
of the Pennsylvanian species appear to tend in the direction of
the Stomatopoda, whose true representatives have been found in the
Jurassic. The Isopoda seem to be the only group of Malacostraca not
readily connected up with the Leptostraca. Their depressed form, their
sessile-eyes, and their antiquity all combine to indicate a separate
origin for the group, and it has already been pointed out how readily
they can be derived directly from the trilobite.

While the Copepoda seem to have been derived directly from the
Hypoparia, the remainder of the Crustacea apparently branched off
after the compound eyes became fully developed, unless, as seems
entirely possible, compound eyes have been developed independently in
various groups. Most Crustacea were derived from crawling trilobites
(Lower Cambrian or pre-Cambrian Opisthoparia), for they lost the large
pygidium, and also the major part of the pleural lobes. In all
Crustacea, too, other than the Copepoda and Ostracoda, there is a
tendency to lose the exopodites of the antennæ.

These modifications, which produced a considerable difference in the
general appearance of the animal, are easily understood. As has been
shown in previous pages, the trilobites themselves exhibit the
degenerative effect on the anterior appendages of the backward
movement of the mouth, and the transformation of a biramous appendage
with an endobase into a uniramous antenna is a simple result of such
a process. The feeding habits of the trilobites were peculiar and
specialized, and it is natural that some members of the group should
have broken away from them. In any progressive mode of browsing
the hypostoma was a hindrance, so was soon gotten rid of, and the
endobases not grouped around the mouth likewise became functionless.
The chief factor in the development of the higher Crustacea seems to
have been the pinching claw, by means of which food could be conveyed
to the mouth. It had the same place in crustacean development that the
opposable thumb is believed to have had in that of man.

An intermediate stage between the Trilobita and the higher Crustacea
is at last exhibited to us by the wonderful, but unfortunately rather
specialized _Marrella_, already described. It retains the hypostoma
and the undifferentiated biramous appendages of the trilobite, but has
uniramous antennæ, there are no endobases on the coxopodites of the
thoracic appendages, the pygidium is reduced to a single segment, and
the lateral lobes of the thorax are also much reduced. _Marrella_ is
far from being the simplest of its group, but is the only example
which survived even down to Middle Cambrian times of what was probably
once an important series of species transitional between the
trilobites and the higher Crustacea.

In this theory of the origin of the Crustacea from the Trilobita, the
nauplius becomes explicable and points very definitely to the
ancestor. According to Calman (1909, p. 23):

     The typical nauplius has an oval unsegmented body and three pairs
     of limbs, corresponding to the antennules, antennas, and mandibles
     of the adult. The antennules are uniramous, the others biramous,
     and all three pairs are used in swimming. The antennæ may have a
     spiniform or hooked masticatory process at the base, and share with
     the mandibles which have a similar process, the function of seizing
     and masticating the food. The mouth is overhung by a large labrum
     or upper lip and the integument of the dorsal surface of the body
     forms a more or less definite dorsal shield. The paired eyes are as
     yet wanting, but the median eye is large and conspicuous.

The large labrum or hypostoma, the biramous character of the
appendages, especially of the antennæ, the functional gnathobases on
the second and third appendages, and the oval unsegmented shield are
all characteristics of the trilobites, and it is interesting to note
that all nauplii have the free-swimming habit.

The effect of inheritance and modification through millions of
generations is also shown in the nauplius, but rather less than would
be expected. The most important modification is the temporary
suppression of the posterior pairs of appendages of the head, so that
they are generally developed later than the thoracic limbs. The median
or nauplius eye has not yet been found in trilobites, and if it is, as
it appears to be, a specialized eye, it has probably arisen since the
later Crustacea passed the trilobite stage in their phylogeny.

The oldest Crustacea, other than trilobites, so far known are the
Branchiopoda and Phyllocarida described by Walcott and discussed
above. It is important to note that while the former have already
achieved such modified characteristics that they have been referred to
modern orders, they retain the trilobite-like limbs and some of them
still have well developed pleural lobes.

Calman (1909, p. 101) says of the Copepoda:

     On the hypothesis that the nauplius represents the ancestral type
     of the Crustacea, the Eucopepoda would be regarded as the most
     primitive existing members of the class, retaining as they do,
     naupliar characters in the form of the first three pairs of
     appendages and in the absence of paired eyes and of a shell-fold.
     As already indicated, however, it is much more probable that they
     are to be regarded as a specialized and in some respects degenerate
     group which, while retaining, in some cases, a very primitive
     structure of the cephalic appendages, has diverged from the
     ancestral stock in the reduction of the number of somites, the loss
     of the paired eyes and the shell-fold, and the simplified form of
     the trunk-limbs.

If the Eucopepoda be viewed in the light of the theory of descent here
suggested, it is at once seen that while they are modified and
specialized, they more nearly approximate the hypothetical ancestor
than any other living Crustacea. Compound eyes are absent, and it can
not be proved that they were ever present, although Grobben is said to
have observed rudiments of them in the development of _Calanus_. The
"simplified limbs" are the simple limbs of the trilobite, somewhat
modified. The absence of the shell-fold and carapace is certainly a
primitive characteristic. Add to this the direct development of the
small number of segments, and the infolded pleural lobes, and it must
be admitted that the group presents more trilobite-like
characteristics than any other. It seems very likely that the
primitive features were retained because of the pelagic habitat of a
large part of the group.

Ruedemann (Proc. Nat. Acad. Sci., vol. 4, 1918, p. 382, pl.) has
recently outlined a possible method of derivation of the acorn
barnacles from the phyllocarids. Starting from a recent _Balanus_ with
rostrum and carina separated by two pairs of lateralia, he traces back
through _Calophragmus_ with three pairs of lateralia to _Protobalanus_
of the Devonian with five pairs. Still older is the newly discovered
_Eobalanus_ of the upper Ordovician, which also has five pairs of
lateralia but the middle pair is reversed, so that when the lateralia
of each side are fitted together, they form a pair of shields like
those of _Rhinocaris_, separated by the rostrum and carina, which are
supposed to be homologous with the rostrum and dorsal plate of the
Phyllocarida. Ruedemann suggests that the ancestral phyllocarid
attached itself by the head, dorsal side downward, and the lateralia
were developed from the two valves of the carapace during its upward
migration, to protect the ventral side exposed in the new position.

This theory is very ingenious, but has not been fully published at the
time of writing, and it seems very doubtful if it can be sustained.

_Summary._

The salient points in the preceding discussion should be disentangled
from their setting and put forward in a brief summary.

It is argued that the ancestral arthropod was a short and wide pelagic
animal of few segments, which so far changed its habits as to settle
upon a substratum. As a result of change in feeding habits, appendages
were developed, and, due perhaps to physiological change induced by
changed food, a shell was secreted on the dorsal surface, covering
the whole body. Such a shell need not have been segmented, and, in
fact, the stiffer the shell, the more reason for development of the
appendages. Activity as a swimming and crawling animal tended to break
up the dorsal test into segments corresponding to those of the soft
parts, and, by adaptation, a floating animal became a crawling one,
with consequent change from a form like that of _Naraoia_ to one like
_Pædeumias_. (See figs. 36-40.) A continuation of this line of
development by breaking up and loss of the dorsal test led through
forms similar to _Marrella_ to the Branchiopoda of the Cambrian, in
which not only is there great reduction in the test, but also loss of
appendages. The origin of the carapace is still obscure, but Bernard
(1892, p. 214, fig. 48) has already pointed out that some trilobites,
Acidaspidæ particularly, have backward projecting spines on the
posterior margin of the cephalon, which suggest the possibility of the
production of such a shield, and in _Marrella_ such spines are so
extravagantly developed as almost to confirm the probability of such
origin. In this line of development two pairs of tactile antennæ were
produced, while the anomomeristic character of the trilobite was
retained. From similar opisthoparian ancestors there were, however,
derived primitive Malacostraca retaining biramous antennæ, but with a
carapace and reduced pleural lobes and pygidium. From this offshoot
were probably derived the Ostracoda, the Cirripedia, and the various
orders of the Malacostraca, with the possible exception of the
Isopoda. I have suggested independent origins of the Copepoda and
Isopoda, but realize the weighty arguments which can be adduced
against such an interpretation.

[Illustration: Fig. 36.--_Naraoia compacta_ Walcott. An outline of
the test, after Walcott. Natural size.]

[Illustration: Fig. 37.--_Pagetia clytia_ Walcott. An eodiscid with
compound eyes. After Walcott. × 5.]

[Illustration: Fig. 38.--_Asaphiscus wheeleri_ Meek. A representative
trilobite of the Middle Cambrian of the Pacific province. After Meek.
× 1/2.]

[Illustration: Fig. 39.--_Pædeumias robsonensis_ Burling. Restored
from a photograph published by Burling. × 1/4.]

[Illustration: Fig. 40.--_Robergia_ sp. Restored from fragments found
in the Athens shale (Lower Middle Ordovician), at Saltville, Va.
Natural size.]

It is customary to speak of the Crustacea and Trilobita as having had
a common ancestry, rather than the former being in direct line of
descent from the latter, but when it can be shown that the higher
Crustacea are all derivable from the Trilobita, and that they possess
no characteristics which need have been inherited from any other
source than that group, it seems needless to postulate the evolution
of the same organs along two lines of development.

I can not go into the question of which are more primitive, sessile or
stalked eyes, but considering the various types found among the
trilobites, one can but feel that the stalked eyes are not the most
simple. While no trilobite had movable stalked eyes, it is possible to
homologize free cheeks with such structures. They always bear the
visual surface, and, in certain trilobites (_Cyclopyge_), the entire
cheek is broken up into lenses. Since a free cheek is a separate
entity, it is conceivable that it might lie modified into a movable
organ.


EVOLUTION OF THE MEROSTOMATA.

It has been pointed out above that the Limulava (_Sidneyia_,
_Amiella_, _Emeraldella_) have certain characteristics in common with
the trilobites on the one hand and the Eurypterida on the other. These
relationships have been emphasized by Walcott, who derives the
Eurypterida through the Limulava and the Aglaspina from the Trilobita.
The Limulava may be derived from the Trilobita, but indicate a line
somewhat different from that of the remainder of the Crustacea. In
this line the second cephalic appendages do not become antennæ and
the axial lobe seems to broaden out, so that the pleural lobes become
an integral part of the body. As in the modern Crustacea, the pygidium
is reduced to the anal plate, and this grows out into a spine-like
telson.

From the Limulava to the Eurypterida is a long leap, and before it can
be made without danger, many intermediate steps must be placed in
position. The direct ancestor of the Eurypterida is certainly not to
be seen in the highly specialized _Sidneyia_, and probably not in
_Emeraldella_, but it might be sought in a related form with a few
more segments. The few species now known do suggest the beginning of a
grouping of appendages about the mouth, a suppression of appendages on
the abdomen, and a development of gills on the thorax only. Further
than that the route is uncertain.

Clarke and Ruedemann, whose recent extensive studies give their
opinion much weight, seem fully convinced that the Merostomata could
not have been derived from the Trilobita, but are rather inclined to
agree with Bernard that the arachnids and the crustaceans were derived
independently from similar chætopod annelids (1912, p. 148).

The greater part of their work was, however, finished before 1910, and
although they refer to Walcott's description of the Limulava (1911),
they did not have the advantage of studying the wonderful series of
Crustacea described by him in 1912. While the evidence is far from
clear, it would appear that the discovery of animals with the form of
Limiting and the eurypterids and the appendages of trilobites means
something more than descent from similar ancestors. Biramous limbs of
the type found in the trilobites would probably not be evolved
independently on two lines, even if the ancestral stocks were of the
same blood.

The Aglaspidæ, as represented by _Molaria_ and _Habelia_ in the Middle
Cambrian, are quite obvious closely related to the trilobites easily
derived from them, and retain numerous of their characteristics. That
they are not trilobites is, however, shown by the presence of two
pairs of antennæ, the absence of facial sutures, and the possession of
a spine-like telson.

The Aglaspidæ have always been placed in the Merostomata, and nearer
the Limulidæ than the Eurypterida. The discovery of appendages does
not at all tend to strengthen that view, but indicates rather that
they are true Crustacea which have not given rise to any group now
known. The exterior form is, however, _Limulus_-like, and since it is
known from ontogeny that the ancestor of that genus was an animal with
free body segments, there is still a temptation to try to see in the
Aglaspidæ the progenitors of the limulids.

The oldest known _Limulus_-like animal other than the Aglaspidæ is
_Neolimulus falcatus_ Woodward (Geol. Mag., dec. 1, vol. 5, 1868,
p. 1, pl. 1, fig. 1). The structure of the head of this animal is
typically limuloid, with simple and compound eyes and even the
ophthalmic ridges. Yet, curiously enough, it shows what in a trilobite
would be considered the posterior half of the facial suture, running
from the eye to the genal angle. The body is composed of eight free
segments with the posterior end missing. _Belinurus_, from the
Mississippian and Pennsylvanian, has a sort of pygidium, the posterior
three segments being fused together, and _Prestwichia_ of the
Pennsylvanian has all the segments of the abdomen fused together. So
far as form goes, a very good series of stages can be selected, from
the Aglaspidæ of the Cambrian through _Neolimulus_ to the Belinuridæ
of the late Palæozoic and the Limulidæ of the Mesozoic to recent.
Without much more knowledge of the appendages than is now available,
it would be quite impossible to defend such a line. It is, however,
suggestive.


EVOLUTION OF THE "TRACHEATA."

The trilobites were such abundant and highly variable animals,
adapting themselves to various methods of life in the sea, that it
appears highly probably that some of them may have become adapted to
life on the land. The ancestors of the Chilopoda, Diplopoda, and
Insecta appear to have been air-breathing animals as early as the
Cambrian, or at latest, the Ordovician. Since absolutely nothing is
yet known of the land or even of the fresh-water life of those
periods, nothing can now be proved.

In discussing the relationship of the trilobites to the various
tracheate animals, I have pointed out such palæontologic evidence
as I have been able to gather. Studies in the field of comparative
morphology do not fall within my province. I only hope to have made
the structure of the trilobite a little more accessible to the student
of phylogenies.


SUMMARY ON LINES OF DESCENT.

In order to put into graphic and concise form the suggestions made
above, it is necessary to define and give names to some of the groups
outlined. The hypothetical ancestor need not be included in the
classification and for reasons of convenience may be referred to
merely as the Protostracean.

The group of free-swimming trilobites without thoracic segments was
probably a large one, and within it there were doubtless considerable
variations and numerous adaptations. While the only known animal which
could possibly be referred to this group, _Naraoia_, is blind, it is
entirely possible that other species had eyes, and that the cephala
and pygidia were variously modified. For this reason and because of
the lack of all thoracic segments, it seems better to erect a new
order rather than merely a family for the group, and _Nektaspia_
(swimming shields) may be suggested. The only known family is Naraoidæ
Walcott, which must be redefined.

_Marrella_ and _Habelia_ are types of Crustacea which can neither be
placed in the Trilobita nor in any of the established subclasses of
the Eucrustacea. They represent a transitional group, the members of
which are, so far as known, adapted to the crawling mode of life,
though it may prove that there are also swimmers which can be
classified with them. To this subclass the name _Haplopoda_ may be
applied, the feet being simple.

The two known families, Marrellidæ Walcott and Aglaspidæ Clarke,
belong to different orders, the second having already the name
Aglaspina Walcott. The name _Marrellina_ may therefore be used for the
other.

For _Sidneyia_, Walcott proposed the new subordinal name Limulava,
placing it under the Eurypterida. While _Sidneyia_, _Emeraldella_, and
_Amiella_ may belong to the group that gave rise to the Eurypterida,
they are themselves Crustacea, and a place must be found for them in
that group. The possession of only one pair of antennæ prevents their
reception by the Haplopoda, and allies them to the Trilobita, but the
modifications of the trunk and its appendages keep them out of that
subclass, and a new one has to be erected for them. This may be known
as the _Xenopoda_, in allusion to the strange appendages of
_Sidneyia_.


_Synopsis._

Class Crustacea.

Subclass Trilobita Walch.

Crustacea with one pair of uniramous antennæ, and possessing facial
sutures.

Order Nektaspia nov.

Trilobita without thoracic segments. Cephala and pygidia simple.

Family Naraoidæ Walcott.

Cephalon and pygidium large, both shields nearly smooth. Eyes absent.
A single species: _Naraoia compacta_ Walcott, Middle Cambrian, British
Columbia.

Subclass Haplopoda nov.

Crustacea with trilobate form, two pairs of uniramous antennæ, no
facial sutures, sessile compound eyes present or absent, pygidium and
pleural lobes generally reduced, large labrum present, appendages of
the trunk biramous.

Order Marrellina nov.

Form trilobite-like, pleural lobes reduced, endobases absent from
coxopodites of body, pygidium a small plate.

Family Marrellidæ Walcott.

Cephalon with long genal and nuchal spines. Eyes marginal. A single
species: _Marrella splendens_ Walcott, Middle Cambrian, British
Columbia.

Order Aglaspina Walcott.

Body trilobite-like, with few thoracic segments, and a spine-like
telson. Appendages biramous.

Family Aglaspidæ Clarke.

Cephalon trilobate, with or without compound eyes, seven or eight
segments in the thorax.

Genus _Aglaspis_ Hall.

Compound eyes present, seven segments in thorax. Upper Cambrian,
Wisconsin.

Genus _Molaria_ Walcott.

Compound eyes absent, eight segments in thorax. Middle Cambrian,
British Columbia.

Genus _Habelia_ Walcott.

Compound eyes absent. Not yet fully described. Middle Cambrian,
British Columbia.

Subclass Xenopoda nov.

Crustacea with more or less eurypterid-like form, one pair of
uniramous antennæ, biramous appendages on anterior part of trunk,
modified endopodites on cephalon.

Order Limulava Walcott.

Cephalon with lateral or marginal eyes and large epistoma. Body with
eleven free segments and a telson. Cephalic appendages grouped about
the mouth.

Family Sidneyidæ Walcott.

Trunk probably with exopodites only, and without appendages on the
last two segments. Telson with a pair of lateral swimmerets.

Genus _Sidneyia_ Walcott.

Third cephalic appendage a large compound claw. Gnathobases forming
strong jaws. Middle Cambrian, British Columbia.

Genus _Amiella_ Walcott.

Middle Cambrian, British Columbia.

Family Emeraldellidæ nov.

Trunk with biramous appendages in anterior part, and appendages on all
segments except possibly the spine-like telson.

Genus _Emeraldella_ Walcott.

Cephalic appendages simple spiniferous endopodites. Eyes unknown.
Middle Cambrian, British Columbia.

[Illustration: Fig. 41.--A diagram showing possible lines of descent
of the other Arthropoda from the Trilobita. The three recognized
orders of the latter are shown separately. The known geological range
is indicated in solid black, the hypothetical range and connections
stippled. The short branch beside the Opisthoparia represents the
range of the Haplopoda. The term Arachnida is used for all arachnids
other than Merostomata, merely as a convenient inclusive name for the
groups not especially studied.]




Final Summary.


It is generally believed that the Arthropoda constitute a natural,
monophyletic group. The data assembled in the preceding pages indicate
that the other Arthropoda were derived directly or indirectly from the
Trilobita because:

(1) the trilobites are the oldest known arthropods;

(2) the trilobites of all formations show great variation in the
number of trunk segments, but with a tendency for the number to become
fixed in each genus;

(3) the trilobites have a constant number of segments in the head;

(4) the position of the mouth is variable, so that either the
Crustacea or the Arachnida could be derived from the trilobites;

(5) the trilobite type of appendage is found, in vestigial form at
least, throughout the Arthropoda;

(6) the appendages of all other Arthropoda are of forms which could
have been derived from those of trilobites;

(7) the appendages of trilobites are the simplest known among the
Arthropoda;

(8) the trilobites show practically all known kinds of sessile
arthropodan eyes, simple, compound, and aggregate;

(9) the apparent specializations of trilobites, large pleural lobes
and pygidia, are primitive, and both suffer reduction within the
group.

The ancestor of the trilobite is believed to have been a soft-bodied,
free-swimming, flat, blind or nearly blind animal of few segments,
because:

(a) the form of both adult and embryo is of a type more adapted for
floating than crawling;

(b) the large pygidium is shown by ontogeny to be primitive, and the
elongate worm-like form secondary;

(c) the history of the trilobites shows a considerable increase in the
average number of segments in successive periods from the Cambrian to
the Permian;

(d) the simplest trilobites are nearly or quite blind.




PART IV.

DESCRIPTION OF THE APPENDAGES OF INDIVIDUAL SPECIMENS.




Triarthrus becki Green.


In order to make easily available the evidence on which the present
knowledge of the appendages of Triarthrus and _Cryptolithus_ rests, it
has seemed wise to publish brief descriptions and photographic figures
of some of the better specimens preserved in the Yale University
Museum. These specimens are pyritic replacements, and while they do
not as yet show any signs of decomposition, it should be realized that
it is only a matter of time when either they will be self-destroyed
through oxidation, or else embedded for safe keeping in such a fashion
that they will not be readily available for study. It is therefore
essential to keep a photographic record of the more important
individuals.


Specimen No. 220 (pl. 3, fig. 2).

     Illustrated: Amer. Geol., vol. 15, 1895, pl. 4 (drawing);
     Amer. Jour. Sci., vol. 13, 1902, pl. 3 (photograph).

This is one of the largest specimens showing appendages, and is
developed from the ventral side. It shows some appendages on all parts
of the body, but its special features are the exhibition of the shafts
on the proximal ends of the antennules, the rather well preserved
appendages of the cephalon and anterior part of the thorax, and the
preservation of the anal opening. In the drawing in the American
Geologist, the right and left sides are reversed as in a mirror, a
point which should be borne in mind when comparing that figure with
a photograph or description.

The shaft of the left antennule is best preserved and is short,
cylindrical, somewhat enlarged and ball-shaped at the proximal end. It
is 1.5 mm. long. The posterior part of the hypostoma is present, but
crushed, and the metastoma is not visible, the pieces so indicated
in Beecher's figure being the rim of the hypostoma. Back of the
hypostoma may be seen four (not three as in Beecher's figure) pairs
of gnathites, the first three pairs broad and greatly overlapping, the
fourth pair more slender, but poorly preserved. The inner edges of the
gnathites on the right side are distinctly nodulose, and roughened for
mastication.

The outer ends of one endopodite and three exopodites project beyond
the margin on the right side. The dactylopodite of the endopodite is
especially well preserved. It is cylindrical, the end rounded but not
enlarged or pointed, and bears three small sharp spines, all in a
horizontal plane, one anterior, one central, and one posterior. The
outer ends of the exopodites show about ten segments each (in 2.5 mm.)
beyond the margin of the test, and from three to five setæ attached to
the posterior side of each segment. These hairs are attached in a
groove, well shown in this specimen. On the anterior margin of the
exopodite there is a minute spine at each joint.

_Measurements:_ Length, 38 mm.; width at back of cephalon, 19 mm.


Specimen No. 210 (pl. 2, fig. 3).

     Illustrated: Amer. Jour. Sci., vol. 46, 1893, p. 469, fig. 1
     (head and right side); Amer. Geol., vol. 13, 1894, pl. 3, fig. 7
     (same figure as the last); Amer. Jour. Sci., vol. 13, 1902, pl. 2,
     fig. 1 (photograph).

This individual supplied the main basis for Professor Beecher's first
figure showing the appendages of the thorax, the head and appendages
of the right side having been taken from it, and the appendages of
the left side from No. 206. Such of the endopodites as are well
preserved show from three to four segments projecting beyond the test,
and the dactylopodites have one or two terminal spines. The antennules
are unusually well preserved and have about forty segments each in
front of the cephalon, or an average of five to one millimeter.

Specimens 209 and 210 are on a slab about 7 × 5.5 inches, and with
them are twelve other more or less well preserved individuals, all but
one of which are smaller than these. Two of the fourteen are ventral
side up on the slab, which means dorsal side up in the rock. Nine are
oriented in one direction, two at exactly right angles to this, and
three at an angle of 45 with the others. If the majority of the
specimens are considered to be headed northward, then seven are so
oriented, two northeast, one east, two south, one southwest, and one
west.

Nine of the specimens show antennules. Five of these are specimens
headed north, and in all of them the antennules are in or very near
the normal position. The antennules of two, one headed east and the
other west, are imperfectly preserved, but the parts remaining diverge
much more than do the antennules of those in the normal position. The
individual headed southwest has one antennule broken off, while the
other is curved back so that its tip is directed northward. Another
one, headed south, has the antennules in the normal position. These
observations indicate that the specimens were oriented by currents of
water, rather than in life attitudes, and that the distal portions of
the antennules were relatively flexible.

_Measurements:_ The specimen (No. 210) is 20 mm. long, 9.5 mm. wide at
the back of the cephalon, and the antennules project 8 mm. in front of
the head. The smallest specimen on the slab is 6.5 mm. long. A
specimen 7.5 mm. long has antennules which project 2.5 mm. in front of
the cephalon.


Specimen No. 201 (pl. 2, fig. 1; pl. 3, fig. 4).

     Illustrated: Amer. Jour. Sci., vol. 46, 1893, p. 469, figs. 2, 3;
     Amer. Geol., vol. 13, 1894, pl. 3, figs. 8, 9.

An entire specimen 17 mm. long, exposed from the dorsal side. It shows
only traces of the appendages of the head, but displays well those of
the anterior part of the thorax, and a number of appendages emerge
from under the abdominal shield. This specimen is of particular
interest as it is the subject of the first of Professor Beecher's
papers on appendages of trilobites. On the right side the pleura have
been removed, so as to expose the appendages of the second, third, and
fourth segments from above. The first two of the appendages on the
right are best preserved, and these are the ones figured. They belong
to the second and third segments. The endopodites of each are ahead of
the exopodites, and the proximal portion of each exopodite overlies
portions of the first two segments (second and third) of the
corresponding endopodite. The coxopodites are not visible, but very
nearly the full length of the first segment of the endopodite (the
basipodite) is exposed. The first two visible segments (the first and
second) extend just to the margin of the pleural lobe, while the other
four extend beyond the dorsal cover. The segments decrease in length
outward, but not regularly, the meropodite being generally longer than
the ischiopodite or the carpopodite. The terminal segment
(dactylopodite) is short and bears short sharp hair-like spines which
articulate in sockets at the distal end. On this specimen the anterior
limb on the right side shows one terminal spine, the second endopodite
on that side has two, and two of the endopodites on the left-hand side
preserve two each. The segments of the limbs are nearly cylindrical,
but the ischiopodites and meropodites of several of the endopodites
show rather deep longitudinal grooves which appear to be rather the
result of the shrinkage of the thin test than natural conformations.

The endopodites on the left-hand side have a number of short, sharp,
movable, hair-like spines, and cup-shaped depressions which are the
points of insertion of others. On the distal end of the carpopodite of
the first thoracic segment there seems to have been a spine, whose
place is now shown by a pit. This same endopodite shows, rather
indistinctly, three pits in the groove of the carpopodite, and the
propodite has two. On the endopodite of the second appendage on this
side, both the carpopodite and propodite possess a fine hair-like
articulated spine at the distal end, that of the propodite arising on
the dorsal and that of the carpopodite on the posterior side. On the
dorsal side of the carpopodite there are three pits for the
articulation of spines, and on the propodite, one.

The exopodites belonging to the thoracic segments are of equal length
with the endopodites, and while the proximal portion of each is
stouter than that of the corresponding endopodite, the exopodites
taper to a hair-like termination, while the endopodites remain fairly
stout to the distal segment. Most of the setæ of the exopodites have
been removed, so that each remains as a curving, many-segmented organ,
transversely striated, with a continuous groove along the posterior
side. The setæ appear to be set in this groove, one for each of the
transverse ridges on the shaft.

A good deal of the test has been cut away on the left-hand side from
the thorax and pygidium, and the appendages exposed from above. Enough
of the dorsal shell has been cut away so that the anal opening is
exposed, and directly behind the pygidium, on the median line, is a
bilaterally symmetrical plate with serrated edges which appears to be
the appendage of the anal segment. (See pl. 3, fig. 4.)

_Measurements:_ The specimen is 17 mm. long, and 8 mm. in greatest
width (at the back of the cephalon). From the median tubercle to the
outer edge of the pleuron of the second thoracic segment the distance
is 3.7 mm. From the point of articulation to the distal end of the
spines on the dactylopodite of the second endopodite on the right-hand
side is 4.3 mm. The basipodite of this appendage is 1.5 mm. long, the
ischiopodite 1 mm. long, the meropodite 1.2 mm. long, the carpopodite
0.5 mm. long, the propodite 0.35 long, and the dactylopodite 0.15 mm.
long. On the left-hand side the endopodite of the first segment
projects 3 mm. beyond the pleuron, the second, 3.2 mm. At the back the
appendages extend a maximum distance of 2.5 mm. behind the pygidium.
The median spinose process of the anal segment extends 0.75 mm. behind
the pygidium, and is 1.6 mm. in greatest width.


Specimen No. 204 (pl. 3, fig. 1; pl. 4, fig. 6; text fig. 42).

     Illustrated: Amer. Jour. Sci., vol. 13, 1902, pl. 2, figs. 4, 5
     (reproduced from photographs).

This specimen, which is developed from the dorsal surface, shows
especially well nine appendages of the left side. The first represent
the last segment of the cephalon; the remainder belong to the thorax.
As is usual, the exopodites of these appendages overlie and curve
behind the endopodites. All the exopodites have lost their setæ and
the segments of the endopodites are flattened by crushing. The
endopodites, while retaining only one or two of the movable spines,
have the cup-like bases of from two to four on each of the visible
segments, namely, the meropodite, carpopodite, propodite, and, in one
case, the dactylopodite. These appendages, although really marvellous
in preservation, are of such small size and react so badly to light
that their study is very difficult, and Professor Beecher, who had
observed hundreds of specimens through all stages of the laborious
process of cleaning the matrix from them, undoubtedly was much better
equipped to interpret them than any other person.

The drawing is made on the assumption that the appendages are
displaced and all moved uniformly outward so that the distal ends of
the coxopodites emerge from under the pleural lobe, whereas these ends
would normally be under the dorsal furrow, and the distal end of the
ischiopodite should reach the margin of the pleural lobe. While it
seems very remarkable that it should happen, that all the appendages
should be so moved that they would lie symmetrically a few millimeters
from their normal position, nevertheless it is found on measuring that
they bear the same proportion to the length and width that the
appendages of other specimens do, thus indicating that Professor
Beecher's interpretation of them was correct. I am unable, however, to
see the coxopodites which he has drawn as articulating with the two
branches of the limb.

[Illustration: Fig. 42.--_Triarthrus becki_ Green. Appendages of
specimen 204. Inked in by Miss Wood from the original tracing. × 10.]

This individual shows, better than any other, the connection of the
exopodite with the endopodite. Even though the coxopodites are gone,
the two branches of each appendage remain together, showing that the
basipodite as well as the coxopodite is involved in the articulation
with the exopodite. Just what the connection is can not be observed,
but there seems to be a firm union between the upper surface of the
basipodite and the lower side of the proximal end of the exopodite, as
indicated diagrammatically in text figure 33.

_Measurements:_ The specimen is 20 mm. long and 9 mm. wide at the back
of the cephalon. From the tubercle on the middle of the first segment
of the thorax to the tip of the corresponding appendage the distance
is 8 mm. The entire length of the exopodite of the first thoracic
segment is 4.6 mm. The exopodite of the appendage belonging to the
seventh segment is only 3.5 mm. long. The pleural lobe is 2.5 mm. wide
at the front of the thorax.


Specimen No. 205 (pl. 2, fig. 4).

     Illustrated: Amer. Jour. Sci., vol. 13, 1902, pl. 5, figs. 2, 3
     (photographs).

This is a small imperfect specimen, developed from the ventral side.
It retains the best preserved metastoma in the collection, but was
used by Professor Beecher especially to illustrate the convergent
ridges on the inside of the ventral membrane in the axial region of
the thorax. These ridges are very low, and on each segment of the
thorax there is a central one, outside of which is a pair which are
convergent forward, making angles of 35 to 45 with the axis.

The metastoma is shaped much like the hypostoma of an _Illænus_. It is
convex, nearly semicircular, with the straight side forward, and there
is a continuous raised border around the curved sides and back. This
border is separated from the central convex body by a deep linear
depression.

The hypostoma is also rather well preserved and has a narrow, slightly
elevated border at the sides and back.

_Measurements:_ The incomplete specimen, from which only a very small
portion of the length is missing, is 9 mm. long. The metastoma is 0.45
mm. long and 0.58 mm. wide.


Specimen No. 214 (pl. 1, fig. 2; pl. 3, fig. 6).

This is a large specimen, developed from the ventral side. It shows
the antennules and some other appendages of the head, but derives its
special interest from the excellent preservation of a few of the
exopodites, which are turned back parallel to the axis of the body and
lie within the axial lobe.

The shaft of the exopodite is made up of numerous short segments which
at their anterior outer angles are produced into spines, and which
also bear movable spines along the anterior border. As shown in
several other specimens, the exopodite ends in a more or less long
spoon-shaped segment bearing on its lower surface a broad groove. No
setæ appear to be attached to this, but both anterior and posterior
margins bear numerous small, apparently movable spines. From the
groove along the ventral side of the remainder of the exopodite arise
numerous long slender filaments which become progressively shorter
toward the tip. This specimen shows that they are not cylindrical, but
are flattened along opposite faces, at least at their distal ends.
While no connection can be seen between adjacent setæ, they seem to
stay together like the barbs on a feather.

_Measurements:_ Length, 33 mm., width at back of cephalon, 16 mm.;
from front of cephalon to back of hypostoma, 6 mm.


Specimen No. 219 (pl. 2, fig. 6; pl. 4, fig. 4).

     Illustrated: Amer. Jour. Sci., vol. 13, 1902, pl. 4, fig. 1, pl. 5,
     fig. 4 (photograph and drawing).

The endopodites of most of the appendages of the thorax are well
shown, and occasional portions of exopodites. The coxopodites are
long, flattened, and do not taper much. The anterior and posterior
edges of the basipodites of the endopodites of the first two segments
are approximately parallel, but on the succeeding endopodites the
basipodites and ischiopodites are triangular in form, with the apex
backward. In successive endopodites toward the posterior end, the
angle made by the backward-directed sides of the basipodites becomes
increasingly acute, so that in some of the posterior appendages this
segment is wider than long. The ischiopodite shows a similar increase
of width and angularity on successive segments, and the meropodites
and carpopodites also become wider on the posterior segments, and even
triangular in outline toward the back of the thorax and on the
pygidium.

Along the median portion of the axial lobe the specimen has been
cleaned until the inner side of the ventral membrane was reached. Here
the test shows on the inner surface at each segment of the thorax a
series of low ridges which are roughly parallel to the axial line, but
which really converge in an anterior direction. Between the ridges
are shallow canoe-shaped depressions, which have the appearance of
areas for the insertion of muscles.

_Measurements:_ Length, 31 mm.; width at back of head, 15 mm.;
distance, in a straight line, from point of insertion of the right
antennule to its tip, 14.25 mm.; it projects 12 mm. beyond the
cephalon.


Specimen No. 218 (pl. 6, fig. 3; text fig. 43).

This specimen is a large one, developed from the lower side, but
retains only the endopodites of a few appendages. The cephalon and
anterior portion of the thorax are missing.

Professor Beecher had a drawing made to show the appendages on the
right-hand side of the last two segments of the thorax, seen of course
from the ventral side. This drawing shows well the broadening of the
basipodite, ischiopodite, and meropodite, while the coxopodite is
thick and heavy, and the inner end of the gnathobase somewhat rugose.
Almost every segment of the endopodites has one or more pits for
insertion of spines, these being along the anterior or posterior
margins. The exopodites lack the setæ, but show no unusual features.

[Illustration: Fig. 43.--_Triarthrus becki_ Green. Drawing to
represent the writer's interpretation of the appendages of specimen
218. Drawn by Miss Wood. × 10.]


Specimen No. 222 (pl. 4, fig. 5).

     Illustrated: Amer. Jour. Sci., vol. 47, 1894, pl. 7, fig. 3
     (drawing).

A small specimen, developed from the lower side, and used by Professor
Beecher to illustrate the form of the segments of the endopodites of
the pygidium. In addition to this, it shows very well the form of the
endopodites of the thorax. All of the appendages on the specimen are
shifted to the left of their normal position. This specimen differs
from most of the others in that the segments of the endopodites do not
lie with their greatest width in the horizontal plane, but were
embedded vertically, with the posterior edge downward. From this
circumstance they retain their natural shape, and it is seen that they
are naturally flattened, with about the same thickness in proportion
to length and width as in some of the modern isopods (Serolis, for
instance). In even the most anterior of these endopodites (that of the
second segment) the ischiopodite, meropodite, and carpopodite are
triangular in shape, with the point backward, but in all the
endopodites at the anterior end of the thorax, the triangle has a very
obtuse angle at the apex, and the base is much longer than the
perpendicular. On the other hand, those of the pygidium, which were
figured by Beecher, have a number of short wide segments, all wider
than long, and, excepting the dactylopodites, triangular in form.

_Measurements:_ Length, 8.75 mm.; width at back of cephalon,
about 4 mm.


Specimen No. 230 (pl. 5, fig. 3; text fig. 44).

     Illustrated: Amer. Jour. Sci., vol. 47, 1894, pl. 7, fig. 2
     (drawing); Ibid., vol. 13, 1902, pl. 2, fig. 2.

[Illustration: Fig. 44.--_Triarthrus becki_ Green. Appendages of the
posterior part of the thorax and pygidium of specimen 230. Inked by
Miss Wood from a tracing made under the direction of Professor
Beecher.]

An entire specimen of medium size, developed from the ventral side. It
seems to have been the first one to yield to Professor Beecher any
satisfactory knowledge of the appendages of the pygidium. There are
five endopodites, all on one side, which appear to belong here. The
segments in this region are characterized by their short, wide,
triangular form. At the apex of each is a small tuft of spines or
short hairs, and the ventral surfaces of some of the endopodites show
pits for the insertion of spines.

_Measurements:_ Length, 21 mm.; width at back of cephalon, 10 mm.




Cryptolithus tessellatus Green.


Specimen No. 233 (pl. 7, fig. 1; text fig. 45).

This is the best preserved entire specimen. It is developed from the
lower side, and shows the hypostoma, antennules, and a few fragmentary
appendages of the cephalon, the outer portions of the exopodites of
thorax and pygidium on both sides, and the endopodites on the left
side.

The hypostoma is imperfectly preserved and is turned completely
around, so that the anterior margin is directed backward, and the
posterior one is so much in the shadow that it does not show well in
any of the photographs. The form is, however, essentially like that
of _Trinucleoides reussi_ (Barrande), the only other trinucleid of
which the hypostoma is known, except that the border does not extend
so far forward along the sides, and it is much smaller.

The antennules are not inserted close to the hypostoma, as in
Triarthrus, but at some distance from it, and, as nearly as can be
determined, directly beneath the antennal pits which are seen near the
front of the glabella in many species of trinucleids.

[Illustration: Fig. 45.--_Cryptolithus tessellatus_ Green. Drawing of
specimen 233, made by Professor Beecher. × 9. Below are parts of two
of the endopodites of specimen 236, showing the interarticular
membranes. × 41.]

The antennules are long, and are composed of far fewer and longer
segments than those of Triarthrus. In this specimen they converge
backward, cross each other and at the distal end are more or less
intertwined.

As is shown in the drawing and photograph, very little can be learned
from this individual about the other appendages of the cephalon. A few
fragments of exopodites on either side suggest that these members
pointed forward and were much like those in Triarthrus, but nothing
conclusive is shown.

The exopodites and endopodites of the left side of the thorax are best
preserved. The exopodites are above the endopodites, and only that
portion exposed from the ventral side which projects beyond the line
at which the endopodites bend backward. The endopodite on the left
side of the first thoracic segment is the best preserved. It shows
seven segments, the outer ones best. The coxopodite is short and
narrow, the basipodite somewhat heavier and longer, while the
carpopodite and propodite are the widest and strongest segments. The
propodite is triangular and flattened, like the segments on the middle
and posterior part of the thorax of Triarthrus. At the inner end of
the ischiopodite and meropodite are tufts of spines pointing inward
and backward. These are not shown on any of the photographs, but may
be seen with the light striking the specimen at the proper angle.

It is not possible to count the exact number of limbs, but one gets
the impression that on the left side of this specimen there are
twenty-one sets of appendages, six of which of course belong to the
thorax. On the thorax and anterior part of the pygidium, successive
endopodites show the propodites and dactylopodites becoming
progressively more slender and shorter, while the ischiopodites,
meropodites and carpopodites become shorter and more triangular, and
with increasingly large numbers of short spines on their posterior
borders. Back of the fourth endopodite on the pygidium it is not
possible to make out the detail, but the appearance is of an
endopodite consisting of short broad segments fringed at the back with
short spines, the ones at the very posterior end appearing to be
exceedingly short and rudimentary.

The exopodites are not so well shown as in some others but the setæ
are flattened and blade-shaped, and often bear numerous small spines.

_Measurements:_ Length (lacking most of the fringe), 10.5 mm. Width of
thorax, 10.5 mm. Length of hypostome, 1.41 mm., width at front, 1.46
mm. The distance from back of fringe to end of antennules is 5.4 mm.
If straightened out, the left antennule would be about 6.1 mm. long.
In the first 3.1 mm., there are only ten segments, so that the average
length of a segment is 0.31 mm. The distance from the inner end of the
endobase of the first segment of the thorax to the outer end of the
meropodite is 2.43 mm., and from that point to the end of the
dactylopodite 2.47 mm. making the total length 4.90 mm. These
measurements are taken from the photograph. Measurements taken from
Professor Beecher's drawing, which was made with the camera-lucida,
give a total length of 4.57 mm., the distance to the outer end of the
meropodite being 2.3 mm. and thence to the tip of the dactylopodite
2.27 mm. Detailed measurements of the segments, on the photograph, are
as follows: coxopodite, 0.321 mm.; basipodite, 0.78 mm.; ischiopodite,
0.68 mm.; meropodite, 0.642 mm.; carpopodite, 0.642 mm.; propodite,
1.01 mm., dactylopodite, 0.825 mm.


Specimen No. 235 (pl. 7, fig. 2; pl. 8, fig. 3; pl. 9, figs. 1, 2).

     Illustrated: Amer. Jour. Sci., vol. 49, 1895, pl. 3, figs. 5, 6.

Specimens 235 and 236 were originally parts of an entire
_Cryptolithus_, but, as Professor Beecher has explained, the specimen
was cut in two longitudinally on the median line, and the halves
transversely just back of the cephalon, so that each now represents
one half of a thorax and pygidium. Both halves have been cleaned from
both upper and lower side, a perfectly marvelous piece of work, for
the thickness is no greater than that of a thin sheet of paper, and
the soft shale of the matrix has a very slight cohesive power.

Both sides of specimen 235 were figured, but the dorsal side was
apparently then somewhat less fully developed than at present. On
plate 9 are two figures in which specimens 235 and 236 are brought
together again, and both dorsal and ventral sides illustrated.

On the dorsal side, specimen 235 shows portions of three exopodites
which lie in a direction roughly parallel to the outer portions of the
endopodites on the lower side, that is, their direction if projected
would reach the axis in an acute angle back of the end of the
pygidium. The setæ stand at right angles to the shaft, and on a
portion of it 0.5 mm. long there are seven of them. This is a fragment
of an exopodite near the front of the thorax, and the setæ, which are
flattened, are about 1.63 mm. long.

On the ventral side this same specimen shows incomplete endopodites
and exopodites of about seventeen segments, six of which would belong
to the thorax and the remainder to the pygidium. The greater part of
the appendages belonging to the pygidium are exceedingly small (about
0.15 mm. long) and so incompletely exposed that the structure can not
be definitely made out.

The endopodites of the thoracic segments all lack the greater part of
their proximal segments and are all of practically the same form. They
turn abruptly backward at the outer end of the meropodite, and the
carpopodite of each is greatly widened, projects inward and is armed
with tufts of spines. The propodite and dactylopodite are wide,
flattened, and taper but slightly outward, the dactylopodite bearing
on its distal end a tuft of spines. On several of the endopodites, the
meropodites are visible and they bear on their inner ends fringes of
spines pointing inward. Behind these well preserved appendages the
proximal segments of several endopodites are visible, and a regular
succession of flattened, oval bodies armed with numerous
forward-pointing spines. These latter bodies Professor Beecher took to
be leaf-like exopodites, which they certainly resemble, and as they
lie beyond the line of endopodites they probably do belong to the
outer halves of the appendages.

The exopodites under the thorax are long, the shaft shows numerous
short segments, and is in each case bent backward, though not through
a right angle. They extend considerably beyond the endopodites. The
setæ do not diverge from the shaft at a right angle as on the dorsal
side of this same specimen, but at an acute angle, indicating that
they were not rigid. The individual hairs are broad and blade-shaped,
frequently with a linear depression along the median line, perhaps due
to collapse of the internal tube.

_Measurements:_ The greatest length of the fragment in its present
state is 5 mm. The dactylopodite of the second endopodite (without
terminal spines) is 0.18 mm. long, the propodite 0.23 mm. long and
0.15 mm. wide; the carpopodite is 0.24 mm. long and 0.38 mm. wide. All
measurements were made on the photographs.


Specimen No. 236 (pl. 7, figs. 3-5; pl. 9, figs. 1, 2; text fig. 45).

The right half of the same thorax and pygidium as specimen No. 235.

The specimen is cleaned from both upper and lower sides and, the
dorsal test being removed, reveals the long blade-like setæ of the
exopodites, each blade being concave along its median line. They are
long on the exopodites of the thoracic segments, but become shorter,
without, however, any visible change of form on the pygidium. Although
the posterior end is not well preserved, one gets no suggestion from a
study of this side of the specimens that the exopodites of the
posterior end are in any striking way different from those of segments
further forward. The tips of some of the setæ show minute spines, one
to each blade.

On the ventral side are a number of endopodites, but they are more
fragmentary than those of the other half of the specimen. Some of the
exopodites are well shown, the blades being in all cases broken from
the shaft. Two of the endopodites of this specimen are of especial
interest, as they have interarticular membranes between the last three
segments. Professor Beecher made a drawing of one of these which he
placed under his pen drawing (text fig. 45).

_Measurements:_ The specimen is 5 mm. long from the front of the
second thoracic segment to the end of the pygidium. The setæ on the
exopodites of the anterior thoracic segments are 1.7 mm. long, as
exposed from the dorsal side. Some of those on the posterior part of
the pygidium, only incompletely exposed, are 0.31 mm. long.

[Illustration: Fig. 46. _Cryptolithus tessellatus_ Green. A part of a
thorax and pygidium, showing appendages. Drawn by Professor Beecher.
Specimen 238. × 10.]

The dactylopodite of the first endopodite showing the articular
membranes is 0.23 mm. long and 0.13 mm. wide. The propodite is of the
same length and 0.17 mm. wide. The interarticular membrane between
them is 0.066 mm. thick. The spines on the dactylopodite of this
appendage are 0.15 mm. long. All measurements were made on
photographs.


Specimen No. 238 (pl. 8, fig. 4; text fig. 46).

A triangular specimen consisting of the greater part of a pygidium and
parts of all the thoracic segments. Under the thorax the specimen has
been so cleaned that the outer portions of the endopodites are well
shown, while under the pygidium the greater part of the endopodites
seem to have been removed, disclosing the setæ of the exopodites. As
in other specimens, the endopodites of the thorax turn backward at the
distal end of the carpopodite, which is broad and curved, and bears a
tuft of spines on the posterior margin. The dactylopodites seem to
preserve their natural shape, and are very nearly cylindrical in form.
Under the pygidium are several sets of overlapping fringes of setæ of
exopodites, and along the edge of the dorsal furrow, a number of
fragments of segments of what may be coxopodites while with them are a
number of fragmentary shaft of exopodites.

_Measurements:_ The pygidium is 3.3 mm. long, the thorax 3 mm.




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       pp. 103-110.


Törnquist, S. L.

  1896 A.--On the appendages of trilobites. Ibid., dec. 4, vol. 3,
       p. 142.

  1896 B.--Linnæus on the appendages of trilobites. Ibid., pp. 567-569.


Tothill, J. D.

  1916.--The ancestry of insects, with particular reference to chilopods
       and trilobites. Amer. Jour. Sci. (4), vol. 42, pp. 373-383. text
       figs. 1-8.


Troedsson, G. T.

  1918.--Om skanes Brachiopodskiffer. Lunds Universitets Arsskrift, n.
       f., Avd. 2, Bd. 15, Nr. 3. pp. 57-67, pl. 1, figs. 19-24.


Valiant, W. S.

  1901.--Appendaged trilobites. The Mineral Collector, vol. 8, No. 7,
       pp. 105-112.


Volborth, A. von.

  1858.--Ueber die Bewegungs-Organe der Trilobiten. Verhandl. russ. k.
       mineral. Gesell. zu St Petersburg, 1857-1858, p. 168.

  1863.--Ueber die mit glatten Rumpfgliedern versehenen russischen
       Trilobiten, nebst einem Anhange ueber die Bewegungs-organe und
       ueber das Herz derselben. Mem. Acad. Imp. Sci. St. Petersburg,
       ser. 7, vol. 6, No. 2, pp. 44-47, pl. 1, fig. 12.

  1866.--Ueber Herrn von Eichwald's Beitrag zu näheren Kenntniss der
       Illænen. Bull. Soc. Imp. des Natural, de Moscou, vol. 39, p. 40.


Wahlenberg, G.

  1821.--Petrificata telluris Suecana examinata a Georgio Wahlenberg.
       Nova Acta Reg. Soc. Scient. Upsala, vol. 8.


Walcott, C. D.

  1875.--Description of the interior surface of the dorsal shell of
       _Ceraurus pleurexanthemus_ Green. Ann. Lye. Nat. Hist. New York,
       vol. II, pp. 159-162, pl. 11.

  1876.--Preliminary notice of the discovery of the natatory and
       branchial appendages of trilobites. 28th Rept. New York State
       Mus. Nat. Hist., adv. sheets, pp. 89-92; published as full report
       in 1879.

  1877.--Notes on some sections of trilobites. 31st Rept. New York State
       Mus. Nat. Hist., adv. sheets, pp. 61-63, pl. 1; published as full
       report in 1879. Reviewed by Dames, Neues Jahrb. f. Min., Geol.,
       u. Pal., Bd. 1, 1880, p. 428.

  1879.--Notes upon the legs of trilobites. 31st Rept. New York State
       Mus. Nat. Hist., adv. sheets, p. 64.

  1881.--The trilobite: New and old evidence relating to its
       organization. Bull. Mus. Comp. Zool., vol. 8, pp. 192-224,
       pls. 1-6.

  1884.--The appendages of the trilobite. Science, vol. 3, pp. 276-279,
       figs. 1-3. Reviewed by Dames, Neues Jahrb. f. Min., Geol., u.
       Pal.., Bd. 1, 1885, Referate, p. 102.

  1894.--Note of some appendages of the trilobites. Proc. Biol. Soc.
       Washington, vol. 9, pp. 89-97, pl. 1; Geol. Mag., dec. 4, vol. 1,
       pp. 246-251, pl. 8.

  1911.--Middle Cambrian Merostomata. Smithson. Misc. Coll., vol. 57,
       No. 2, pp. 17-40, pls. 2-7.

  1912 A.--Middle Cambrian Branchiopoda, Malacostraca, Trilobita, and
       Merostomata. Ibid., No. 6, pp. 145-228, pls. 24-34, text figs.
       8-10.

  1912 B.--New York Potsdam-Hoyt fauna. Ibid., No. 9, pp. 251-304,
       pls. 37-49.

  1913.--Eastman-Zittel Text-book of Paleontology, ed. 2, vol. 1,
       figs. 1343, 1376, 1377.

  1916.--Ann Rept., Secretary Smithsonian Inst, for 1915, pl. 9.

  1918.--Appendages of trilobites. Smithsonian Misc. Coll., vol. 67,
       No. 4, pp. 115-226, pls. 14-42.


Watase, S.

  1890.--On the morphology of the compound eyes of arthropods. Johns
       Hopkins Univ., Studies from Biol. Lab., vol. 4, no. 6, p. 290
       (footnote).


Woodward, H.

  1870.--Note on the palpus and other appendages of _Asaphus_, from
       the Trenton limestone, in the British Museum. Quart. Jour. Geol.
       Soc., London, vol. 26, pp. 486-488, fig. 1. Abstract in Geol.
       Mag., dec. 1, vol. 7, p. 292, also in Nature, vol. 2, p. 94.

  1871.--On the structure of trilobites. Geol. Mag., dec. 1, vol. 8,
       pp. 289-294, pl. 8.

  1884.--Notes on the appendages of trilobites. Geol. Mag., dec. 3,
       vol. 1, pp. 162-165, 2 text figs.

  1895.--Some points in the life history of the Crustacea in early
       Palæozoic times. Quart. Jour. Geol. Soc., London, vol. 51,
       pp. lxx-lxxxviii, 1 pl.



       *       *       *       *       *


PLATE 1.

Photographs of _Triarthrus becki_, made by C. E. Beecher.

Fig. 1. Specimen 213. The dorsal test has been removed from the
glabella, revealing the outline of the posterior end of the hypostoma,
the proximal ends of the antennules, the gnathites, and incomplete
endopodites of some appendages, × 5.43.


Fig. 2. Specimen 214. The head of a complete large specimen. Part of
the thorax is shown on pl. 3, fig. 6. Note especially the form of the
segments of the endopodites and of the anterior coxopodite on the
right side, × 7.33.

Fig. 3. Specimen 217. This specimen shows better than any other the
form of the gnathites of the cephalon. Note also the setæ of the
exopodites under the cheek at the right. The appearance of a hook on
the posterior gnathite on the right may be accidental, but it does not
show broken edges, × 6.85.

Fig. 4. Specimen 215. The ventral side of the cephalon of a small
entire specimen. Shows well the form of some of the gnathites and a
few of the endopodites. Note the unusual position of the antennules. ×
7.63.

Fig. 5. Specimen 226. This specimen did not photograph well, but is
important as showing the exopodites and endopodites emerging from
under the cephalon. × about 6.


PLATE I.


HELIOTYPE CO. BOSTON

       *       *       *       *       *


PLATE 2.

Photographs of _Triarthrus becki_, made by C. E. Beecher.

Fig. 1. Specimen 201. The entire specimen, details of which are shown
in pl. 3, fig. 4 and pl. 4, figs. 1, 2. The dorsal test has been
removed from the anterior segments on the right side. × 4.12.

Fig. 2. Specimen 206. A small individual with the endopodites, and the
exopodites minus their setæ; well preserved on the left side. Note the
position of the antennules. The course of the facial suture is
unusually well shown. × 10.

Fig. 3. Specimen 210. The specimen which served as the main basis for
Professor Beecher's first figure of the appendages of the thorax,
specimen 206 (fig. 2, this plate) having supplemented it. Note the
"normal" position of the antennules and the extension of the
appendages from beneath the pleural lobe. Specimens with the
antennules in this position may possibly be males. × 4.

Fig. 4. Specimen 205. A small specimen with some of the appendages
preserved, especially toward the posterior end, but particularly
valuable for the unusually well preserved metastoma. × 11.

Fig. 5. Specimen 211. A small cephalon, cleaned from the ventral side,
and showing well the gnathites which approach each other unusually
closely on the median line. × 10.5.

Fig. 6. Specimen 219. An entire specimen of medium size, developed
from the ventral side. It shows particularly well the "normal"
curvature of the antennules, the change in form of the segments of the
endopodites from cephalon to pygidium, and, along the axial lobe, the
apodemes of the ventral integument. See also pl. 4, fig. 4. × 3.6.


PLATE II.


HELIOTYPE CO. BOSTON

       *       *       *       *       *


PLATE 3.

Photographs of _Triarthrus becki_, made by C. E. Beecher.

Fig. 1. Specimen 204. See also text fig. 42 and pl. 4, fig. 6. The
exopodites and endopodites of the first few segments of this specimen
are better preserved than those of any other revealing them from the
dorsal side, × 9.5.

Fig. 2. Specimen 220. A large individual exposed from the lower side.
It shows well the endopodites and part of the exopodites, and, rather
better than any other specimen, the endobases of the coxopodites. ×
2.4.

Fig. 3. Specimen 216. A small entire specimen showing considerable of
the detail of the appendages of the cephalon, and some of those of the
remainder of the body, × 7.4.

Fig. 4. Specimen 201. This figure shows the details of the appendages
of the left side and of the pygidium. Note the plate on the median
line back of the pygidium, the sockets for spines, and the terminal
spines on the anterior endopodites. See also pl. 2, fig. 1 and pl. 4,
figs. 1, 2. × 7.1.

Fig. 5. Specimen 207. One half of the posterior part of the thorax and
pygidium, showing exopodites and endopodites as seen from the dorsal
side, × 7.6.

Fig. 6. Specimen 214. The exopodites have been turned back nearly
parallel to the axis of the shell. Notice particularly the long
flattened setæ and the spinose spatula-shaped terminal portion of each
shaft. See also pl. 1, fig. 2. × 7.


PLATE III.


HELIOTYPE CO. BOSTON

       *       *       *       *       *


PLATE 4.

Photographs of _Triarthrus becki_, made by C. E. Beecher.

Fig. 1. Specimen 201. Another photograph, similar to fig. 4, pl. 3,
but showing more clearly some details of spines on the endopodites. ×
12.66.

Fig. 2. Specimen 201. Three appendages on the right side of the
thorax. See also pl. 2, fig. 1 and pl. 3, fig. 4. × 12.66.

Fig. 3. Specimen 223. A small crushed specimen which nevertheless
shows well the appendages of the right side of the thorax, developed
from the ventral side. Note coxopodites, exopodites, and endopodites,
and that all appendages are moved equally laterally from their
original position. × 11.4.

Fig. 4. Specimen 219. Another photograph, with different lighting, of
the individual shown in pl. 2, fig. 6. This print brings out better
the coxopodites and the folds of the ventral membrane. × 3.23.

Fig. 5. Specimen 222. This specimen is interesting, because it shows
the endopodites in what is probably their natural position, that is,
in a plane nearly vertical to the plane of the body, instead of being
flattened down, as is usually the case. The appendages under the
pygidium are unusually well preserved. × 12.

Fig. 6. Specimen 204. Photograph of the entire specimen of which a
part is shown in text fig. 42 and pl. 3, fig. 1. × 4.5.


PLATE IV.


HELIOTYPE CO. BOSTON

       *       *       *       *       *


PLATE 5.

Photographs of _Triarthrus becki_, made by C. E. Beecher.

Fig. 1. Specimen 209. Photograph of the pygidium shown in pl. 6, fig.
2. This specimen shows especially well the way in which the exopodites
of the pygidium decrease in length backward, × 11.5.

Fig. 2. Specimen 229. The under side of the posterior end of a
medium-sized specimen, showing the appendages, especially the
endopodites. On and among the limbs are scattered numerous minute
spheres of pyrite, of the kind usually known as "trilobite eggs." They
do not show very well in the photograph, but can be made out much more
clearly with a hand lens, × 12.

Fig. 3. Specimen 230. A specimen showing the appendages of the
posterior part of the thorax and the pygidium. The same individual is
also shown in text fig. 44. Note particularly the form of the segments
of the endopodites, and the spines on them, × 13.

Fig. 4. Specimen 227. The small doubly curved bodies shown in this
figure lie under the axial portion of the cephalon and anterior part
of the thorax. The specimen still has a very thin coating of matrix
between it and the shell. Whether the curved bodies have anything to
do with the trilobite is not known, × about 12.

Fig. 5. Specimen 221. A small individual which shows well the
exopodites of the posterior part of the thorax. Note the spatulate
terminations and the spines of the shaft, × 11.

Fig. 6. Specimen 202. Posterior part of the thorax and pygidium,
showing endopodites and exopodites projecting under the dorsal test.
Note the spiniferous plate on the median line, and the large opening
in the anterior portion of it. × 9.75


PLATE V.


HELIOTYPE CO. BOSTON

       *       *       *       *       *


PLATE 6.

All figures except 4 and 5, from photographs by C. E. Beecher.

Fig. 1. _Triarthrus becki_. Specimen 203. A well preserved small
individual, showing the appendages of the right side of the thorax. ×
11.46.

Fig. 2. _Triarthrus becki_. Specimen 209. A well preserved individual,
showing the antennules and some appendages of thorax and pygidium. For
detail of the pygidium, see pl. 5, fig. 1. × 4.

Fig. 3. _Triarthrus becki_. Specimen 218. Ventral side of the pygidium
and greater part of the thorax of an individual of medium size. Note
especially the relation of exopodites to endopodites of the last two
thoracic segments. A drawing of these appendages is shown on text fig.
43. × 4,3.

Figs. 4 and 5. Endopodites, probably from a species of _Calymene_.
These specimens, with several others, are on a small slab of limestone
from the Point Pleasant (Trenton) beds opposite Cincinnati, Ohio.
Specimen in the U. S. National Museum. Photographs by R. S. Bassler.

Fig. 6. _Acidaspis trentonensis_ Walcott. Both the specimen, No. 245,
and the photograph are poor, but show that in this genus the
endopodites are like those of Triarthrus. × 8.5.

Fig. 7. _Cryptolithus tessellatus_ Green. Specimen 234. This specimen
shows well the backward directed antennules and also the outer
segments of some of the cephalic endopodites. × 11.


PLATE VI.


HELIOTYPE CO. BOSTON

       *       *       *       *       *


PLATE 7.

Photographs of _Cryptolithus tessellatus_ Green, made by C. E.
Beecher.

Fig. 1. Specimen 233. The best preserved individual, the one from
which Professor Beecher's drawing (text fig. 45) was made, and which
served as the principal basis for the restoration (text fig. 20). Note
the long, backward directed antennules, the abrupt backward turn of
the outer portions of the endopodites, the way in which the exopodites
extend beyond the endopodites, and the fact that alt are beneath the
cover of the dorsal shield. The hypostoma is turned entirely around.
× 10.9.

Fig. 2. Specimen 235. Half of the thorax and pygidium, with the
appendages revealed from the ventral side. Note the abrupt manner in
which the outer portions of the endopodites are turned backward. See
also pl. 8, fig. 3, and pl. 9, fig. 1 (right half). × 14.45.

Fig. 3. Specimen 236. Detail from fig. 4, to show the blade-like setæ
of the exopodites and the numerous terminal spines of the endopodites.
× 30.

Fig. 4. Specimen 236. The appendages of the thorax and pygidium, seen
from the lower side. Specimen 236 is the right half of the same
individual from which specimen 235 was obtained. Note the
interarticular membranes between the segments of the endopodites and
the blade-like setæ of the exopodites. See also pl. 9, fig. 1 (left
side). × 19.

Fig. 5. Specimen 236. The same specimen, seen from the dorsal side,
showing, when the test is removed, the long blade-like setæ of the
exopodites. See also pl. 9, fig. 2 (right half). × 19.


PLATE VII.


HELIOTYPE CO. BOSTON

       *       *       *       *       *


PLATE 8.

Photographs of _Cryptolithus tessellatus_ Green, made by C. E.
Beecher.

Fig. 1. Specimen 231. A nearly complete individual, cleaned from the
ventral side and showing obscurely the hypostoma and fragments of
numerous appendages. Note the lines of appendifers along the sides of
the axial lobe. × 11.

Fig. 2. Specimen 232. Although this is not very well preserved, it
shows more of the cephalic appendages than any other. Even so, only
just enough is shown to indicate that they were similar to those on
the thorax. × 12.

Fig. 3. Specimen 235. Dorsal side of the appendages of the thorax and
pygidium. See pl. 7, fig. 2 for the ventral view. On pl. 9, fig. 2
(left side) is a drawing taken from the same specimen. × 11.

Fig. 4. Specimen 238. Part of a thorax and pygidium, seen from the
ventral side. The series of heavy segments shown in the upper part do
not belong to one appendage, but are the distal ends of several
endopodites. See also text fig. 46 for a drawing of this specimen.
× 18.

Fig. 5. Specimen 237. Pygidium and part of the thorax, with some of
the appendages. × 11.


PLATE VIII.


HELIOTYPE CO. BOSTON

       *       *       *       *       *


PLATE 9.

_Cryptolithus tessellatus_ Green. Upper drawing by C. E. Beecher;
lower drawing by Miss F. E. Isham, under the direction of C. E.
Beecher.

Fig. 1. Appendages of the thorax and pygidium, seen from the ventral
side. These are not restorations, but drawings from the halved
individual numbered 236 (right side of drawing) and 235. For
photographs of these specimens, see pl. 7, figs. 2, 4. × 20.

Fig. 2. Appendages of the thorax and pygidium, seen from the dorsal
side. Same specimen as in fig. 1. For photographs, see pl. 7, fig. 5,
and pl. 8, fig. 3. × 20.


PLATE IX.


HELIOTYPE CO. BOSTON

       *       *       *       *       *


PLATE 10.

From photographs made by C. E. Beecher.

Fig. 1. _Isotelus latus_ Raymond. Ventral surface of the specimen in
the Victoria Memorial Museum at Ottawa, Canada. Note the large,
club-shaped coxopodites and the more slender endopodites. The first
large coxopodite back of the hypostoma belongs to the last pair of
cephalic appendages. The coxopodite of the appendage in front of it is
seen turning in beneath the tip of the hypostoma. × 2.

Fig. 2. _Isotelus maximus_ Locke. The ventral side of the specimen
described by Mickleborough and now in the U. S. National Museum. The
tips of the hypostoma may be seen at the front, and the first two
pairs of coxopodites behind them belong to the last two pairs of
appendages of the cephalon. Note how much stronger the coxopodites are
than the endopodites. The appendages of the pygidium show but poorly,
× 1.45.


PLATE X.


HELIOTYPE CO. BOSTON

       *       *       *       *       *


PLATE 11.

_Ceraurus pleurexanthemus_ Green. A restoration of the ventral surface
and appendages, made by Doctor Elvira Wood, under the supervision of
the writer, from data obtained from the translucent slices prepared
and described by Doctor Walcott. × 5.


PLATE XI.


HELIOTYPE CO. BOSTON




       *       *       *       *       *


Transcriber's Notes

    Small captioned text was not converted to ALL CAPS.
    The numer 1 and capital I both look alike in the printed version.
      Therefore, some of the volume, plate and other roman numerals may
      have been incorrectly converted to 1.
    Some tables were reformatted due to space considerations.