Transcriber Note

Text Emphasis denoted as _Italics_ and =Bold=. Magnificatons as 30/1.
The » is used in Swedish for quotes and dittos. Numeric abbreviations
N:o, 1:o, 2:o, etc. for Number, primo, secundo, etc. Commas are used
here as decimal separators (ex., 0,06 = 0.06.





   KONGL. SVENSKA VETENSKAPS-AKADEMIENS HANDLINGAR. Bandet 34 N:o 8.

                              RESEARCHES

                                  ON

                           THE VISUAL ORGANS

                                  OF

                            THE TRILOBITES

                                  BY

                             G. LINDSTRÖM.

                            WITH SIX PLATS

         COMMUNICATED TO THE ROYAL SWEDISH ACADEMY OF SCIENCES

                         SEPTEMBER 12TH 1900.


                               STOCKHOLM

             KUNGL. BOKTRYCKERIET. P. A. NORSTEDT & SÖNER

                                 1901.




CONTENTS.

  Introduction                                  Page  6.
  Blind Trilobites                                »   9.
  The eyes of the trilobites                      »  26.
  On the maculæ of the hypostoma                  »  35.
  Conclusions                                     »  71.
  Explanation of the plates                       »  75.


At the outset of this memoir it must be said, that it is in fact
a joint work by the first discoverer of the remarkable hypostomic
structure to be described, Herr G. LILJEVALL, and by myself. A
considerable portion of this work belongs to him as he with great
discernment searched for a large amount of the material, prepared it
as to be fit for the many microscopic sections which he executed, and
at last with his well known artistic skill drew the figures. My share,
again, in the common work consists in the supervision of the figures,
in the arrangement of the observations and details in a coherent form,
in which suggestions and remarks of Herr LILJEVALL have been of great
use, further to write the descriptions and to make all the necessary
researches in literature, so whatever may be faulty in these respects
must be laid to my charge.

  September 12th 1900.

                                                          G. LINDSTRÖM.




INTRODUCTION.


It is well known amongst palæontologists that the hypostoma of the
trilobites has been the particular object for research and description
by a few authors, as BARRANDE, NOVÁK and BRÖGGER and some passing notes
concerning it are given in the descriptive works of other authors. In
the following pages reference is often made to them and I may here give
a list of the chief works and the abbreviations with which they are
cited below in this paper.

  ANGELIN Palæontologia Scandinavica = ANG.
  BARRANDE Système silurien de Bohéme = BARR.
  BRÖGGER Die silurischen Etagen 2 & 3 im Christianiagebiet = BR. 1.
  ---- Die Ausbildung des Hypostoms bei einigen Asaphiden BR. II.
  HALL, JAMES, Palæontology of N. York, vol. I, II, VII = H.
  HOLM Trilobiter ur Dalarnes graptolitskiffer = HM I.
  ---- Ostbaltische Trilobiten, Illæniden, = HM II.
  NOVÁK, Studien an Hypostomen Böhmischer Trilobiten. Three
     different papers between 1884-1886. = N. II, N. III, N. IV.
  SALTER Monograph British Trilobites.
  SCHMIDT Ostbaltische Trilobiten I, II, IV. = SCHM.
  WOODWARD, HENRY, Carboniferous Trilobites. Pal. Soc. 1883-84.

       *       *       *       *       *

[Illustration: _Hypostoma of Bronteus, front and side views._

_a._ anterior margin. _b._ posterior margin. _c._ anterior pair of
wings. _d._ posterior pair of wings. _e._ anterior groove. _f._
posterior groove. _g._ lateral grooves, _h._ maculæ.]

Before proceeding further I shall give a general representation of the
shape of the hypostoma. This enigmatic part of the trilobite skeleton
resembles as to its outline a heraldic shield being broad at its
anterior margin, gradually tapering towards the posterior extremity,
which generally is acuminated or, as in the Asaphidæ, bifid. In a few,
as Deiphon, Remopleurides, it is nearly square. Its exterior surface
may consist of a single field or he divided transversally into two or
three fields, according to the presence of one or two shallow grooves.
These are transverse, curved forwards near the lateral margins and may
be designated as the anterior groove and the posterior groove. Beside
these grooves there are in some instances others, one on each of the
lateral margins, parallel with these. On both sides of the anterior
margin a protuberance, sometimes of considerable length, stands out.
They are bent backwards towards the interior side of the hypostoma and
in some genera, as Dysplanus, assume the shape of long, acuminated,
halfways hollow horns. These protuberances were called »_ailes_» by
BARRANDE and it is best to retain this term, in english »wings», in
spite of its being not quite expressive. A smaller posterior pair
lies nearer to the posterior margin and these wings are so much
bent backwards, that they are seldom visible from the outside. They
are prolongations from the »duplicature» or the narrow fold of the
posterior margins round the interior side of the hypostoma.

For the rest the whole exterior surface is covered with terrace lines
running concentrically and varying according to the different genera
and even species. Many other genera, again, as Acidaspis, Calymmene,
Homalonotus, Dalmanites etc., are devoid of them and instead granulated
or smooth.

In the plurality of species there are two tiny patches or maculæ,
sometimes elevated above the surrounding surface like tubercles and so
they have also been called by some authors. But I have preferred to use
the name »macula» for them, as the plurality does not form tubercles.
They are generally entirely smooth and glossy and situated next to the
anterior groove, either above it or in it, at a regular distance from
each other and the lateral margins. They may form a sunk spot or, as
commonly, an ovoid or elliptic area surrounded by a linear elevated
border. Thus amongst the Asaphidæ. In others, again, (Bronteus, Illænus
etc.) they form a moderately elevated tubercle. The direction in which
they are oriented in relation to the longitudinal axis of the hypostoma
is quite as variable. It is to the closer consideration of the nature
of these macula that this memoir will be chiefly devoted. They have
been delineated several times, but very seldom has any accurate
attention been paid to them by previous authors. It is highly doubtful
if BARRANDE ever alluded to them, when he in his »Système Silurien
de Bohéme» vol. I, p. 156, in describing the hypostoma, says: »Cette
plaque bombée porte souvent des saillies et des empreintes creuses,
dans le voisinage de la bouche. Leurs formes varient suivant les
espèces mais en conservant toujours les caractères génériques. Nous les
considérons comme les points d'attaches des muscles et des màchoires.»
In the numerous specific descriptions in his grand work there is
never any mention made of the macula and in contemplating the figures
and reading the explanations of these, we shall find that he meant
something more comprehensive with his statement, to wit, the entire
groove where these tubercular macula are situated.

In NOVÁK'S third paper, p. 4, we see the figures of two Phillipsiæ with
tubercles on the hypostoma and he says only that in the anterior groove
there are two such symmetrically placed ... glandular intumescences.[1]

[Footnote 1: »In der Mittelfurche sitzen 2 kleine symmetrisch gelegene,
nicht immer dentliche drüsenartige An schwellungen.»]

BRÖGGER again in his paper on the »Ausbildung des Hypostoms» devotes a
page (p. 19) to explain the nature of these maculæ and regards them as
the exterior marks of two muscular impressions or points of attachment
on the inside for muscles which have attached the hypostoma to the
walls of the glabella or the head. The real nature of these so variable
tubercular maculæ is, however, a quite different one, as we hope to be
able to demonstrate in describing all their varying shapes in several
genera.

The interior surface of the hypostoma shows all the protuberances of
the exterior surface (tubercular maculæ, spines and granulations) as
shallow pits, the exterior grooves being on the contrary elevated
ridges. For the rest the interior surface is smooth and its margins are
posteriorly covered with a more or less broad duplicature, from the
lateral margins of which the posterior wings project.

The manner in which LILJEVALL became aware of the interesting
structural features of which I am going to give an account, is as
follows. We were about to describe and delineate some new or not
sufficiently known Upper Silurian trilobites from the island of
Gotland. In order to draw a specimen of Bronteus polyactin ANGELIN,
which is rarely found in the Wenlock and Ludlow strata of Gotland,
LILJEVALL was preparing and cleaning its hypostoma. The shape of this
hypostoma, as represented in pl. II fig. 22, is clypeiform, tapering
towards the posterior margin, which is regularly tongue shaped.
The anterior margin is faintly curved and at both sides elongated
in a broad, pointed wing. Its exterior surface is divided in three
transverse fields, defined through two shallow grooves, the anterior
and largest field occupying more than the moiety, the median one is
crescent-shaped and narrow, and the posterior one is nearly of the
same size and confluent with the lateral borders of the hypostoma,
which near the median transverse axis of the latter project in a blunt
angle on each side. The surface, for the rest smooth, is covered by
concentric, irregular terrace lines. On the inferior edge of the upper
groove two small tubercles are situated, one on each side closer
to the lateral margins than to the central axis. The size of their
longitudinal axis amounts to 0,96 mm. in the shortest and to 1,07 mm.
in the longest specimens. This axis is parallel with the groove and
consequently oblique to the longitudinal axis of the hypostoma. They
are oblong or ellipsoid, their inferior apices bluntly pointed or
rounded, varying a little as seen in the three specimens figured, (Pl.
II figs. 23, 24, 25). For about two thirds their surface is perfectly
smooth or rather glossy, and the lower third is covered with a compact
accumulation of small granules. The extension of this peculiar
accumulation varies, being somewhat larger in some specimens, and
the limit which runs oblique towards the glossy part is more or less
curved. Such a granule taken single is of the extremest minuteness,
measuring 0,055 millimeter at the highest. Their outline is perfectly
circular and semiglobose.

When LILJEVALL saw this group of granules he was at once struck with
the perfect, outward similarity between them and the facets of the
compound eyes on the upper surface of the head of this same species,
and I fully concurred in the same view. It now became of importance to
ascertain whether the intimate structure also corresponded in both, and
microscopic sections were accordingly prepared. Although not so clear
as in other species of this genus a vertical section of a cephalic
eye (Pl. II fig. 21) shows a median row of black spots which compared
with other sections (Pl. II fig. 9) must be the centres of the badly
preserved lenses. This is confirmed by the horizontal section (Pl. II
fig. 20) where the well known image of the polyedric facets appeared.
In the same manner the granulated spot of the hypostoma shows in a
vertical section even more clearly (pl. II fig. 27) the elongate,
coherent lenses with a black centre and the polyedric form of the
lenses in a horizontal section (Pl. II fig. 26). The assumption of a
pair of small adventive eyes on the exterior side of the hypostoma
became thus based on the clear evidence of the perfect structural
agreement between them and the eyes of the head. But it was necessary
to strengthen this evidence through extended researches on the
hypostoma of as many trilobite genera as possible and also to study the
intimate structure of the cephalic eyes in as many genera as could be
accessible.

We shall now give first a general account of the structure of the eyes
in the trilobites as far as we have been able to study them, and then
proceed to describe the maculæ of the various genera, which we have
observed.

With regard to the presence or non existence of visual organs, we must
remark that a considerably greater number of genera than those which
unanimously have been regarded as blind, also are devoid of eyes though
they still by many authors are ranked amongst the oculate ones and we
then had better first to make a review of them and thus to eliminate
them from the number of the oculate species.




=Blind Trilobites.=


In the chaos of generic forms and in the great disagreement which
prevails as to the systematizing of the trilobites of the Cambrian
time, there is a thorough revision of them highly needed by a person
having access not only to the literature, but also to the original
specimens. It is almost impossible in the present state of things to
tell with any degree of certainty how many well established genera had
been living during that period. Hence the difficulty of fixing the
systematic names of many specimens the visual organs of which are to be
described.

My researches on the visual organs of the Cambrian trilobites are
founded on the Angelinian Collection in the Swedish State Museum
together with collections of foreign species, but also largely on the
waste European and American literature, though we have to deplore the
often occurring inexactitude of the figures, especially in the older
works, and constructed or schematized figures in some of the newer
ones, which give a quite false notion of the structure. There is no
lack of figures to show how it ought to be, according to preconceived
notions and, on the other hand, a great scarcity of representations,
to show how it really is. In spite of all this there is a sufficiently
great number of well established facts to demonstrate the organization
of the Cambrian genera.

The trilobites of this division may be called blind only in so far
as they have no eyes on the upper surface of the head, but they may
have been provided with visual organs, though more imperfect, on the
hypostoma as really seems to have been the case with some of them.

According to the structure of the head shield the blind trilobites may
be subdivided into the following well characterized groups. These are:

  I. Without facial ridge:
    1) The Archæan Trilobites.

  II. With facial ridge:
    1) The Olenellidæ.
    2) The Olenidæ and related.


=I. Blind trilobites without facial ridge (= »eye lobe»).=

=Group 1. The Archæan Trilobites.=

In these the head shield is in one piece without any facial suture
and facial ridge, and without the least trace of anything that might
be called a visual organ and they must consequently be considered as
totally blind. In contradistinction to the following groups, excepting
the oldest Olenellidæ, the head consists of only three parts, 1)
glabella, and 2-3) the two fixed cheeks. These genera range from the
oldest zone in which hitherto trilobites have been found, that of
Olenellus (Holmia) Kjerulfi, to the zone of Paradoxides Forchhammeri,
and some, as Agnostus, even continue as high in the Lower Silurian
series as in the Brachiopod schists. Beside Agnostus the other genera
are Conocoryphe (seven species in the Swedish Cambrium), Toxotis,
Ctenocephalus (?), Elyx, Aneuacanthus, Conophrys and Microdiscus.

»Harpides» breviceps ANG., also belongs here. Anopocare of ANGELIN
should also be regarded as one of this group. But it cannot be retained
any longer because it is founded on two other, well known forms, being,
according to LINNARSSON, Peltura scarabæoides (pl. 27 fig. 1, a in Pal.
Scand.) and young specimens of Sphærophthalmus alatus (ibid. figs. 1 &
2).

It is remarkable that some of the Conocoryphidæ have an imperfect
facial ridge, to be compared with the commenced one in Sao BARR. I (pl.
7 fig. 9). So the American Con. trilineata and reticulate. WALCOTT U.
S. Geol. Survey 10th Rep. pt. I, pl. XCV f. 5 & 6. It is, as it were,
arrested in the development and these adult trilobites had stopped,
where the larva of Sao was proceeding in its second stage. They are
the forerunners of the blind trilobites with facial suture, belonging
to the third group. It needs scarcely be mentioned that the genera now
enumerated have hardly anything in common, beside the general character
of the head, and that real affinity exists only between Agnostus and
Microdiscus, and probably also between Conocoryphe, Ctenocephalus and
Elyx.

BEECHER[2] asserts that there is a suture in Agnostus, but in vain
we have searched for it in numerous well preserved specimens and Dr
HOLM also denies its presence. Nor are there any signs of closed up
sutures, which also could not possibly be expected in so early a stage
of evolution. It may then be taken as well settled that a fundamental
character in these the oldest[3] of all known trilobites is the total
want of a facial suture and a compactness of the whole head shield
which later is broken up in several parts through the disjunction
of the free cheeks. In the Lower Silurian formation there are a few
genera sharing in the same structure of the head shield, though by no
means else related. Such are Dindymene, Areia, Carmon and in the U.
Silurian Cromus. The two species forming BARRANDE'S genus Dindymene are
so dissimilar that Dind. Friderici Augusti had better to be removed
to a new genus and the first described one to be retained as type of
the genus Dindymene. The same is the case with Carmon, where the type
species C. mutilus is blind and without free cheeks while the other
species, known only by its fixed cheeks and glabella is one of the
Olenidæ.

[Footnote 2: Nat. Classification of the Trilobites, p. 183.]

[Footnote 3: Oldest in that sense that they are the descendants of an
archaic precambrian stock, the chief characteristics of which they have
retained in the main unchanged and persisting long ages after the close
of the Cambrian times, some, as Agnostus, continuing high up in the
Lower Silurian.]


=II. Blind trilobites with facial ridge.=

This large division embraces the second and third groups or, with a
few exceptions, all the rest of the Cambrian trilobites on account of
a feature in the cephalic sculpture common to them all, though widely
different as to its first origin in both. What forms the prominent
and common characteristic of these two groups is the presence of the
_facial ridge_, which emanates from the basis or the front of the
foremost segment of the glabella and in a great variety of different
shapes continues backwards near to the posterior border of the
head. It has received several names as _eye-line, palpebral lobe,
ocular ridge, eye-lobe, ocular fillet_ (MATTHEW). In German it is
named _Augen-leiste_, in French _filet_ (BARRANDE) and in Swedish
_ögonlist_.[4] Some authors make a difference between the more narrow
part, calling it eye line, and the thicker posterior node, which they
name the palpebral lobe proper.

[Footnote 4: That name is the most current amongst the swedish authors,
together with »_palpebrallob_»; HOLM says _ögonlob_ and _frontallob_.]

As this peculiar ridge exists before any facial suture has made its
appearance and separated the head shield in five parts, viz. the median
glabellar part, the two fixed cheeks and the two free cheeks, and
as it occurs in genera which never possessed any facial suture, and
where no eye ever was formed, it is not adequate to call it an ocular
ridge etc. the more so, as it, at least during a long series of genera
succeeding each other, has had no connection whatever with any eye. I
therefore propose to call it _facial ridge_ (in swedish _faciallist_).
It occurs on the head of almost all Cambrian trilobites, excepting
the archaic ones, and it is retained in the later Cambrian Peltura,
Sphærophthalmus etc., which have real, compound eyes, as well as in a
few Lower Silurian genera as Triarthrus, Pliomera,[5] Euloma, in the
Upper Silurian Arethusina and Acidaspis and most persisting in Harpes,
ranging from the Lower Silurian Lower Red Orthoceratite Limestone into
the middle Devonian beds.

[Footnote 5: Pl. Törnquisti HOLM.]

It presents itself in the most variable shapes, and as it in fact can
be followed through its development in the oldest known species, it is
suitable to begin its description together with the characteristics
peculiar to the oldest or second group in this large subdivision, that
of the _Olenellidæ_.


=Group 1. The Olenellidæ.=

In this group we have two families of different age, the older, less
developed the Olenellidæ proper, and the younger the Paradoxidæ.
The former consists of the genera Olenellus, Holmia, Mesonacis
and Schmidtia, and we shall attend to them first. They have no
facial suture[6] and consequently a tripartite head shield like the
Archaic ones, no eyes, but there is that strongly developed and most
characteristic facial ridge. As seen in Olenellus Thompsoni[7] the
crescent like ridge starts as a direct outflow from the base of the
first segment of the glabella, and is in direct continuation with it
as an integral part. It is roundbacked, regularly curved and at its
starting point as broad as one of the posterior glabellar segments. It
is regularly semilunate, tapers posteriorly and ends near the occipital
segment. From the narrow second segment of the glabella, inside the
just described larger ridge, a smaller ridge emanates, broader and
flatter than the former, slightly curved, and ends between the third
and fourth glabellar segment. Already in Holmia Kjerulfi the ridge
is modified. There it is only one ridge, the anterior one, nearly as
doubled through a shallow groove running along its back. The second
one has dwindled away so as to be seen only as a narrow stripe near
the occipital segment and ending outside this in a point. The anterior
ridge, represented by HOLM as consisting of two nearly independent
parts, is indeed in one piece, though its dorsal groove sometimes is
deep, and it is with its total breadth joint to the first glabellar
segment. Along its outer edge, where it lies close to the cheek, a
narrow slit runs, and I suppose that it is to be considered as the
first indication of the forming of a facial suture, which however does
not reach longer than the ridge. As in Paradoxides it is so tight to
the cheek, that there has been no place for an eye. It is no accidental
break, its edges being too regular and unbroken in all specimens. This
ridge is by far much shorter than in Olenellus and terminates opposite
the third segment.

[Footnote 6: BEECHER Natural Classification of the Trilobites, p. 191,
pretends that in Olenellus and Holmia real sutures »in a condition of
symphysis» occur. He seems to deny the facial sutures and to accept as
»real sutures» the »internal sutures» described by HOLM in Olenellus
Kjerulfi. It is highly doubtful if these interior elevated lines are
to be regarded as sutures. They are indeed no sutures, but in reality
elevated linear ridges, inclosing, as it were, narrow canals. The real
sutures known, the facial sutures, never form elevated lines, be it
on the outside of the head or on the inside. Probably these lines are
derived from some now unknown interior organization and it may be fit
to remind of the somewhat similar though more numerous linear canals of
the branchiæ on the interior surface of the great head shield of Apus.
(ZADDACH De Apodis cancriformis Anatome ... pl. II fig. 1) or what
HUXLEY (Anatomy Invertebr. Animals p. 281) calls the convoluted »shell
gland» in the carapace. A quite different structure is what I suppose
to be the incipient facial suture in Holmia.]

[Footnote 7: WALCOTT in 10th Ann. Rept. U. S. Geol. Survey, pl. 82 f.
2.]


[Illustration: Larva of Olenellus asaphoides, chiefly according to FORD
and WALCOTT.]

  1. Hypothetical figure, based on fig. 2. represents a stage
     preceding the next. There are three pair of pleura and three
     segments of the rhachis.

  2. Copy of WALCOTT'S fig. 5 in »Bullet. 1886», pl. XVII. Three
     pleura at right, two at left, the larger formed through fusion
     of the two hindmost in fig. 1.

  3. Copy of WALCOTT'S fig. 6: the second and third pleura have been
     united into one.

  4. Hypothetical figure as a further development, following upon
     the stage represented in fig. 3. Fusion completed between all
     posterior plectra (2, 3, 4), thus forming a single large pleuron
     composed of the three mentioned.

  5. Copy of FORD'S fig. 1, pl. IV, Amer. Journ. Science 1877,
     enlarged to the same size as Walcott's. First sign of the
     pygidium.

  6. Copy of FORD'S fig. 2 (1877) enlarged. The pygidium has been
     added to the head shield.

  7. Copy of FORD'S fig. 3, enlarged.

  8. Copy of FORD'S fig. 1 page 251 in Am. Journ. Oct. 1881, enlarged.

  9. Copy of FORD'S fig. 2 page 251. Amer. Journ. Science 1881.
     Slightly enlarged.

  10. Copy of FORD'S figure 5, pl. IV, Amer. Journ. Sci. 1877,
     slightly enlarged.

Already LINNARSSON observed the second ridge as he tells in a paper
where he describes Olen. Kjerulfi for the first time.[8] It has been
called an »ornamental» spine, but in the following we shall learn what
it really is. This ridge in connection with its spine has not been
observed in any other of the Olenellidæ, at least not in their adult
stage.

[Footnote 8: Öfversigt Vet. Ak. Förhandl. 1871, tafl. XVI fig. 1.]

Thanks to the discovery of very early larval stages of the American
Olenellus asaphoides, which FORD[9] and WALCOTT[10] have described and
figured, we can through combination of these decipher the development
and signification of the facial ridges. To facilitate my explanation
I here join a series of cuts from the earliest stage to the more
developed, with addition of two schematic stages to complete in a
certain degree the series of WALCOTT and FORD.[11] See figures p. 13.

[Footnote 9: In American Journ. of Science 1877 p. 265 and 1881 p. 250.]

[Footnote 10: Bullet. U. S. Geol. Survey N:o 30, 1886, pl. 17 fig.
5-6.--Tenth Annual Rept. U. S. Geol. Survey, 1888-89, printed 1890,
plate 88, fig. 1, 1 a.]

[Footnote 11: In his paper on »The larval stages of Trilobites» p. 175
BEECHER gives a new figure (f. 6), original from FORD'S collection,
of the larva of Ol. asaphoides, but it is so sketchy that I cannot
with certainty make out what it means. I cannot agree with him when he
speaks of free cheeks and eyes in these and he is completely wrong when
he says that the outer pair of spines belong to the free cheeks etc.
(p. 176).]

The figure 1 has been hypothetically composed as a deduction from fig.
2, which presupposes an earlier stage of development like that in fig.
1, when there existed three or four pair of lateral appendages in
the larva. This then consists of a central portion of five segments.
The large anterior crescentlike segment does not, however, show any
distinction between a central part and lateral appendages, it is
nearly as large as the three next taken together and its backwards
bent side horns embrace the next two posterior segments and attain
with their narrow pointed tips the back of the fourth segment. The
central portion consists of five segments, when the somewhat not well
definite posterior marginal segment is taken in account. Each of these
segments excepting the fifth one has lateral appendages, those of the
second and third segment being quite as broad as the central part and
bent backwards in a curve ending in a small pointed spine. The lateral
appendages of the fourth segment are largest of all, more than double
the length of the two next in front, triangular and standing out beyond
the posterior border of the shell as a broad spine.

In fig. 2 a great change has set in. There is no distinction between
the lateral appendages of the third and fourth segments at left. These
two have been fused together, they have united, so as to make the left
triangular spine look larger than it was originally. The appendages
of the right side are still in the same state as before. But this
fusion of the lateral appendages also takes place in another direction,
as shown in another specimen (fig. 3), so that the second and third
appendages on both sides coalesce into one piece. Now it is easy to
imagine that at last a complete fusion has set in between all lateral
appendages and that instead of the original three on each side, there
is only one large piece, reaching beyond the shell as a broad spine, as
represented in the hypothetical figure 4.

In the figure 3 a progressive change is also seen in the transformation
of the first central segment. From occupying the whole foremost space
of the shell it has been lessened in size, more distantiated from the
anterior border of the shell and rounded off, more prominent and
definite from the lateral appendages, which have become narrow, though
of the same length as before. The two first central segments seem to
have been united into one.

Between the hypothetical figure 4 and fig. 5, there is evidently a
great lacuna, not yet filled up. In the interval of time the two
appendages, which we saw in fig. 4, have been much modified, the
posterior one having lost so much in bulk, and the anterior being
lengthened and stretching out beyond the border of the shell in a
narrow spine alongside the posterior one. The central segmented part
has now assumed a shape, which on comparing it with the following
stages of development makes its true nature evident and that it indeed
is what in the adult animal becomes the glabella. The meaning of the
previous stages then also is easily understood. The central segmental
piece in them is the glabella or we may, as BERNARD already has done,
call it for rhachis and the side appendages for pleura, as this little
larva represents the whole body of the future trilobite, and embodies
all its parts _in nuce_. Through the great changes which these pleura
undergo, it results, as we have seen, that two pair vanish, being
incorporated with the large fourth pair and that only two rest for a
while, the anterior one being the so much renowned frontal-lob or eye
lobe and the second one the so called »ornamental spine», which in fact
is a compound of the original second third and fourth pleura of the
corresponding segments.

It is to be borne in mind that this larva, which represents the future
trilobite in its earliest stage, is nothing but the head, or what
in the adult takes the place of the head, and especially its dorsal
surface and that it thus solely consists of the future head.

In the figure 5 (FORD'S fig. 1.) there are the first signs of the
pygidium coming and in the fig. 6 it is well developed, but the thorax
is still non apparent. In the stage fig. 6 both pleura have increased
in length and the compound one also in bulk. They project with spiny
points beside the beginning pygidium and the anterior pleura have
united across the first segment of the glabella through a narrow ridge,
which seems to cut that segment in two. The sequence in the order
of development or growth thus is first the head, then the pygidium
and last the thorax. At least it is so in these the oldest of all
trilobites. But in nearly all trilobites of which there are good data,
the head is the part first developed.

Evidently a large hiatus exists between the stages represented in
figs. 6 and 7 (FORD'S figs. 2 & 3, 1877), in the latter of which the
animal, though not adult, has had the thorax and pygidium added to
the head. The modifications in the size of the pleura are the chief
changes. The anterior pair is reduced and retired within the posterior
border of the head forming a semilunar arch joined with the occipital
ridge in an angular bend. The posterior pair is enlarged and its spine
is by and by reduced (figs. 8, 9, 10) till it quite disappears and
only the wide semicircular field between the first pleuron and the
glabella is left behind as a remnant of its dilated body. It is to
be remarked, that while in the plurality of the adult Olenellidæ all
traces of the spiny projections of the second pleuron have vanished,
they are still retained in the adult Holmia Kjerulfi, though not in
the American Holmiæ, and thus give it at the same time a more ancient
and a more larval stamp. The shallow groove along the back of its
first pleuron indicates strongly its pleural nature, as the thoracic
pleura commonly are divided through such grooves. The same peculiarity
is also observable in several of the American Olenellidæ. It is much
the same with the posterior pleuron, the pleural nature of which is
revealed through its spine, that is homologous and identical with the
spiny terminations of the thoracic pleura. We have thus through the
remarkable finds of FORD and WALCOTT combined received an explanation
of the morphological origin and nature of the facial ridge, the
so called eye lobe and found that it has nothing whatever of the
character of a visual organ. But it must be borne in mind, that these
developmental changes are peculiar only to the Olenellidæ, the origin
of the facial ridge in the later trilobites is, as we shall see, quite
a different one.

The Olenellidæ belong chiefly to the oldest of the Cambrian beds with
trilobites, and none of them has as yet been found higher up.

Next in order of evolution we have the important tribe of the
Paradoxidæ. These are preeminently distinguished from the Olenellidæ
through the well developed facial suture, which without exception in
them all runs outside the facial ridge and separates this from the
free cheek.[12] This is a great step forwards in the evolution and
establishes the fact, demonstrable also by other evidence, that the
formation of the facial suture is subsequent to the appearance of
the facial ridge. This preexisting ridge seems to have had no small
influence on the development of the suture, it checked its progress
from the front or from the sides toward the fixed cheeks and directed
its course against the genal angles. It lay as a protection for the
glabella against this disseverance, causing the separating line to run
along its outside.

[Footnote 12: They have thus a quinquepartite cephalic shield, as the
later trilobites.]

This group consists of the genus Paradoxides proper, as well as of
Centropleura, Metadoxides and Hydrocephalus, if this is an adult form
and not the larva of an unknown Paradoxides. Perhaps such forms as
»Conocephalites» Emmrichi BARRANDE, as well as Anomocare limbatum, An.
excavatum, Bathyuriscus and Dolichometopus may on account of the shape
of their facial ridges be considered as related to the Paradoxidæ.
But this must be left for coming researches to decide. Some American
Cambrian forms also share in this characteristic and may upon closer
inspection be ranged here. So Zacanthoides. In these as in the true
Paradoxidæ the facial suture follows the ridge along its whole length,
while in the trilobites of the third group the facial suture is in
contact only with the posterior end of the ridge, the so called eye
lobe. Remopleurides does not show characters, that as BEECHER thinks,
could unite it with the Paradoxidæ. These are blind and Remopleurides
has well developed eyes and an organization that gives it an isolated
position in the system.

The facial ridge continues in a great variety of shapes, short or long,
but always forming the fraction of a circle, always of nearly equal
thickness, only slightly tapering towards one of the extremities, and
always when in direct connection with the glabella, starting from the
base of its foremost, largest segment. As a rule the ridge is more
developed in the young or larval individuals, continuing from the
glabella to near the posterior cephalic border in an uninterrupted
arch[13] quite as in several of the adult Olenellidæ of the oldest
Cambrian. It can be taken as granted that its origin is the same as
in the Olenellidæ though at present the only evidence at hand is the
small larva of Paradoxides oelandicus, which LINNARSSON called Parad.
aculeatus.[14] In this we see the anterior pleuron or the facial ridge
alone present, elongated downwards like the same pleuron in the figure
6 of Olenellus asaphoides and terminating like this in a fine spine
stretching backwards outside the posterior border. Of the second pair
of pleura there is nothing to be seen. This must then have been aborted
at an earlier stage than in the Olenellidæ.

[Footnote 13: G. F. MATTHEW had before me, as I now find, pointed out
this distinction in his memoir »Illustrations of the Fauna of St. John»
N:o IV, p. 163. When he speaks of »the embryonic stage» in this and
other passages he evidently means »larval stages», as the embryonic
stages of necessity must remain unknown to us. (Later remark.)]

[Footnote 14: Om faunan i lagren med Paradoxides ölandicus (1877), p.
359, pl. 14, f. 11.]

The connexion between the free cheeks and the middle part of the head
has been very lax not only in the Paradoxidæ, but on the whole in
nearly all Cambrian trilobites with free cheeks. When the free cheek
is dissevered it shows no trace of the ridge, there is only a large
scallop on the spot where it embraced the ridge. In the Paradoxidæ the
rim of the indenture and the ridge are in so close contact that there
is not the least place for an eye between them, as can be seen in the
few specimens with a complete head. In all oculate trilobites again
without any exception the facial suture separates that part of the eye
which is the real visual organ with corneal facets, from the interior
often elevated portion, opposite it, the so called palpebral lobe. The
eye is _always_ placed on the free cheek,[15] the lobe again always
on the fixed cheek of the head shield. No real eye exists without the
palpebral lobe, and, on the other hand, that part of the facial ridge
which later in the development changes to a palpebral lobe, occurs
alone without any eye in a great number of Cambrian trilobites, and
consequently these are blind and such is the case with the Paradoxidæ
and a great number of the succeeding.

[Footnote 15: Excepting in Harpes, which has no free cheeks.]

There is not the least evidence to support the suggestion that the
»ocular ridge» is homologous, with the eye of Apus[16] and that the
real crystal cones lay sunk beneath the surface in a »water sac». As
we, for instance, in Peltura have an »ocular ridge» (= facial ridge
_mihi_) on the fixed cheek and opposite its posterior extremity, the
»eye lobe», a real eye with facets on the free cheek it is not likely
to suppose that the »ocular ridge» nor the »eye lobe» ever functioned
as a visual organ or that two widely different sorts of eyes were
placed in closest vicinity opposite each other.

[Footnote 16: BERNARD, The systematic position of the Trilobites. Qu.
Journ. Geol. Soc. 1894, p. 411.]

It may be worth while here to remind of the great similarities, whether
analogous or homologous, in the formation of the superior surface of
the head in the trilobites and the embryons and the newly hatched
larva of Limulus. The latter have the head shield separated into
five portions, partly through a facial suture which, as in Peltura,
Dalmanites and others, forms a continuous line around the glabella.
This suture divides the eye node in a similar way as in the trilobites,
that is, sectioning it in two parts, of which one adheres to the
central fixed cheeks and the other to the free cheeks.[17] And to judge
by the figures of KINGSLEY the former, the whitish moiety is the first
developed and sometimes for a while quite alone as the facial ridge of
the trilobites and probably also anterior to the suture, as this is not
complete at this stage. This white node reminds of the small facial
ridge in Arionellus ceticephalus BARR.

[Footnote 17: PACKARD, Development of Limulus polyphemus. Memoirs
Boston Soc. Nat. H. vol. II, pl. V, fig. 25. Nothing is said about the
exterior structure of the eyes.

DOHRN, Zur Embryologie and Morphologie des Limulus Polyphemus.
Jenaische Zeitschrift 1871. A very good figure (pl. XIV, f. 4) shows
clearly the two parts of the eyes, the interior one being larger.

KINGSLEY also (Devel. of Limulus, Journal of Morphology, vol. 7, 1892,
pl. VI, fig. 34) has in the last larval stage the suture and the eye in
two parts, of which one is white lying inside the suture and the eye
proper, black, outside.]

As KISHINOUYA[18] has already pointed out and as I have anticipated
above the head of the most developed trilobites in their adult state,
and the head of the larval Limulus consists of five parts, viz. 1)
the glabella in the centre, 2 & 3) the fixed cheeks, 4 & 5) the free
cheeks. An elevated ridge in the adult Limulus shows where the suture
once lay and it is on the outside thereof that the eye of the adult is
placed. What other authors call the ridge or the eye ridge in Limulus,
KISHINOUYA rightly names a suture.

[Footnote 18: Journal College of Science Tokio, vol. V, p. 53, 1892.]

It was the renowned Swedish naturalist WAHLENBERG, who first
recognized the importance of the facial suture, which he called »linea
ocularis»,[19] but to another Swedish palæontologist DALMAN[20] the
exact definition of this suture is due, to which he gave the name
still in use. He expressly remarks that the suture crosses the ocular
node and limits the outside of the »lobus palpebralis» and he makes a
clear distinction between that lobe and the »tuberculi and eminentiæ
oculares» (= facial ridge) of which he says that they are the more or
less evident elevations situated in the _blind_ Palæades on the place
of the eyes (in which he is wrong) »and which perhaps are an indication
of such organs». But then he says doubtfully (p. 255) »tuberculorum
ocularium veram naturam determinare haud ausi sumus, etsi oculorum
formam sat bene exhibere videantur» and he adds concerning Paradoxides
»oculi nulli, eorum loco autem tuberculi duo».

[Footnote 19: Additamenta quædam ad petrificata telluris Suecana, in
Acta Upsaliensia, vol VIII, 1821, p. 294.]

[Footnote 20: Vet.-Akad. Handl. 1826 p. 126.]


Group 2. The Olenidæ and related families.

Next in the ascending order we have the largest group of trilobites in
the Cambrian, of which the greatest part is formed by the Olenidæ. A
facial ridge different in shape and different in development from that
in the former group characterizes them. Unlike the semilunate ridge of
the Olenellidæ it issues mostly from the front of the first segment
of the glabella and goes generally backwards till it meets the facial
suture. It is narrow and fine as a thread, but for the rest assumes
a great variety of forms. It may be curved as a circle segment as in
Sao, Liostracus, it may be long and straight, standing out in a right
angle to the glabella as in Eurycare, it may be short and straight in
an acute angle to the glabella as in Parabolina and so forth. In the
same genus, as for instance Olenus, straight and curved ridges occur
in the various species, and consequently the form of this sort of ridge
cannot be used as a generic character. It moreover differs from the
ridge in the Olenellidæ and the Paradoxidæ in having the posterior
extremity widened as a tubercle or node, which commonly has been called
the »palpebral lobe», while in the older groups the ridge is only at
the most a little pointed in both ends or of equal thickness.

Some persistent archaic genera as Conocoryphe and Elyx inform us how
this ridge has originated and how widely different it is from that
in the Olenellidæ. On the surface of their head shield no ridge is
visible, but on the interior side of it we behold, as the figures show
(Pl. VI, figs. 43 & 44) on each side of the glabella a ramifying system
as of the most minute vessels, which spread and cover the whole surface
of the cheeks. This reticulation issues from a main trunk that goes
nearly straight in an oblique direction from the foremost segment of
the glabella and emits narrow branches forwards and backwards on its
both sides. And these branches go on dividing till they occupy the
entire interior surface of the fixed cheeks. The figures representing
this are taken from Elyx laticeps fig. 43, and an almost similar image
has been obtained from a species of Conocoryphe fig. 44.

Now the question may be asked, what does this network of branching
canals mean? I think we cannot gain a more plausible answer than that
given us through the inspection of the soft parts that lie hidden
behind the glabella of Limulus and its fixed cheeks, the only living
crustacean which offers the greatest homologies with the head shield
of the trilobites. Next below the shell of the glabella and the cheeks
of Limulus there is a complicated stratum of muscles and behind this
the heart and the great central circulatory system spreading from that
centrum towards the sides of the head shield, the vessels being the
more fine and minute, the more they are elongated from the centre. I
now suppose that likewise in these trilobites the narrow and prominent
glabella has been the receptacle not only of a strong mass of muscles,
but also for the central part of the circulation. This centre has there
sent out two relatively strong ducts or canals, one on each side of
the glabella feeding all soft tissues inside and near the cheeks and
probably also other important parts of the body. It is also clearly
seen in Elyx how the orifice of the main trunc opens in the hollow of
the glabellar apex.

In all crustaceans, as far as is now known, their more or less hard
calcareous or chitinous skeleton is moulded by the subjacent tissues
and glands. The sculpture of the surface consequently is an outcome
of the fashioning procedures showing what has been going on below and
what is still going on. If we then on the exterior surface of the head
shield of the trilobites see the radiating lines in relief we must
conclude that they are due to a subjacent system of almost capillary
vessels causing hollow impressions on the inside and elevated ridges
exteriorly. In Elyx the vessels have made no strong impression as to be
visible on the outside. In a couple of American Conocoryphæ again (C.
reticulata and C. trilineata) the main trunc of the vessels has formed
a short faintly elevated ridge. In Solenopleura (Pl. VI, f. 45) the
facial ridge has been fully developed, and by the casts of the inside
it is clearly seen that the main trunc of the vessels makes the inside
of the ridge and has been much incrassated. On the other hand the
smaller branches issuing from it have been reduced in size and number,
but are still visible. Thus, if I am right, the two, the facial ridge
and the main trunc, are in causal connection and the former has been
moulded by the later, when it had gained in size sufficiently, and when
a richer affluxion of nutritive fluid was directed backwards towards
the point where the eye at last originated. The further changes in this
ridge thus are related to the development of the eye. As far as I am
aware, there are as yet no data to tell us whether the appendages or
pleura of the glabella in the larval (or primordial) Olenellidæ have
the same origin as the ridge of the Olenidæ or are homologous with it.
Perhaps it may be so in respect to the foremost one, which also is
persisting. It is, however, not known how the pleura in the olenellid
glabella have been developed, whereas it is well ascertained by the
growth of the glabella in Sao, Liostracus and others that such pleura
never have been developed in them.

What the phylogenetic evolution has taught us concerning the formation
of the facial ridge is confirmed by the ontogenetic development
of individuals of some species as for instance Sao hirsuta BARR.,
Liostracus sp. BRÖG. and others.

This development can be followed in detail through the excellent
figures of the larval stages of Sao hirsuta which BARRANDE has given
in his magnificent work, vol. I, pl. 7. In the _first_ stage figured
(figs. 1 d-e) the whole animal consists only of the head shield which
is completely smooth, the glabella scarcely segmented, no facial ridge,
no facial suture. In the _second_ stage, according to me (figs. 2-4
a), the glabella has become distinctly segmented, and the pygidium and
partly the thorax have been added. In the _third_ stage, 4 c, d-9, one
small narrow string exits from each side of the front of the glabella,
making the first faint beginning of the facial ridge. They form a right
angle with the glabella. In continued growth they become by degrees
a little more curved (figs. 5 c, 6 b) and the lengthened ridge bends
parallel to the outer lateral margin of the head (fig. 9 b). In fig.
9 d it has become so far complete that it reaches nearly back to the
posterior cephalic margin, but is still of the same narrowness all
along. In the _fourth_ stage, in a specimen (fig. 10 a) of 3 mm. in
length, the facial suture makes its first appearance, setting in from
the posterior margin of the head and meeting in its forward progress
the posterior extremity of the facial ridge which now begins to swell
out. It seems that both have a mutual influence on each other, the
suture being deviated from a straight course[21] to take a bend outside
the ridge, and the extremity of the ridge again at this contact to
increase in size so as to form the elongated tubercle, often called
palpebral lobe (fig. 14 b). This now augments in the same rate as the
whole body. The characters of the four stages of the development of the
larva consequently are: 1:o The archaic stage, only head shield with
ridgelike glabella. 2:o The coming and growth of the pygidium and the
complete segmentation of the glabella. 3:o The coming and growth of the
facial ridge. 4:o The coming and development of the facial suture.

[Footnote 21: In Trinucleus and Ampyx where there is no facial ridge,
the suture has a straight direction along the lateral margins.]

BARRANDE regards the whole ridge as a prolongation of the eye and
the tubercle at its posterior extremity as the eye itself.[22] But,
again, in page 399 he says »Sa surface (of the eye) est toujours mal
conservée, pour nous permettre de voir si elle était réticulée.» And
he also confounds the ridge, »filet» as he names it, with the eye
itself, and the first faint beginnings of this ridge in his fourth
stage he considers as the eye.[23] By a partly schematical figure
of the free cheek (fig. 25) he places the eye on this cheek, and in
the same manner in fig. 29, »restaurée d'après divers fragments» he
figures a reticulated surface of the eye on the free cheek, outside the
tubercle. I have sought for a reticulated surface on sufficiently good
specimens, but never found any, and I must consider Sao as one of the
blind genera. BARRANDE himself also in the table on the eyes of the
trilobites places Sao in the group with »Surface visuelle inconnue», p.
131.

[Footnote 22: Page 383 »l'œil argue est prolongé par un filet en
relief, vers le front de la glabelle».]

[Footnote 23: p. 389.]

[Illustration: (0 is new, added here to supplement fig. I.)]

BRÖGGER has also succeeded[24] in finding a series of small larva,
which he considers as belonging to a species of Liostracus. As the
figures drawn by BRÖGGER twenty five years ago may now be very little
known, I here reproduce them with the kind permission of the Editors
of »Geologiska Föreningens Förhandlingar», where they were published
in 1875. This development proceeds nearly upon the same plan as in
Sao. The first stage, however, (I) seems to be much earlier than any
of Sao, the rhachis or glabella consisting of an unsegmented ridge of
more primitive appearance. Before this first stage of BRÖGGER'S larva a
still older phase of development can be imagined, (0) a simple rounded,
smooth head shield without any indication of a glabella at all. This
stage might correspond to the head of certain species of Agnostus, as
A. glandiformis, A. nudus, which have no glabella. This stage 0 is
also valid for the larva of Sao. Several stages are evidently wanting
between I and II, in the later of which the thickened glabella is
divided in four segments. In III we have six segments, all these three
stages consisting only of the ovate head with narrow fixed cheeks. In
IV the pygidium has been added to the primitive head, but the segments
of the glabella have been reduced to four and in V slightly altered in
shape In VI, again, we see the head with five glabellar segments and
scarcely the first sign of the facial ridge. Between VI and VII there
must be links missing, as the change can not be so abrupt, and likewise
between VIII and IX as in VIII there are still no free cheeks nor any
facial ridge. This interesting discovery of BRÖGGER confirms, together
with those of several other authors[25], the supposition that the
development of the later Cambrian and older Silurian forms is a quite
different one from that of the Olenellidæ and the Paradoxidæ. They have
a rhachis, but no pleura proper, as the single facial ridge has a
quite different signification and appears at a comparatively much later
stage than the facial ridge of the Olenellidæ, which is present from
the earliest stages known.[26]

[Footnote 24: Fossiler fra Öxna og Klettna, Geol. För. Förhandl. 1875,
p. 572, pl. 25, fig. I-X.]

[Footnote 25: Foremost among these stands MATTHEW in »Illustrations
of the Fauna of St. John» IV, where he, p. 143, pl. II figs. 1 f.
etc., describes a few stages very like those given by BRÖGGER, so the
glabella as an unsegmented, narrow ridge etc.]

[Footnote 26: A deviating form of the ridges is shown by »Liostracus»
tener HARTT, Acad. Geol. 2d Ed. p. 652 (see also MATTHEW Illustrations
of the Fauna of St. John [1887] p. 132 p. 1 f. 3 a-3 c). Beside the
usual facial ridge there is a second pair of ridges between the first
and the glabella, arching in an opposite direction. See also WALCOTT,
Bulletin U. S. Geol. Survey, N:o 10, 1884 pl. V, figs 6, 6 a, 6 b, new
figures and copy of HARTT'S description.]

This form of facial ridge, although prevalent in the middle and later
Cambrian times, dates back so far that genera coeval with Holmia, viz.
Ellipsocephalus and Arionellus show it along the facial suture. This
early occurrence of the ridge coeval with the less developed Olenellidæ
leads to the assumption of a different origin of these trilobites as a
branch, which already far back in the oldest Cambrian or precambrian
times had deviated from a common ancestor and I have tried to give a
view in tables of these two coordinated lines of evolution further
on, at pages 24 and 25. The most remarkable genera which, as far
as I have found, belong to this fourth group are Ellipsocephalus,
Arionellus, Liostracus, Olenus, Leptoplastus, Parabolina, Corynexochus,
Parabolinella, Sao, Ptychoparia, Doropyge, Oryctocephalus etc.

I have not been able with an absolute degree of certainty to recognize
whether some of these genera now enumerated, have been oculate or
blind like Sao, like the Paradoxidæ and similar. The precarious state
of preservation prevents all definite conclusions in that respect. It
seems, however, that the evidence gathered through the examination
of numerous specimens rather points in a negative direction. As the
free cheeks in these old Cambrian trilobites have been in a very loose
connection with the fixed cheeks and generally deciduous, contrary
to the condition in the Silurian ones, it is in many instances very
difficult to tell whether species with facial ridge, especially those
from the earlier Olenus schists have been blind or provided with eyes.

The order of succession of the genera in the Swedish uppermost
Cambrian, in the Olenid slates is, according to S. A. TULLBERG'S
researches[27] at Andrarum in Scania as follows:

  1. Parabolina (oldest division of the Olenus slates).
     Olenus.
     Liostracus.
  2. Eurycare.
     Leptoplastus.
  3. Peltura.
     Sphærophthalmus.
     Ctenopyge.
  4. Cyclognathus (uppermost).
     Acerocare.

[Footnote 27: Om Agnostusarterna vid Andrarum.]

It is already in the second division that we find the earliest oculate
genus in Eurycare and in the higher strata. Sphærophthalmus and
Ctenopyge with enormous hemispherical eye balls, while Olenus and
Parabolina, their earliest predecessors, probably were blind. We
have not amongst hundreds of specimens of these genera found a single
specimen showing an ocular globe covered with facets. Olenus and the
nearly related genus Parabolina are found in innumerable specimens
in the thinly laminated alumschists of Scania and other provinces of
Sweden. But rarely a perfect head shield, or nearly so, is to be found
with the free cheeks in place. If so, the semicircular scallop in the
free cheek is entirely filled up by the posterior lobe of the facial
ridge and there is no place left for any eye.--If we, again, find a
non compressed glabella and fixed cheeks likewise, the facial lobe
(-»eye-lobe») is in some elevated so much as to leave a little space
between it and the scallop of the free cheek, which space must have
been an empty lacuna if not filled up by an eye ball. But it may also
be that the free cheek has been somewhat put out of its order and that
consequently some space has been left between it and the free cheek.
It is quite as much with older genera, as Solenopleura especially,
in which the posterior lobe of the facial ridge (_vulgo_ »eye lobe»)
has attained a great development, and in which one just could expect
to find a sphærical eye resting between the elevated lobe and the
scallop. The elevated rim of this scallop does not in fact constitute
a proof for its having clasped an eye as the elevated scallop in
Sphærophthalmus did. In Paradoxides, again, in Ellipsocephalus, where
there is absolutely no trace of an eye ever having been present,
whenever you succeed to find the free cheek in juxtaposition with the
fixed cheek it is evident that the elevation of the scallop rim is
due to the impact of the posterior lobe of the facial ridge. In many
specimens of Dolichometopus and Corynexochus etc. no free cheeks have
ever been found and to judge by the shape of the facial ridge it may be
concluded that these also were deprived of eyes.

On seeing this great number of trilobites, that on account of their
organization must be considered as blind, the first suggestion that
strikes the mind, is that they must have lived in abyssal depths of the
Cambrian sea, where the most intensive darkness prevailed. Nor does
the nature of the strata contradict such an assumption as evidently
this fine sediment must have been deposited far beyond the reach of the
influence of the wave motion, a depth amounting to more than a thousand
metres as now calculated, for else it could not have preserved unbroken
such delicate parts of organisms as that free cheek of Ctenopyge pecten
with its extraordinary long and delicate horn, figured on plate III
fig. 27 and many others. But it seems incredible that such a state of
things should have prevailed during the deposition of all the Cambrian
strata, although they in Sweden amount to only 160 feet in thickness,
according to the evaluation of S. A. TULLBERG, not considering what has
been lost through denudation. The length and duration in time can in
this instance not be measured by the thickness of the beds, but by the
great changes in the faunas which there have succeeded one another. The
physical conditions, to judge by the composition of the rocks, seem in
the main to have lasted during immeasurable periods and still the fauna
has changed in no little degree. During that enormous length of time,
embracing in Sweden eight well separated periods, there must, however,
have been minor changes in the conditions of depth and consequently
in the nature of the depositions. To a certain extent the physical
agents must have influenced the organization of the animals, but not
essentially. There are sure evidences of another factor being the chief
agent and that is the evolution.

We have from the lowest Olenellus beds to the lowest Lower Silurian
strata followed the clear traces of the changes from the eyeless
Olenellidæ past the Paradoxidæ with the facial suture to the great
tribe of the Olenidæ in which the facial ridge with its lobe is so
prominent, and to the oculate genera at the top of the formation.

As stated above there seems to be evidence enough for accepting two
different lines of evolution in these the oldest trilobites. For the
oldest, Olenellus is the type and for the second Sao may be taken as
the representative.

These two evolutional lines could be represented with their phases in
chronological succession as follows:


A. =The Olenellus line.=

1. The trilobite consisting only of the head with rhachis and pleuræ,
no sutures, no ridge, no eyes. Adult forms precambrian. Corresponding
larval forms (Olenellus asaphoides) in the Lowest Cambrian. Still older
and simpler forms may be presupposed as preceding these.

2. Semilunate facial ridge on the tripartite head-shield, no sutures,
no eyes, but a hypostoma fused to the rostrum of the head and provided
with maculæ. The adult animal with thorax and pygidium in the oldest
Cambrium, the Olenellus zone. Genera: Olenellus, Holmia (with beginning
suture), Mesonacis, Schmidtia.

3. The head quinquepartite with facial suture, short semilunate ridge
in the adult, a long ridge in the young, no eyes. The genera occur
in the Paradoxides-zones of the Cambrian formation. These genera are
Paradoxides, Centropleura, Metadoxides, Hydrocephalus, which are direct
descendants of the Olenellidæ.

This line of evolution, in which the species never acquired eyes, was
extinct in the sixth zone of the Swedish Cambrian, Centropleura Lovéni
being the last.


B. =The Sao line.=

I call this so because we see in the development of Sao, as represented
by BARRANDE, most of the different' phases in evolution, which the
ridge bearing trilobites of this second group have experienced during
the Cambrian times.

I.

The body consists only of the head, neither pygidium, nor thorax yet
developed.

1. _Psilocephali_ (ψιλος, bald), primeval, precambrian, adult stage not
found, supposed to be like the head shield of Agnostus glandiformis and
A. biplicatus quite round and bare without any glabella nor sutures.
The corresponding stage in development also wanted.

2. _Glabellate._ The entire animal only head with a mesial ridge, which
at first is entire or unsegmented and later is metamorphosed into the
segmented glabella.

a. _unsegmented._, Precambrian, adult form unknown, supposed to be
like the first larval stage of Liostracus according to the figure I
of BRÖGGER and also as the head of Agnostus parvifrons LINNARSSON.
Corresponding larval stages Liostracus (I).

b. _segmented._ Probably precambrian, not represented as adult in the
Cambrian strata. Supposed to be like the larval forms II and III of
BRÖGGER'S Liostracus, consisting of the round or oval head shield and
the segmented glabella. Also BARRANDE'S first stage of Sao (BARR., fig.
1 pl. 7).

As a transition to the next phase forms may be imagined having head and
the beginning pygidium, nearly as Agnostus atavus, minus the thorax.
The corresponding larval stages are Liostracus (BRÖGGER'S figures IV,
V, VI) and Sao (BARRANDE, pl. 7 fig. 2-4 a, b). Nearly so, though the
pygidium is more developed, are the larva of Agn. bibullatus and Agn.
nudus (BARR., pl. 49) both without thorax.

II.

These have the three chief parts of the body developed.

3. _The facial ridge._ Cambrian, adult with glabella and facial ridges,
short or long, emanating from the top of the glabella, thorax and
pygidium. Several American Conocoryphæ. Corresponding larval stage,
Sao, stage 3, BARR. pl. 7 figs 4 d-9.

4. _Suture._ A suture dividing the fixed cheeks in two pair viz.
two fixed cheeks and two free cheeks. Fully developed facial ridge.
Oldest known adult forms are Ellipsocephalus, Arionellus, already in
the Lowest Cambrian, the Olenellus beds, what presupposes a long,
antecedent lineage far back in the precambrian times. Corresponding
larval stage in Sao, BARR. pl. 7 figs 10-13. The plurality of Cambrian
trilobites belong here. An intermediate stage leading to the next is
seen in such forms in which the ridge posteriorly is widened into the
»eye lobe», which rests in the scallop of the free cheek. So it is in
Liostracus and many others besides. Solenopleura possibly oculate.

III.

5. Globular eyes. Cambrian, in the youngest zone, the Olenus schists.
The oldest at present known oculate trilobite Eurycare is found in the
second division of these schists.

In the lowest Lower Silurian division, the Ceratopyge limestone, Euloma
and Ceratopyge occur as the last survivors of the blind, partially
ridge bearing genera. The Trinucleidæ and Ampyx belong to another group
of trilobites.

Even among other exclusively Lower Silurian genera, in which the
plurality of the species is oculate, there are species entirely blind.
So with Illænus, in which genus Dr HOLM has not found any eye in Ill.
Angelini, I. leptopleura and Ill. cæcus. The free cheek in these three
species is much narrow, as the facial suture lies near the margin of
the head.




=The eyes of the Trilobites.=


If, as is probable, to judge by the conformity of their cornea with
that of recent crustacea, the trilobites like these were provided with
crystalline cones beneath the corneal lenses or facets, only the latter
have been preserved in a fossil state. Although the crystalline cones
in consequence of their solid consistence might have been petrified
as well as the cornea, they must, imbedded as these tiny cones lie,
entirely wrapped up in delicate tissues, fall away and be lost, when
the dissolution of the dead body had set in. Consequently the curious
appendages on the inferior side of the lenses in Dalmanites vulgaris
(Pl. III f. 50) or Phacops quadrilineata (Pl. V fig. 38) can noways
be considered as belonging to the original structure of the eye,
apart from their great dissimilarity with anything appertaining to
the eyes of the Arthropoda. The cornea on the contrary cohered with
the integument of the body, and it has been well preserved in a great
number of trilobites.

I subdivide the trilobites in respect to the form of their eyes in the
following manner:


=I. Genera with compound eyes.=

1. _With prismatic plano-convex cornea facets._

  Acidaspis, as a transitional form to the next group.
  Asaphus.
  Bumastus.
  Cyphaspis.
  Dysplanus.
  Encrinurus.
  Illænus.
  Megalaspis.
  Nileus.
  Phillipsia.
  Niobe.
  Proetus.
  Ptychopyge.
  Symphysurus.

2. _With round or biconvex transversally elongate lenses._

  Acerocare.
  Bronteus.
  Chirurus.
  Ctenopyge.
  Cyrtometopus.
  Eurycare.
  Peltura.
  Sphærophthalmus.


=II. Genera with aggregate eyes of biconvex lenses.=

  Acaste.
  Chasmops.
  Dalmanites.
  Phacops.


=III. Genera with isolated eyes, one or several stemmata at the
extremity of a straight facial ridge.=

   Harpes.
   Harpides.
  (?) Trinucleus in the larval state.

Only a few authors have before now occupied themselves with the
intimate structure of the trilobite eye. PACKARD gave in 1880, in the
»American Naturalist» a note on the structure of the eye of trilobites
(p. 503). There are some rough and inexact sketches of the eyes of
Limulus and Asaphus, and although he seems to have known the beautiful
researches of GRENACHER he still »claims that the trilobite eye was
organized on the same plan as Limulus». This statement is altogether
wrong, and as I hope to show the trilobites have had eyes entirely
different from that of Limulus and instead agreeing with those of the
Isopoda and perhaps also with a few other Crustacea. In 1889 J. M.
CLARKE published an account[28] on the »Structure and development of
the visual area in the trilobite Phacops rana GREEN». The aggregate eye
described by him are of the type forming my third group. His holochroal
division embraces my first and second groups and the schizochroal my
third.

[Footnote 28: Quart. Journ. of Morphology, vol. II p. 253.]

The latest contribution to the knowledge of these eyes is found in
EXNER'S »Physiologie der facettirten Augen von Krebsen and Insecten»,
1891, where he gives good figures of the lenses of Phacops fecundus,
pl. II figs 18, 19. He says that the palpable difference in the
structure of these eyes and those of Limulus point to a change in the
function of these eyes.


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. In the plurality it is, however,
difficult to discern the lenses from the outside.

The lenses, as seen in a vertical section of the eye of Asaphus
expansus, (pl. I fig. 12), are columnar prisms, like the pillars of
basalt, attaining a length of 0,2 mm and at the point where the eye
joins the test of 0,3 mm. At their interior extremity they have a
breadth of 0,066 mm. On that point the surface is convex and at the
exterior surface plane. They are closely packed and in a transverse
section resemble a pavement of regular hexaeders. But they also assume
other shapes and become rhombs or even quadrates, as seen in a specimen
of Asaphus fallax (pl. I fig. 18), where the hexaeders and quadrates
lie side by side without transitional forms. As a rule the lenses
become more and more irregular in the vicinity of the surrounding
frame or near the suture, nearly blotted out, as it were, and without
any definite border line mingled with the confused, spongy mass that
like a belt or a frame surrounds they eye in Asaphus and is sharply
limited from the other part of the free cheek. This remarkable zone
which is almost only present amongst the Asaphidæ (Asaphus, Megalaspis,
Ptychopyge, Isotelus) retains in a confused manner somewhat of the
prismatic structure of the eye as shown in the section (Pl. I, fig. 11,
b). The eye of Bumastus also is environed by a similar zone, with a
structure like that of the eye (Pl. II fig 35, 41).

In an undetermined species of Asaphus the lenses, although somewhat
apart, are of an elongated hexaedric outline, which passes into a
regular circular one farther away and on the surface of the eye they
are slightly convex (Pl. I figs 27-29). In other genera belonging
to this group the shape of the lenses are like those of Asaphus, so
for instance in Illænus (I. chiron and I. Esmarki) and in Niobe. In
Dysplanus centrotus they are shorter and broad, and their interior
or lower surface strongly convex. It is likewise so in Nileus, where
Nileus armadillo has an exceedingly thick exterior integument above
the lenses. Such an integument has in a still higher degree increased
in Bumastus sulcatus so as to exceed in thickness the stratum of the
corneal prisms and it may in fact be doubted if the eyes of this
species ever were able to function as visual organs. Proetus nearly
resembles Bumastus in the thickness of the integument covering the
prismatic lenses, which are interiorly convex, with a diameter of 0,03
mm.

In all genera belonging to this group a horizontal section gives the
image of the hexaeders as in Asaphus with some change to squares or
rhombs.

In scrutinizing a horizontal, somewhat extensive section of an eye in
this group of trilobites, it will be perceived, as for instance in the
figures (Pl. V fig. 16, 22) that the regular and evidently homogenous
and intact prismatic lenses by and by have been altered and in a part
of the section, a little distant from the intact ones disintegrated
in their interior, showing various aspects of alteration. I cannot
but think that this is a destruction which has set in long after
the fossilization. It has revealed certain states of the intimate
structure, certain delicate details, that now with an astonishing
regularity come in sight and probably also lie hidden in the intact
prisms. In the specimens of Asaphus, which we have studied, the
alteration has taken the shape of a concentric stratification forming
the body of the prisms, which is well discernible in a horizontal
section, but not easy to detect in the longitudinal one (Pl. I figs
9-10, 11). It is likewise so in Niobe. In the other genera again the
decomposition makes the prisms look like empty tubes in which a few
irregular traverses and trabecular remains of their solid mass radiate
towards the interior. They thus assume the aspect of a composite coral
with its septa in the calicles (Pl. VI fig. 31). This is also evident
in Nileus palpebrosus and Dysplanus.

=2. Eyes with biconvex lenses.=

The surface of the eye is, as in Chirurus glaber ANG., a mass of
contiguous hemispherical lenses, probably once covered with a membrane,
as is still to be seen in well preserved specimens of Bronteus
laticauda. Both in Chirurus and Bronteus the lenses seen vertically are
globular and ordinated beside each other either continuous or separated
only through a faint dividing line. In a horizontal section passing
right through the point of contact they show the common hexaedral
shape and when somewhat corroded the interior radiate structure also
comes forth, the radii directed towards a little black point in the
centre. The lenses of the Brontei have the same stellate structure
as in Bumastus (Pl. II fig. 7). In Cyrtometopus the lenses are in
size the fourth of those in Chirurus and they form an extremely
thin stratum in strongest contrast with the adjoining cheek, which
surpasses them more than six times in thickness (Pl. III fig. 19). The
lenses of Cyrtometopus are more flattened and irregular than in the
former genera. The free cheek around the eyes does not form a border
zone, somewhat imitating the eye structure as in Asaphus, but is more
compact, composed of vertical elements which give to the test of the
trilobites in general a tendency to split up in vertical prisms.

Of a peculiar interest are the eyes in the oldest of all oculate
trilobites, at present with certainty known, Eurycare, Peltura,
Sphærophthalmus and Ctenopyge.

Of these genera Eurycare is the oldest (see table p. 22). Amongst the
many free and detached cheeks only a single, very little one has been
found with the eye ball fixed. It seems to be of the same structure as
in Sphærophthalmus. In Sphærophthalmus and Ctenopyge the eye globes
are enormous, considering the size of the cheek in which they are set
and occupy more than a third of the length of the free cheek (Pl. III
f. 26, 31). They are hemispheric, blackish and glossy, more so in the
former genus. The spheroidal lenses, projecting on the surface, are in
Ctenopyge larger near the facial suture and small at the opposite side
where the eye is fixed in the free cheek. For the rest, in both genera
(Pl. III fig. 34) the lenses form a thin stratum, where they in a
vertical section lie elongated, flattened and biconvex, slightly joined
with each other at the point of contact. The fine form which they
exhibit reminds of the lenses of Sphæroma.[29] They are in diameter
thrice as long as they are high. Seen in a horizontal section passing
through the point of contact they show hexaeders with a curiously
jagged outline (Pl. III fig. 33).

[Footnote 29: BELLONCI Atti dei Lincei. Memorie, vol. X, 1881,
Sphæroma, pl. II fig. 11.]

Peltura which is coeval with these, has a narrow semiglobose visual
field (Pl. III figs 35-41), the superior surface of which is quite
smooth and evenly rounded. On its interior side there stand out,
somewhat distantiated, in a low relief semiglobular facets, quite as
regular incrassations of the cornea, thus not forming free lenses, but
rather reminding of the for the rest differently formed quasi-lenses
of Limulus. In a vertical section they appear as the inferior moiety of
real ovate lenses (Pl. III f. 40-41).

The much younger Acerocare has a similar cornea. A very little
specimen, the head of scarcely more than one millim. in length, retains
both eyes, of which one shows the slightly convex lenses and the other
a cast of the interior side as in Peltura. These both genera should in
consequence of their peculiar limuloid cornea be ranged for themselves
apart from the real lenticulate genera, but any material sufficient for
doing this properly, is at present not at hand.


II. =Aggregate eyes.=

These are found solely in the family of the Phacopidæ, unless the
Lichadidæ were also provided with this sort of eyes, but we have had
no opportunity to study them. It seems, however, not likely that they
had aggregate eyes. BARRANDE has represented them quite as finely
reticulate as the eyes of any Asaphid. We have sectioned and figured
the eyes of Dalmanites vulgaris and D. obtusus from the Silurian of
Gotland and found that these have truly aggregate eyes, each consisting
of a regular biconvex lens, lying enclosed in a socket of its own and
covered by a cornea of its own. The distance between the eyes is much
variable and in a few instances they are nearly contiguous. Extremes
are seen on pl. III figures 43, 47. The lenses are comparatively large,
and have always had a covering membrane, though this in many instances
has been lost. This membrane which is an immediate continuation of
the general integument of the body covers the lenses all round their
superior moiety. In its prolongation downwards between the lenses (Pl.
VI fig. 3, 4) it is free from the contact with them and hangs alongside
and around much incrassated, so as to take in a section a lengthened
lancet like shape. It lies thus alongside the other interstitial test,
and is like this perforated by longitudinal canals. In a horizontal
section taken a little below the surface it encircles the lens as a
wall like ring (Pl. VI fig. 1, 2). In a vertical section the lenses
lie in direct contact with the cheek without any intervening zone and
the cheek has the structure so common amongst the trilobites, being
perforated by vertical tubes going straight down from the surface (Pl.
III fig. 44, Pl. VI f. 5).

In Dalm. vulgaris and also in Phacops quadrilineata there is as already
before mentioned a peculiar structure beneath the lenses, consisting
of narrow, threadlike, straight lines, twice as long as the lenses
(Pl. III 49-50, pl. V fig. 38). In a horizontal section they are found
to be irregular prisms closely packed. It can not be any structure
peculiar to the eyes or the lenses, rather some parasitic growth added
since the death of the animal. The lenses are in several specimens
composed of clear calcareous spar. In others again they have been
filled with a dark muddy calcareous rock excepting in the lower moiety
where there is left a residue of the white spar, having in all lenses
assumed a regular shape which I consider as organic (Pl. VI fig. 5).
This spar covers the whole bottom and its upper rim is incrassated and
bent inwards. In horizontal sections this residue is a whitish ring
close inside the interior ring wall (Pl. VI f. 2). I would suggest
that this curious conformation is due to the original structure of
the lens, supposing that it in these crustaceans has been built upon
the same plan as in several other Arthropoda. In Cymothoa[30] and in
Sphæroma[31] for instance the lenses are built up of thin strata, which
are parallel with the convex outside, so that on the inferior surface
of the lens they are arched downwards and on the superior side upwards,
being not strictly concentric. In the spiders they are constructed
upon this same plan[32] perhaps more evidently. If now in Phacops the
lens consisted of such semiconcentric strata and the upper moiety has
been destroyed, the rest must have taken the shape as we find it. It
is moreover peculiar that the destruction has been exactly similar in
all lenses of that specimen. Can it be due to the circumstance that the
power of resistance in the inferior strata has been greater?

[Footnote 30: BULLAR Philos. Transact. 1878, pt. II, pl. 46, fig. 12.]

[Footnote 31: BELLONCI Atti dei Lincei, Memorie X 1881, pl. II, fig.
11.]

[Footnote 32: See GRENACHER pl. II f. 18 Epeira.]

In Phacops quadrilineata the lenses are more elliptic than in the
former. On their interior surface beneath the spiny tufts mentioned
large hexaedral prisms of clear calcareous spar issue, one prism for
each lens (Pl. V fig. 38), having thus a very deceptive appearance,
but no doubt of inorganic origin with the lenses as a basis for their
crystallization, quite as in the Cystoids where the interior often is
converted into a mass of crystalline prisms, issuing from the interior
surface of the plates.


III. =Genera with stemmata and ocelli.=

In a little group that has retained larval or ancestral characters
during a great part of the palæozoic period, the genus Harpes stands
as a type. It has ranged from the oldest Lower Silurian, if we join
the related Harpides, to the middle Devonian. From near the top of
the glabella, though not so much forward as the facial ridge of the
blind trilobites (Olenus, Liostracus etc.) a straight ridge of much
varying length stands out on both sides and at its extremity two or
three globular stemmata with glossy surface lie encased. Probably this
ridge has the same origin as in the Olenidæ, the more so, as there are
indications of an extensive circulatory system. On plate IV fig. 18-19
the right hand ocelli of Harpes vittatus BARR. from Lochkow, Bohemia,
are represented. They are two, lying isolated near each other, quite
globular with circular outline, smooth and glossy as to exhibit a
shining surface. Their size is 0,4 mm in diameter. Being cut vertically
in the direction of the longitudinal axis of the head they resemble
elongated hemispheres, convex on the exterior surface, slightly concave
on the interior. The test of the head lies between them as a saddle and
covers them only partially and on the outer sides they lie with their
margins encased in the head shield. Seen in thin sections of the right
lens and magnified the whitish mass is traversed by vertical, blacker
streaks, standing somewhat radiating towards the sides and cancellate.
The other lens has a horizontal row of black dots. All this is
probably not of any structural value, only due to later changes.

The remarkable genus Harpides from the lowest Lower Silurian belongs
also to this group and has beside the fixed peduncle a peculiar
elongated ridge going from the eyes to the lateral margins of the
head[33], a ridge which is also present in some of the true Harpes.

[Footnote 33: Harpides breviceps cannot belong to this genus and is
rather related to Erinnys SALTER, as also MATTHEW holds it.]

As BEECHER has shown[34] the larva of Trinucleus possesses quite
the same transverse ridge with intumescent eyelike extremities, and
although the smallness of the specimens has not permitted to ascertain
the presence of a true eye, it may be apposite to suppose it on the
homology with the eyes of Harpes, a genus with which Trinucleus is
related. But as well known, in the adult Trinuclei there is no trace
of these ocular ridges nor of real eyes so that BEECHER in his paper
»Blind Trilobites»[35] numerates Trinucleus amongst these. Out of the
nine Scandinavian species of Trinucleus no less than seven have a well
marked little tubercle on each side of the glabella placed exactly on
the same spot where the larval Trinuclei had their much larger eyes
placed. In this case it may be allowed to suppose that the tubercles
are the direct successors of the larval eye and that they are true
ocelli. REEDE seems to be willing to regard them as possessing a visual
function.[36] BEECHER[37] holds the eye nodules in the larva and the
ocelli in the adult to be identical.

[Footnote 34: Structure and appendages of Trinucleus.]

[Footnote 35: Geol. Magazine 1898. pp. 439, 493, 552.]

[Footnote 36: l. c. p. 447.]

[Footnote 37: l. c. p. 309.]

But, as BARRANDE has shown,[38] there is a certain species of
Trinucleus, the larva of which wants a facial ridge and eyes, as there
also are several adult forms without ocelli. These have remained on
a much ancestral stage, while the larva with eyes are more highly
developed in such species, where the adult have been subject to a
retrograde development. Ampyx and Dionide, though completely blind,
evidently belong to this group, and once, as is to be hoped, larval
forms may be discovered showing their development. In a certain way
Arethusina shows characters proper for this group, in having the
straight ocular ridge, quite as in Harpes, but eyes of the reticulate
type and probably prismatic. It thus like Harpes conserved an ancestral
characteristic long periods since it had disappeared in most of the
other genera.

[Footnote 38: Sys. Sil. de Bohéme I, pl. 30, figs 41-50.]

Of the groups, in which JOH:S MÜLLER[39] long ago classified the
Crustacean eyes, his second »Hauptgruppe» (»Aggregate von einfachen
linsenhaften Augen») and the fourth »zusammengesetzten Augen ...
facettirte Hornhaut» the former corresponds with my third and the
latter with my two first divisions. In so far as the cornea and its
facets or lenses are to be regarded, there is the greatest analogy with
the Isopoda. In vertical sections of Sphæroma we have the same sort of
elongated, flattened biconvex lenses as in Sphærophthalmus and others.
Since GRENACHER and EXNER and others have published their excellent
works on the eyes of the Arthropoda, there can be no foundation for
speaking of the resemblance of the trilobitic eye with that of Limulus,
as this genus stands completely isolated amongst all Arthropoda in that
respect. There is, as stated above, a certain resemblance between the
cornea of Peltura and that of Limulus, but this is not yet ripe for a
discussion. Nor is there any evidence for correlating the eyes of the
trilobites with the eyes of the Phyllopoda. BERNARD thinks that the
so called eye of the Paradoxidæ has been formed upon the same plan as
that of Apus. There is nothing to prove this hypothesis that the facial
ridge or any part of it ever had been a visual organ, and the evidence
at hand rather tends in a contrary direction.

[Footnote 39: In Merkels Archiv 1829 p. 46 and in Treviranus
Zeitschrift für Physiologie Bd IV p. 97.]

There are signs of long physiological and anatomical efforts to prepare
the development of the eyes on the free cheek, as revealed through the
long series of blind trilobites. A system of radiating blood vessels,
similar to those described above as covering the inside of the head
in some older genera, all issuing from the scallop in the free cheek,
where later the eye had to find its place, have left their stamp,
their mark on the surface of the free cheek. They attest the great
vital activity which was so intense at the point were the eye was to
be formed. We give the figures of two such cheeks of different types.
One from Parabolina spinulosa (pl. V fig. 31) is the more common, where
six or more isolated trunks radiate from the semilunar ridge round the
indenture and subdivide in branchlets which cease near the lateral
margin of the cheek. It may be that it is an annular vessel near the
indenture that feeds them all and that this probably is in connection
with the great central circulatory system. In Olenus (pl. V fig. 29)
the vessels are partly anastomosing and form a reticulate system and
they are studded with minute wartlets. Another sign, which may be taken
as a preparation, is the elevated rim around the scallop, which is so
prominent in several of the Cambrian genera, but which does not embrace
any facet bearing cornea.

From what has been stated above the following conclusions have been
arrived at.

1. The plurality of the genera living during the Cambrian period were
blind and it was first at the close of that period, in the Olenus
schists, that genera with real visual organs appeared. There may have
been oculate trilobites earlier, as Solenopleura, but we know nothing
of their eyes.

2. The primordial glabellar pleuron which was metamorphosed into a
facial ridge is no visual organ. It is in the Olenidæ nothing but the
elevated line made in the test by the subjacent main trunk of the
circulatory system. It swells out in a node, »palpebral lobe», but not
before the facial suture has been formed. In the genera where there is
no facial suture, there is no node. In the Paradoxidæ where the ridge
is of a different origin, there is no node, though there is a suture.

The four types of eyes in the trilobites have probably succeeded one
another in the following chronological order:

1) with stemmata or ocelli; 2) biconvex or lentiform; 3) prismatic;
4) aggregate. The oldest known representatives for each type are for
1) Harpides rugosus in the Ceratopyge limestone of the Lower Silur.,
for 2) Eurycare, in the Cambrian Olenid schists, division 2, for 3)
Megalaspis, in the Ceratopyge limestone of the Lower Silurian, for
4) Phacops in the Lower gray Orthoceratite limestone. The eyes of
the trilobites show the greatest conformity with those of the recent
Isopoda.

The most perfect eyes amongst all the trilobitic eyes may be those of
the Phacopidæ, which are also geologically the youngest, the least
developed again those of the Proetidæ or rather of the Bumasti. The
great thickness of the cornea in these must have weakened their power
of vision and they had probably only a faint perception of light.




=On the maculæ of the hypostoma.=


We shall now turn our attention to the visual organs which LILJEVALL
discovered on the hypostoma of Bronteus. In doing this, we may bear
in mind, that the genera in which we really have found lenses on the
maculæ are relatively few, but that we shall review the maculæ in all
genera, which we have been able to examine, and try to show that even
most of these may, although in an inferior degree, have acted as visual
organs. At the same time some more details shall be given about the
cephalic eyes for comparing them with the hypostomic ones.


=Acidaspis= MURCH.

The hypostoma is of a peculiar type, deviating from that of the other
genera, squarish or rectangularly transverse, entirely without terrace
lines and no maculæ proper. BARRANDE has given no less than nine
different samples of these hypostomas.


=Acidaspis crenata= EMMR.

(Pl. I figs 1-6.)

There are certainly no maculæ of the same sort as in so many other
trilobites and the interior side of the hypostoma does not bear the
least traces of impressions which might be taken as the reverse of the
maculæ and still more less as muscular impressions. In fact, the total
absence of such in Acidaspis militates against the interpretation of
the macula impressions as muscular scars. When the maculæ fail, also
the so called muscular impressions fail. Moreover we are able to see
still clearer in this matter through what we know about the structure
and position of the extremely similar hypostoma of Apus, of which we
have given a description in the end of this memoir. This skeletal part
of Apus is without any connection with the surroundings, excepting
at its anterior margin, and is consequently movable in a direction
outwards and upwards. Along that anterior margin it is fixed to the
outside of the ventricle through three pair of muscles, three muscles
on each side of the margin. ZADDACH De Apodis cancriformis anatome (pl.
II fig. XIV p. 68).

For the rest the marks of the attachment of the muscles are as a rule
in the Crustacea, at least those of the head, elevated small platforms,
so in the Trilobites, of which I have excellent specimens in a Bumastus
and others. On the inside of the head of Limulus they are faintly
elevated patches. The small hollows on the inside of the hypostoma
formed by the maculæ are indeed the sockets in which the soft parts
of these more or less developed hypostomic eyes were sheltered. But
there is still a feature in the hypostoma of Acidaspis which merits our
attention and which perhaps may have a significance akin to that of the
maculæ.

The hypostoma is square, with two short pointed wings, one on each side
of the slightly bent anterior margin and likewise two smaller ones
at the corners of the posterior margin. A groove follows on a short
distance the lateral margins and the posterior margin and disappears a
little below the anterior one. In the same direction, distally, though
a little more inside and unconnected with them there are two small
grooves, the bottom of which consists of a shell substance of different
colour and structure than the other parts. Having been a little ground
and seen in transmitted light it exhibits the shape of a club and a
homogenous yellow spot, tapering posteriorly and swelling out distally
(Pl. I fig. 4). It must be left an open question whether these maculæ
share in the nature of visual organs as the quite different maculæ of
the other trilobites, but it may be possible that it is so. It must,
however, be remarked that there are two types of hypostoma in the genus
Acidaspis as shown by the illustrations of BARRANDE. One has the small
grooves, possibly all sheltering the claviform maculæ, disposed as in
the now described A. crenata. This group embraces five species of the
Bohemian Silurian formation. The other group of three species again has
the hypostoma of the same quadratic or rectangular shape, but the two
short grooves, which may be expected to contain the maculæ, are placed
midways between the anterior and the posterior margins, nearly as the
maculæ bearing grooves of other trilobites. We have however not had
material for pursuing our researches in this genus, the other species
of the Swedish Acidaspidæ being unknown as to their hypostoma.

It may here be added an observation concerning the ornamentation of
the exterior surface of the hypostoma of Ac. crenata. It is covered
by a great number of diminutive circular or oblong wartlets occupying
the whole surface excepting the lower third of the central field just
above the posterior groove which is smooth. These wartlets seen through
transmitted light (Pl. I fig. 5) show in their interior something like
a peculiar black spiculum rising from a bifid rootlet and confined
within the wartlet and not extruding from it. In a longitudinal section
(Pl. I fig. 6) the spicula perforate the wartlets reaching through
their whole length. As in Calymmene these interior pseudo-spicula are
tubes, filled with iron-pyrites. It is probable that these tubes were
once bearing setæ and quite as in Apus formed a fur of bristles.

The structure of the cephalic eyes (Pl. 1 f. 1-2) is prismatic, but the
separate prisms are rather short and broad. Their lower or interior
end is convex. The separating lines between the single prisms are not
always distinct.


=Agnostus= DALM.

Pl. 1 fig. 7.

Agnostus glandiformis ANG. Although there is not the slightest evidence
of eyes in this the largest of its genus, nor any free hypostoma
hitherto has been found, we may here give a little account of our
researches into this species. The scantiness of material forms a chief
obstacle to our knowledge. Only three entire, rolled up specimens have
been found, and it is by sectioning and preparing such that any hope
can be entertained to gain reliable results.

A rolled up specimen from Andrarum Scania was sectioned lengthwise.
The tail-piece closed tight against the head-shield, so there had
been little chance for foreign matter to penetrate into the interior
which, however, is filled. Close below the cephalic shield there is a
remarkable structure, mostly resembling an elongated intestine with
swellings joined by more narrow ducts and anteriorly the coherence is
interrupted. As in crustaceans and Arthropods in general the stomach
and the intestine are situated on the dorsal side of the body, there is
nothing unlikely in assuming that this in reality may be the remnants
of the intestine. This may also be compared with the observation
made by VOLLBORTH in his memoir »Ueber die mit glatten Rumpfgliedern
versehenen Trilobiten», 1863, p. 46, tab. 1 fig. 12 where the
heart-tube probably is delineated. BARRANDE has also given figures of
what he considers as the intestine in Trinucleus.

Below this organ in Agnostus, there lies a section of a vaulted
calcareous plate with its convexity turned against the dorsal side of
the head shield, that is to say quite the reverse what might have been
expected if it had been the hypostoma in its true position. This may,
however, not be any objection against considering it as a sectioned
hypostoma, loosened from its connection with the cephalic shield,
disturbed in its original position and turned round, when the shell
became filled with mud. At the distal end there is a much distinct
duplicature.


=Asaphus= BRONGN.

The wellknown hypostoma of this genus has the same characteristic
shape, though more pronounced, as in Ptychopyge, with its posterior
margin deeply indented, so as to form two large, pointed lobes. There
is properly only one median field, surrounded by flat, lateral borders,
continuing down into the posterior lobes, from which it is separated
through a shallow groove. The macula are situated in this groove, on
each side of the inferior border of the central field. They are more
or less prominent, but whatever their form may be, their surface is
always entirely smooth, lying well circumscribed amidst the surrounding
terrace lines.

The following list enumerates all the species in this genus, of which
previous authors have delineated the tubercles, though they in the
descriptions only in very few instances have mentioned their presence.

_A. acuminatus_ NIECZKOWSKI, »Zusätze zur Monogr. der Trilobiten 1859»,
tab. I, f. 6. Maculæ most prominent, but no mention made of them in the
description.

_A. (Isotelus) canalis_ J. HALL, Pal. N. Y. I, pl. 4 bis, f. 18-19.
Though this and a following species probably on account of the
deviating conformation of their body, belong to a different genus
or subgenus, I mention them along with the Asaphi, as there is the
greatest similarity in their hypostoma. Fragment of an interior cast;
still more strange is the fragment showing part of the interior surface
and the duplicature.

_A. expansus_ L. BR. I pl. VII f. 3, BR. II pl. I f. 2, 2 c. In the two
first figures there are no tubercles nor maculæ marked, in the last
figure there are distinct tubercles.

_A. fallax_ A. pl. XXVIII f. 3, c. BR. II pl. I, f. 3, both nearly
congruent.

A. (Isotelus) gigas J. HALL, Pal. N. Y. 1, pl. 60, f. 7 g, pl. 66 f.
5, the inside of an entire, uncommonly large hypostoma gives a general
good view with the macula; which are large and evident. He mentions the
maculæ as »two circular spots». »These probably indicate the points for
the attachment of muscles and tendons upon the inside,» he adds.

_A. ingens_ BARR. NOVÁK II, pl. I, f. 7, a good figure showing two
semiglobular maculæ. BARRANDE'S figures pl. 33, f. 7, 8 are not
distinct.

_A. ludibundus_ TQT. BR. II, pl. 1, f. 7. Two semilunar sulci on the
cast of the interior surface, being only the posterior borders of the
maculæ which are a little more raised and distinct than the rest.

_A. nobilis_ BARR. pl. 32, f. 6. Indistinct traces of maculæ. In the
figure 6, pl. 31 representing the hypostoma of a young specimen, there
are no tubercles at all. If the figures were to be relied upon, it
might be assumed, that the tubercles appear at a more mature age.

_A. Powisii_ SALTER, pl. 23, f. 6. Two semilunar, narrow tubercular
maculæ, their interior apices converging towards the anterior border of
the hypostoma.

_A. raniceps_ A., pl. XXVIII, f. 2 c., BR. II, pl. 1, f. 4. A good
figure.

_A. raniceps_ var. _maxima_ BR. II, pl. 1, f. 6. Semilunar impressions
on a cast and thus far incomplete.

_A. striatus_ BOECK. To this belongs probably A. expansus M. SARS
»Ueber einige neue oder unvollständig bekannte Trilobiten» in Isis
1835 p. 333, and especially p. 340 and the following (Bemerkungen über
die untere Seite von einigen Trilobiten). SARS there gives a very
good description of the exterior side, where he also mentions the two
maculæ, pl. IX, f. 9 a, b. (»2 kleine Knoten»), BR. I, pl. VIII, f. 4
a, and BR. II, pl. I, f. 9. These latter figures differ in so far, that
the former has the maculæ excavated or rather tubular below, in the
latter again they are regular.

_A. trinucleorum_ BR. II, pl. I, f. 16 a large specimen with linear
maculæ arranged rectangularly in respect to the longitudinal axis of
the hypostoma and both on the same level.

_A. tyrannus_ MURCH. SALTER in Mem. Geol. Survey, Brit. Foss. Dec.
II, 1849 pl. V, f. 4, excellent figure. On p. 2 he says: »there is
an oval circumscribed tubercle at the origin of each (fork) most
distinct on the inner surface». This figure is again reproduced in the
»Monograph», pl. 22, f. 6, where it is said »Two linear tubercles with
their interior apices converging towards the posterior margin of the
hypostoma».


_Asaphus_ sp., probably A. raniceps, POMPECKJ, Trilobitenfauna Ost- and
West-Preussens, taf. VI, f. 7. p. 80 »an seinem Hinterrande liegen zwei
kleine, flache Höckerchen, die als Reste des Hinterlappens aufzufassen
sind»(!?).

We shall now describe the Asaphi which we have been able to examine
more in detail.


=Asaphus expansus= L.

Pl. I figs. 8-17.

In the numerous specimens of which we have examined the hypostoma,
there are always small, nearly circular maculæ which by their lighter
colour are marked out from the surrounding smooth space of the inferior
sinus of the lateral grooves where they are situated. They do not rise
at all above the surrounding surface from which they are separated only
by a fine, scarcely perceptible line. On the interior surface of the
hypostoma they are better seen, and the enclosing line is deeper and
more distinct. Their surface is there quite as smooth and even as on
the exterior side of the hypostoma. They do not consequently in the
least manner resemble muscular impressions as observable on the inside
of the glabella of other trilobites. These are on the contrary elevated
above the surface and finely striated in various ways.

The peculiar nature of these macula is revealed through the various
sections we have made. In pl. I fig. 15 a vertical section across the
entire hypostoma is represented. The sectioned, dark maculæ (a) lie on
both sides of the faintly curved central field, in the sinus of the
lateral grooves, and the strongly developed terrace lines continue
sidewards just a little on the interior side, where the duplicature
turns round. As seen in a magnified vertical section (fig. 16) the
maculæ consist of horizontal, whitish, straight lines, probably lines
of successive growth, and these are crossed rectangularly by more
irregular whitish lines separated from each other through dark spaces.
The whole thus gains the aspect of a dark surface cancellated by white
lines. This reticulated or spongious macula is enclosed as to its
superior as well as to its inferior part in the compact and homogenous
test of the hypostoma and occluded from the influence of the light. Its
value as a visual organ consequently is insignificant. In a horizontal
section (fig. 17) the structure is still more bewildering. There the
whole macula is a confused, spongious white mass with dark spaces
between the white meshes, and only at one side, the left one, some
obscure indications as of polygones are visible. It would indeed have
been impossible to interprete what this means, had not the study of the
cephalic eye given a clue thereof.

The eye of this species is covered with a delicate perfectly smooth
and glossy integument (f. 12) which is reposing immediately on the
prismatic lenses of the cornea. In a few instances, depending on the
colour or the state of preservation of that integument the lenses are
indistinctly translucent. Owing to their state of preservation their
aspect is greatly variable. They are in many instances, as seen in
fig. 9, six-sided or some rhombic, foursided or even quadratic (f.
8, 10). They are all of the same length, 0,2 mm. along the surface
proper of the eye, but are lengthened to 0,3 mm. towards the border
of the eyes, f. 12, which will be described further on; they are of
equal breadth, amounting to 0,066 mm. Their inferior ends are slightly
convex or nearly plane when well preserved, else, when as often is the
case, disintegrated as to be scooped out and vaulted. Their interior
structure, as revealed by sections, shows a cylindrical core, f. 8-10,
composed of concentric strata. This cylinder fills nearly the whole
interior space of the prisms, there being, however, in many instances
a compact dark mass between the cylinder and the walls of the prisms.
There are also sections in which the prisms are filled with a uniformly
black mass without any concentric structure. In longitudinal sections
the outlines of the individual prisms are not clearly discernible (f.
11). There are longitudinal, white lines of varying thickness with
lateral irregular offshoots, which may join with those from opposite
walls, and give the interior a sort of spongious or cancellate
appearance.

Towards both sides of the ocular surface, towards the superior and
inferior side, a change sets in as to the shape of the lenses, as best
seen in horizontal sections (figs. 8, 10) they are lengthened and
become more and more indistinct, and at last in the upper and lower
marginal zones pass over into a reticulate, spongious mass, which
seen in a longitudinal section presents almost the same aspect as in
the regular prismatic surface of the eye. Fig. 11 _b_, _a_ being the
visual field. It is, however, more densely reticulate, but a prismatic
arrangement is quite as much evident as in the ocular surface proper.
In the rule the passage from the prismatic surface to the reticulate
is gradual, the prisms becoming by and by irregular in their outline
and diminishing in size (fig. 12, _a_ the eye proper, _b_ the border
zone), but there are also instances where the distinction between these
two fields is sharp and without any gradual transition. In the inferior
reticulate zone there are generally some oblongue, funnelshaped pits.
I am uncertain whether they are to be regarded as regular parts of
the eye structure or rather as burrows of some parasite. They do not
continue deep down.

If we now compare the reticulate zone of the eye with the maculæ of the
hypostoma, for instance the vertical section fig. 16 with fig. 11 _b_,
we find the most complete identity in structure. In the same manner
the horizontal section of the border zone of the eye, fig. 8 _a_, fig.
10 _b_, and of the macula, fig. 17 are similar. The same chaotic,
spongious mass in both, with some tendency to form prisms more evident
in the longitudinal sections, where the same reticulate structure with
predominant white, longitudinal streaks is so palpable. That there is
a complete identity in structure between the two, the macula of the
hypostoma and the border zones of the eye, is as evident as anything
can be, but as to the functional identity or what this function may
have been it is difficult to decide anything with certainty, at least
it seems to me that the capacity of vision must in both have been far
more restricted than in the eye proper. They rather give the impression
as of rudimentary visual organs.


=Asaphus raniceps= DALM.

Pl. I f. 23-26.

The maculæ of the hypostoma are placed obliquely in the saline manner
as in the preceding species. They are more prominent and the oblong
macula, with the longest diameter of 1,2 mm. is on the top of a
little mound and surrounded by a fine, elevated marginal line (fig.
23). Its somewhat convex surface is entirely smooth and if sectioned
horizontally exhibits the same sort of spongy texture as A. expansus
(pl. I f. 24). In a vertical section (fig. 25) the macula does not
occupy so large a space as in As. expansus, but rather lies as a
lenticular disk in the hypostomic test closer to the superior surface
than the inferior. With sufficiently high power the same sort of
pillars, divided by horizontal strata is seen. Upon the whole the
vertical section is not so clearly developed. The reticulate zone of
the eye is more definitely separated front the prismatic zone than in
A. expansus. There can be no doubt that there is a correspondence in
this species between the structure of the macula! and the spongious
zone of the eyes.


=Asaphus cornigerus= SCHLOTH. (A. Kowalewskyi LAWROW).

Of this strange species with its enormous eyestalks, more than 2
centimeters in length I have through the kindness of Akademiker FRIEDR.
SCHMIDT in St. Petersburg had occasion to study some specimens.

The rather large maculæ, obovate-circular, are like those of several
other Esthonian species oriented inwards and upwards instead of inwards
and downwards as in the Swedish species described. As the specimens
have suffered through corrosion of the surface there are only faint
traces of a marginal line. Their microscopic structure is badly
preserved, and they look pale and transparent with only few indications
of the spongious texture. The lenses of the cephalic eye are nearly
square prisms sometimes with a slight approach to hexaedral pillars.
There is no clear transition into a spongious or reticulate border
zone. But this may depend upon the bad preservation.


=Asaphus fallax= DALM.?

Pl. I fig. 18-22.

It is very difficult to distinguish this species or rather variety from
A. expansus, but if we have found genuine specimens, there are some
points in the shape of eyes and maculæ which make it different. The
hypostoma (f. 21) is rather more broad and the duplicature is large
with an upturned margin. The little macula is placed on the top of a
smooth rounded elevation and enclosed by an elevated rim.

The cephalic eyes are rather short, regular hexaedral prisms (f. 18-20)
and as shown in fig. 18 change into irregular squares near the border
of the eve.


=Asaphus= sp.

from Brunsby kanal, Segerstad parish, isle of Öland. The maculæ are
large pale whitish and of an uncommonly fine-meshed reticulation, well
limited from the surrounding hypostoma.


=Asaphus= sp.

Pl. I fig. 27-30.

from the islet of Sandö, north of Gotland. Although we cannot give
any account of its hypostoma, not having had sufficient material, the
structure of the eyes is so peculiar that it seems worthy of being
recorded. The integument is extremely thin and transparent and the
subjacent lenses are clearly seen, and through their impact on the thin
integument they make this to stand out in a very low relief above them
(fig. 27). These lenses are uncommonly short, forming at the surface
rather oblong, slightly hexaedral prisms with a narrow interspace
between them. In a section lower down they have the shape of hollow,
white rings filled with black mud and in a longitudinal section the
white walls of the lenses look like short pointed spikes and interiorly
they are completely empty. We here find also the same gradual change
from regular cones to the spongious border zone as in the above
mentioned species f. 29. The border zone is finely reticulated.


=Asaphus (Isotelus) gigas= J. HALL.

Pl. II figs. 1-3.

The enormous hypostoma of this giant resembles in a high degree that of
the Asaphi, but is at the same time the most evident verification of
the experience that the hypostoma _per se_ cannot be regarded as the
sole criterion for determinating the generic affinities of different
species. The whole structure of this trilobite in other respects gives
it a quite independent position, distinct, from Asaphus.

We have had at our disposal several specimens of the large hypostoma,
the dimensions of the largest, fragmentary in its anterior margin,
being as follows: breadth 48 mm. length 41 mm., probably 47 mm. when
entire, breadth of each of the posterior lobes at their bases 19 mm.,
length of the same 23 mm. The maculæ which are placed on the flat
surface of the hypostoma without being at all elevated, are prominent
through their great size and their pale, whitish colour. They are
somewhat oblongue having their longest axis directed inwards and
downwards. They attain a diameter of four millimeters. Horizontally
sectioned, f. 2, they show the spongious texture and vertically the
quasi prismatic reticulate structure richly developed, f. 3. The
pillars are very distinct and continue without interruption all
through the macula and in the interstices there are traverses joining
between two pillars or partially filling the darkish interspaces, thus
giving the whole the aspect of some »tabulate» coral sectioned. By the
inspection of the horizontal section alone, presenting the irregular
spongy texture it would have been impossible to imagine the ordinated
arrangement which the vertical section reveals to us. The eyes consist
of regular hexaedral prisms, and there is a very sharp boundary line
between them and the spongious zone, which is very narrow and without
distinct separating lines joins with the cephalic test. For the rest
there are in all probability at least two different species sent from
America under the name of Isotelus gigas, of which only the largest,
almost like a Homalonotus, and the hypostoma of which has been partly
delineated in this memoir, seems to be the real one. They differ both
in the shape of the hypostoma as well as in other respects.


=Barrandia= MAC COY.

According to SALTER Monogr. pl. 19 fig. 9 his B. Portlocki, of which
there is a fragmentary and broken hypostoma, shows feeble traces of
oblique macula, nearly in the fashion of Asaphus. In the description at
page 139 is told about »the usual pair of tubercles».


=Bronteus= GOLDFUSS.

BARRANDE delineates the maculæ on the hypostoma of

_B. palifer_, Tab. 45 fig. 17,

_B. planus_, Tab. 48 fig. 7, and

in the supplementar volume of

_B. rhinoceros_, Tab. 9 f. 16,

_B. furcifer_, Tab. 11 fig. 16, but there is not the slightest
indication of granulation on any of them, nor is there in the
descriptions, generic or specific, the least mention made of the
tubercular maculæ. It is also remarkable that in the works of ANGELIN
and NOVÁK where several hypostomas belonging to species of this genus
are delineated, not a single one shows these tubercles. We shall now
continue the descriptions of the Swedish Brontei, already begun with
Br. polyactin in the introductory part of this memoir.


=Bronteus irradians= LINDSTR.

Pl. II figs. 4-5.

has a hypostoma that much resembles that of Br. polyactin. In its
general shape it is similar and the two concentric grooves with the two
maculæ placed in the same way, just below the superior groove. These
maculæ are much larger than in Br. polyactin, nearly thrice their size.
They are also more ovate or rather like a bean, the smooth surface
is larger and the granulated spot restricted to a more narrow space
forming an oblique patch. The granules or lenses are also individually
larger than in the allied species, double their size or 0,06
millimeters. We have not succeeded in making sections of the cephalic
eye nor of the maculæ.


=Br. platyactin= ANGELIN.

Pl. II f. 14-19.

The hypostoma has a transversally triangular form, and is divided only
in two fields through a shallow semicircular groove near the posterior
margin. The two maculæ are situated above the groove near its superior
sinuses. They are elongated, fig. 17, elliptic with the narrow pointed
end directed outwards and the broad rounded end inwards. The chief
surface is scooped out as a shallow depression. The granulated spot is
situated on the broader end and covering it completely. The relatively
large lenses are arranged in five regular rows, the uppermost one
being the longest. On the interior surface of the hypostoma there
are the corresponding sockets of both maculæ with smooth surface.
The horizontal sections of the granules figs. 18, 19 present the
image of white rings in close contact, without, however, to occasion
a prismatic structure, a dark interspace lying between each ring.
These lenses are filled with a dark mass, and in some the same sort
of radiated structure is perceptible as in the lenses of the cephalic
eye. In horizontal sections of the cephalic eye the lenses approach
the polyedral shape. In another section near to the surface of another
specimen the lenses are decidedly hexaedral. The vertical sections,
fig. 14, reveal their real nature as lenses where they lie as a string
of beads with a dark nucleus enclosed within a thin whitish shell. They
are covered by a thin membranous lining. When seen in transmitted light
the lenses proper are dark, and the shell white and in reflected light
the lenses are lighter than the rest.


=Br. laticauda= ANGEL.

Pl. II figs. 6-13.

Lower Silurian from Dalecarlia. The hypostoma, f. 10, is of a broad
clypeate shape, the anterior margin rounded without any large
projecting wings. On the exterior surface there are two grooves
parallel with the rounded inferior margin. As in Br. polyactin the two
elliptic tubercles are situated on the inferior edge or slope of the
upper groove and they are deepened by a shallow depression as in Br.
platyactin. The granulated spot situated along the posterior margin of
the macula deviates much in shape from that of the other species. It
consists of a long and narrow stripe ending in a fine point outwards
and widening inwards, where it is rounded, forming thus a claviform,
curved elevation. Around the granulated area the maculæ are quite
smooth, and show in a horizontal section an irregular structure of tiny
black dots nearly resembling the structure of the hypostomic shell. We
have not been able to obtain any good vertical section, but by casts of
the interior side of the tubercles it is found that the granules form
polyedric facets like those of the eyes though perhaps not so regular.
There is a specimen with the granules intact, and the polyedric shape
is then not so distinct.

Sections of the eyes elucidate the structure, which partially is
obscure in other species of Bronteus. There is a thin membranous
coating covering the subjacent well formed lenses (fig. 8). When this
membrane in some instances has been peeled off the lenses lie bare
(fig. 9). These have a dark nucleus and in some instances it seems
as if there were two. In a vertical section they look spheroidal, in
a horizontal section again they are polyedric, especially when taken
somewhat below the surface or near the middle line (fig. 6). In fig. 7,
some lenses are delineated in a horizontal section, highly magnified
and the corallian appearance is evident. The lenses are well separated
by distinct lines, and from their inner tubes whitish reticulations
radiate towards the centre.

If we now compare these sections, where the lenses are so distinctly
seen, with the more obscure sections of other Brontei, it is evident
that the dark points in them are nothing but transformed and
deteriorated centres of the lenses. In Br. platyactin they are a little
more distinct than in the other species. And it is with these that the
granulated spots on the maculæ of the hypostoma are most concordant.


=Bronteus= sp. indet.

Pl. II figs. 28-30.

This species, of which only the hypostoma is known, found at Lansa,
Gotland, is likely, to judge by that, to be nearly related to Br.
polyactin. The shape of this hypostoma is exactly the same and the
maculæ are placed in the same position, but they are larger, a little
sunk on the blind surface and the group of the lenses is different. Its
superior margin forms an ingoing arch and the lenses themselves are not
convex, but rounded or slightly polyedric, and separated through thick
interspaces.


=Bumastus= MURCHISON.

The only recorded hypostoma with maculæ belongs to B. insignis HALL
as described by SALTER Monogr. p. 208 pl. 27 fig. 7. He says: »A
pair of compressed tubercles occurs at the lower third: they are
transverse-ovate, and more than their own diameter apart.» This is
nearly in concordance with what is seen on the species to be described
below.

Although it can in reality be said that the species of Bumastus are
the most common of all trilobites in the Silurium of Gotland, in so
far that their head pieces and pygidia are met with everywhere in the
limestone rocks of the island, the find of entire specimens or of
detached hypostomas is amongst the rarest events when collecting there
and amongst thousands of fragments, in several places entirely filling
large portions of the limestone rock, there have in the Swedish State
museum been acquired only three or four hypostomas, which we are now
going to describe.

At first we must point out a certain resemblance which prevails
between the hypostomas and maculæ in Bumastus and Bronteus. Compare
for instance Bum. sulcatus with Br. platyactin and Br. laticauda. The
general form is nearly the same in both and the shape of the maculæ
almost identical but for the complete want of granulations on those of
Bumastus. To this resemblance must be added that there are, as known,
forms of trilobites which by some authors have been regarded as Brontei
and by others, again, ranked amongst the Illænidæ.

Three species of Bumastus have been recorded as found in the Gotland
strata, but of one of these no hypostoma is known. Besides, to judge by
the head shields and hypostoma, there are a few new species. To begin
with the most common


=B. sulcatus= LDM.

Pl. II figs. 33-40.

The hypostoma is broadly shield-formed and the anterior margin is most
characteristic being in its central part elongated in a short evenly
rounded projection. The side wings are triangular acuminate, bent a
little backwards and towards the interior surface. There is only a
single rounded field passing into an evenly bent border, in front of
which the projecting, elliptic maculæ lie. A second pair of wings
stretches backwards near the posterior margin. We have not obtained any
sections of the maculæ.

The cephalic eyes figs. 36-38 are covered with an uncommonly thick
stratum of homogenous shell, one and a half times as thick as the
subjacent lenses. These are comparatively broad, above passing
indistinctly in the covering stratum, below, at their basis, convex.
In a horizontal section figs. 33, 34 they exhibit the common hexaedral
or pentaedric aspect, their exterior sides being white and confluent
without any separating lines. Their interior is dark and from the
polygonal sides white lines go in giving the whole the appearance as
of a starry, composite coral with its septa. The correspondence in
structure with Bronteus laticauda pl. II fig. 7 is striking. A narrow
space separates the surface of the eye from a nearly similar patch
below the eve on the cheek 35 _b_, 38 _b_. But there the position
of the composing strata is singularly reversed and the spongious or
pseudo-prismatic stratum lies close to the outside and the homogenous
stratum on the inside. The white longitudinal lines are well marked
out and regular in the vertical section, fig. 38 _b_, indicating the
prismatic structure most clearly.


=B. barriensis= MURCH.

Pl. II figs. 31, 32.

The hypostoma fig. 31 is of so transverse a shape that the breadth much
surpasses the height, the former being 6 millim., while the latter
attains only 3,5 millim. The anterior margin is so much arcuated and
sloping backwards that it forms an obtuse angle. The side wings are
triangular and acuminated and from them a thick pad runs backwards and
forms the posterior margin. Below the largest field of the exterior
surface the maculæ rest on a ridge having a much elongated, vermiform
shape and sunk in the surface (fig. 32) a little more than three times
longer than broad. In its shape it approaches much to the hypostoma of
Bum. insignis HALL as delineated by SALTER pl. 27 fig. 7.

In another, undetermined Bumastus (pl. II figs. 47, 48)[40] the
hypostoma (from Klints, Othem, isle of Gotland) is of a more elongate
obovate form, the anterior margin elevated, rounded, the terrace lines
faint, the tubercles are larger than in the former Bumasti, oblong,
curved, beanshaped, evenly rounded and smooth.

[Footnote 40: Probably the same as Bum. sulcatus pl. XII fig. 12 in my
paper on the Trilobites of Gotland 1885.]

Another of the new species is one that LILJEVALL has found at Korpklint
near Wisby (Pl. II figs. 41-46). The hypostoma is more elongate than
transverse with the anterior margin regularly arcuated, by far not
so prominent as in B. sulcatus and the former. By a shallow groove
the exterior surface is divided in two fields, the anterior one the
largest and covered with a few, distantiated, concentric terrace
lines. The posterior field is smooth, nearly even and the maculæ lie
just in the groove, with their axis oblique to the longitudinal axis
of the hypostoma, beanshaped and smooth. The cephalic eyes show in
a horizontal section hexaedral white walls enclosing a dark space
in which some indistinct radii emanate, fig. 41 _a_. In the vertical
sections the prisms are narrow and elongate and the integument by far
not so thick as in Bumastus sulcatus, fig. 42, 43. The border zone
of the eye in this (fig. 41) as well as in B. sulcatus, as seen in
horizontal sections, agrees with that of the Asaphidæ.


=Bumastus= sp. inlet.

Pl. III fig. 1, 2.

In a detached piece of limestone, found in Gotland, at Norderstrand,
near Wisby, two hypostomas and a free cheek lay embedded along with
a coral of the genus Acantholithus and a Bronteus probably belonging
to a new species. As the coral has not been found above the uppermost
beds of the Lower Silurian it is probable that the trilobites also are
derived from the same horizon.

The hypostoma, figured pl. III fig. 1, is broadly tongueshaped, with a
short, blunt, triangular wing on each side of the anterior margin. The
exterior side is nearly completely occupied by a single large field,
which is convex and decorated by sparse terrace lines. Below this a
smooth, nearly even plane, reaching to the inferior, elevated border
of the hypostoma. The two tubercular macula' are situated exactly on
the boundary line between the convex and the plane field. They are
elongately ovate, pointed outwards, rounded inwards, with smooth and
glossy surface. That portion, on which in the species of Bronteus
the granular spot rests, is transversally wrinkled by some faintly
elevated, sigmoid lines (fig. 2). It would have been of great interest
to investigate the interior structure of these curious maculæ, but the
scarcity of the material has prevented our doing so.

The other hypostoma, pl. III fig. 3-5, is larger and has the terrace
lines more distantiated and forming more open curves, but for the rest
it is of the same shape as the smaller, so that we may not ascribe
these small deviations to a specific difference. The maculæ are more
elongated, narrow, curved tubercles, thus differing from the next
preceding. A section running through its left tubercle does not show
any structure, but the shell of the maculæ is extremely thin, thus
contrasting strongly with the surrounding thicker shell, fig. 4 _a_.
The cephalic eye, again, remaining on the free cheek found in the same
piece of limestone and probably belonging to the same specimen, has its
surface well preserved and exhibits semiglobular lenses in low relief,
fig. 5.


=Calymmene= BRONGN.

In the following species of this genus hypostomic tubercles have
formerly been recorded.

_Cal. Blumenbachi_ SALTER Monogr. pl. 8, fig. 9. The identity of the
species is dubious.

_Cal. duplicata_ SALTER Mon. pl. 9, fig. 22. The tubercles rather more
linear and approximated than in the subsequent. SALTER calls them »two
small transverse lobes ... forming a nearly continuous ridge».

_Cal. tuberculata_ ANGEL. Tab. XIX fig. 5 c from the inside.

It is remarkable that the eyes or that part of them where the cornea
and the lenses should be found, has been destroyed and entirely lost in
all specimens of the Calymmene known to us, many hundreds having been
searched and every one with a lacuna on that spot. It must have been
of an excessive thinness. We know only of one instance, the Bohemian
Calym. Arago, in which BARRANDE has seen the eye intact and provided
with a small number of about eleven sphærical lenses.


=Calymmene intermedia= LDM.

Pl. III figs. 6, 7.

The clypeiform hypostoma is divided in two fields, one anterior larger,
in the centre elevated in a short, blunt knob, and a posterior, bearing
the two elevated rounded maculæ, just below the semicircular groove
which separates it from the anterior field. These fields are surrounded
by a large border which is posteriorly emarginated through a short
rounded sinus. The surface is entirely covered by small semiglobular
wartlets perforated by a straight pore or duct, which being filled
with a black mineral, probably iron pyrites, gives a peculiar and
characteristic aspect to the sections. In the same manner the whole
mass of the hypostoma is pierced by straight, black lines, also filled
up pores, starting from the outside and ending before attaining the
inner surface.

The maculæ are prominent, separated from the hypostoma through narrow,
distinct grooves, ovate, with the pointed end directed obliquely
outwards, against the margins of the hypostoma. They are covered with
wartlets and leaving only a little oblong spot free, quite smooth, this
being consequently the macula proper. But in a longitudinal section
there is not the least distinction between this macula and the other
hypostoma, only, that it is free of pores.


=Calymmene tuberculata= BRÜNNICH.

Pl. III figs. 8, 9.

We have succeeded to prepare a horizontal section of the macula. This
is oval, perfectly homogenous and without any structure proper, showing
only indistinctly a mottled medley of pale, brownish spots in the clear
mass. There is in both maculæ a small dark, angular spot placed in
their inferior part, fig. 9.


=Centropleura= ANGELIN.

This author has delineated most distinct maculæ in Centr. Steenstrupi
pl. III fig. 4, b. two, small elongate darkish spots, near the
posterior margin. I have not seen any specimen of this trilobite and
have consequently not been able to follow up ANGELIN'S observation.

=Chasmops= M'COY.

Pl. III fig. 10.

In his work on the Russian Phacopidæ FRIEDR. SCHMIDT has given the
figures of Chasmops Eichwaldi and Ch. Wesenbergensis showing, as it
were, patches of black maculæ. But as there is no mention of them, and
as they are quite unlike the macula? which are described here, I am in
doubt of their real nature. We have figured a macula from the hypostoma
of a Chasmops, probably Ch. macroura, from a drift block near Rostock.
It is elongate and narrow, more than four times longer than broad. Its
smooth surface is a little scooped out and no structure is observable.


=Chirurus= BEYRICH.

We find the hypostomic maculæ annotated and delineated as occurring in
the following species.

_Ch. bimucronatus_ SALTER pl. 5, fig. 5 as two narrow elongated pits,
surface seems intact. No mention of them in the letter-press.

_Ch. exsul_ SCHMIDT pl. VI fig. 9. Indications of the oblong, narrow
maculæ.

_Ch. macrophthalmus_ SCHM. pl. VII, figs. 1 c, 2. Faint indications of
narrow pits.

_Ch. Quenstedti_ BARR. Tab. 42 fig. 3. Maculæ near lower border as two
narrow slits.

_Ch. spinulosus_ SCHM. pl. VII fig. 10. The lateral pits well visible.

_Ch. tumidus_ SCHM. pl. VIII, fig. 22. The pits are short, but present.

The exterior surface of the hypostoma is entirely covered with small
wartlets, closely set, of different size in the various species,
and spread between them lie a great number of elongate smooth spots
dispersed, on the interior side of the hypostoma showing an excavated
surface (pl. III fig. 16).


=Chirurus (Cyrtometopus) clavifrons= DALM.

Pl. III figs. 17-21.

The shape of the hypostoma is shown in fig. 21 pl. III. It is regularly
shield-formed with posterior margin rounded. A shallow groove running
parallel with the margins forms a large, faintly elevated border. The
anterior margin projects on both sides in a short, backwards directed,
broad and flat wing, hollowed out with a pit on the front side, and
there is a posterior lateral wing, midways between the anterior and
posterior margin short, blunt, directed obliquely backwards. The whole
exterior surface is covered with diminutive tubercles, the above
mentioned smooth spots interspersed on the central disk. A little below
the middle there are two lengthened pits, so shallow that they often
are not discernible. In a specimen of Chirurus spinulosus NIECZKOWSKI
from Estland there are two, almost 3 mm. long dots, one on each side
above the maculæ as the fig. 21 shows and in several others of this
generic group there are also indications of similar. I cannot compare
these dots with anything more than the lengthened spots visible in
Acidaspis crenata.

The maculæ are wanting in this species, but the cephalic eyes are well
developed. A section of them fig. 19 shows the enormous difference of
the shell in the eye and the surroundings where the shell surpasses it
many times in thickness. The eye consists in fact only of the ovate,
beadlike lenses of which a string is seen sectioned in the figure
mentioned. More enlarged (fig. 20) a nucleus is visible in each lens,
and in a horizontal section (fig. 18) a little below the surface, where
they are more pressed against themselves, they have a polygonal shape.
The figure 17 seems to represent lenses that have been much changed
interiorly, having only a narrow zone left of the primary structure.


=Chirurus glaber= ANGELIN.

Pl. III fig. 11.

As a sample of the shape of the surface of the eyes in this genus, we
have given a figure, showing the small rounded lenses.


=Chirurus ornatus= DALMAN.

Pl. III figs. 12-14.

The cephalic eyes have globular lenses, of more than double the size of
those in Chir. clavifrons. The difference between the lenses and the
surrounding shell amounts at the highest to thrice the former, while
in Ch. clavifrons it is at least seven times as much. In a horizontal
section the lenses have a little, darkish irregular nucleus surrounded
by a radiated structure (fig. 12).

The maculæ (fig. 14) are slightly concave, oblong, smooth, surrounded
by comparatively large granulations.


=Chirurus speciosus= DALM.

Pl. IV fig. 1.

The exterior surface of the hypostoma is sparingly covered by granules
of larger size than in other Chiruri. The maculæ as seen in a cast
are prominent near the posterior border, sunk in their centre and
surrounded by a narrow distinct border line. In a horizontal section
it as not been possible to detect any peculiar structure, only a dark
border surrounding the interior clear surface.


_Chirurus spinulosus_ NIECZKOWSKI.

Pl. III fig. 15.

We have had the interior surface of a hypostoma at our disposal from
the lower Siluria, of Esthonia (Kuckers C^2) belonging to Dr. G. HOLM.
The macula are well preserved, standing out black on the white surface,
oblong, 1,5 millim. in greatest length (fig. 15). There are indications
as of a great number of small lenses on these maculæ. Remarkable are
the two elongated spots, almost 3 mm. in length, above these maculæ, of
which already mention has been made under Ch. clavifrons.


=Chirurus= sp. indet.

Pl. III fig. 16.

The hypostoma of an undescribed species from Öland. The maculæ are
lengthened, ellipsoid and smooth tubercles lying in a groove. The
finely granulated surface of the hypostoma shows rare smooth spots
interspersed, being on the inside smooth pits, from which again in
casts of the hypostoma larger tubercles are moulded. Of the same nature
are those that are visible on the nucleus of Chir. speciosus and
others. There are still some undescribed Lower Silurian species, for
instance one nearly related to Ch. conformis with tubercular maculæ. A
large hypostoma from the Leptæna limestone of Dalecarlia, 33 millim. in
length, has a longitudinal macula, 2 millim. in length, as seen on the
inside of the fragment.

On pl. VI fig. 10 a little hypostoma is delineated that shows some
resemblance with that of the Chirurus, but probably belongs to some
other, unknown genus. It is Upper Silurian, found at Mulde in Fröjel,
Gotland. It is of an elongated ovate shape, with the anterior border
faintly arched, the anterior wings broad, truncate. A narrow elevated
border surrounds the lateral and posterior margins. The somewhat
vaulted surface is covered by fine granulations, and the macula,
situated on equal distance from the anterior and posterior margins are
ovate, smooth and directed obliquely inwards. There are two minute
pointed processes on each side of the lateral borders and one on each
side of the pointed posterior margin.


=Ctenopyge= LINSN.

=Ct. spectabilis= BR. (I, pl. XII, fig. 12 a).

A small lengthened, apparently smooth hypostoma with two globular
tubercular maculæ near the posterior border. No mention made of them
in the description of BRÖGGER. Cambrian. I am not, however, quite sure
whether this really belongs to a Ctenopyge, as it rather more resembles
the type of a Peltura as shown in Pelt. scarabæoides. As Ctenopyge is
so nearly related to Sphærophthalmus it could be expected to see its
hypostoma of the same type and the maculæ entirely wanting. The eyes of
this genus have been described above at page 29.


=Cybele= LOVÉN.

=Cyb. bellatula= DALMAN.

Pl. IV fig. 2.

We give a new figure of the hypostoma to complete that of FR. SCHMIDT,
pl. XIII fig. 9. The anterior wings are larger and obtuse, excavated
below and the border rim is not so distinct all round as he has
figured. There are two pair of oblique, lengthened grooves above each
other. The opposite do not, however, join to form a coherent groove
across the median field of the hypostoma, which is level between them.
No maculæ. Nearer the anterior border close below the wings, there is a
small roundish, dark spot. The surface of the hypostoma is granulated.


=Cyphaspis= ANG.

=Cyph. elegantula= ANG.

Pl. III figs. 22-25.

The hypostoma is elongated, anterior margin evenly arched, anterior
wings triangular, lateral margins faintly curved as well as the
posterior margin, at both sides of which there is one diminutive
spine. The posterior wings are short forming a spiny process from the
lateral margin. The surface of the hypostoma is smooth with a shallow
transverse groove on the inferior moiety and two corresponding lateral
impressions below it, one on each side. The elevated lateral borders
are longitudinally striated by a few terrace lines. There are no maculæ
visible. The eyes consist f short hexagonal prisms (figs. 22, 23)
nearly resembling those of Proetus.


=Dalmanites= EMMRICH.

In this large genus the hypostoma has been figured in a great number
of species. But it is only in the following that we, at least in the
figures, if not in the descriptions, are able to detect the maculæ.

_D. atavus_ BARR. Suppl. pl. 5 fig. 14.

_D. Calypso_ J. HALL, vol. VII, pl. XI A. fig. 21 p. 66
»Postero-lateral pits, moderately strong and elongate». There are casts
of the two well distinct maculæ.

_D. caudatus_ BRÜNN.

a. FORBES and SALTER in British Organic remains, Dec. II pl. 1, fig.
3 give a good figure, showing the oblique pitlike maculæ. On page 2
is told that »a pair of lateral strong indentations indicate a second
furrow above» (»the transverse furrow between the tip»).

b. SALTER, pl. 3, figs. 7, 8, bad figures, the description verbally the
same as above. What ANGELIN pl. VIII, fig. 2 c gives as the hypostoma
of »Ph. caudata» is in fact the hypostoma of a species of Lichas.

_D. Mac Coyi_ BARR. Supl., pl. 13 f. 32. This magnificent hypostoma
belongs to the same group as D. micrurus HALL as to which we may
hesitate whether there not be a double pair of maculæ above each other
near the posterior border. In this species, however, there is the
transverse lower groove and above probably the two maculæ united by a
groove so closely as to resemble the lower groove.

_D. micrurus_ J. HALL, Pal. N. Y. III pl. 74 fig. 20. Shows, as it
were, a double pair of maculæ, very like D. spinifer BARR.

_D. rugosus_ BARR. pl. 24 f. 23. The two maculæ in the ordinary place,
without being united by a groove.

_D. socialis_ BARR. pl. 26 f. 21. Rather of an uncommon shape, not so
triangular as in the other species, the two maculæ distinct as narrow
slits. P. 553 »Vers l'extrémité du corps central, on aperçoit de chaque
côté, près du bord, une impression oblique, alongée et arquée».

_D. spinifer_ BARR. Pl. 25, fig. 20. A large beautifully preserved
hypostoma, shows what I think we positively must interpret as a double
pair of maculæ close above each other. As stated above there is
reason to believe that in other species of Dalmanites there also are
indications of four maculæ, though not so evident as in this species.
BARRANDE says nothing about this remarkable feature.


=Dalmanites imbricatulus= ANGELIN.

Pl. III figs. 43, 44.

The interesting image, as given by the agglomerated eyes, is
represented in the magnified figure 43 Plate III. The lenses lie there
separated from each other at much varying distance, some in close
contact. The presence of the delicate covering membrane is clearly
seen, being of white colour contrasting with the black, glossy lens.
It is much lacerated and only preserved around the periphery. The size
of the ocelli is on an average 0,5 mm., a little one has 0,3 mm. The
surface between the ocelli is most finely granulated. The granules
scarcely attaining the fourth of the granules in Dalm. vulgaris.

In a vertical section the regular biconvex lenses are seen to be
covered with the extremely thin integument, which is a direct
continuation from the test. The test between the lenses is perforated
by some longitudinal tubes, as usual in the skeleton of the trilobites.
We have not found any hypostoma with the maculæ.


=Dalmanites obtusus= LDM.

Pl. III figs. 45, 46.

We have not succeeded in finding any hypostoma, but as the structure
of the cephalic eyes is sufficiently well preserved, we here describe
it. The lenses seen from the surface look globular, and in a vertical
section they are ovate and in a horizontal circular. They are
covered by a very thin and delicate membrane, that envelops their
superior moiety completely and between the eyes it is enclosed by the
surrounding test and growing out from it. In the horizontal section,
fig. 45, it looks as a circular frame around the lenses and the
membranes of two contiguous lenses sometimes lie so close, that they
entirely fill up the interspace between the lenses, leaving only a
narrow slit between them marking their boundaries. The space between
the lenses, fig. 46, is porous, being perforated by tubes, which
continue vertically down through the shell of the cheek.


_Dalm. sclerops_ DALMAN.

Pl. IV fig. 3.

Commonly this species has been ranged with Phacops, but it is evident
that it as to the conformation of its glabella and the head in general
is highly discrepant, and in these respects is more concordant with
Dalmanites, though still deviating as for instance in the pygidium and
some details of the head. Therefore it may be justified with FRIEDR.
SCHMIDT[41] to consider it as a generic division, though not, as he has
it, as an independent genus, but as a subgenus of Dalmanites.

[Footnote 41: Revision, I, p. 76.]

The granulated hypostoma differs from that of Dalmanites proper in
having the anterior margin strongly arched, the pointed wings almost
on the median line of the hypostoma. There is one groove near the
posterior border and the maculæ lie as two, narrow long, crescentic
grooves close below the wings. The few sections give no clear idea of
any structure.

The cephalic eyes are constructed on the same plan as in the previous
species, the lenses being surrounded by a frame.


_Dalm. vulgaris_ SALTER.

Pl. III figs. 47-52.

The hypostoma has the common shield-like shape, fig. 51, anteriorly
slightly arcuated, the short obtuse wings curvated towards the interior
surface. It is finely granulated. The groove does not go so near the
cuspidate posterior margin as in other species. The two maculæ, fig.
52, are narrow and elliptic, somewhat convex and lie in a little
concavity, a narrow smooth space around is devoid of the granules,
which cover the surface of the rest. They show absolutely no structure,
excepting a few indistinct blackish spots.

The cephalic eyes have been often described and delineated. Most
detailed are the descriptions and figures given by SALTER in the
Memoirs of the Geological Survey Dec. II. pl. 1. In the figure 4 a
part of the surface is shown in well preserved state. He says that
»the cornea is ... present and distinctly convex over each lens, the
intermediate portions being ornamented with tubercles and granules».
He thinks also that there are undeveloped or small lenses between the
larger. His fig. 5 represents the frames around the lenses, these being
probably lost. His explanation of fig. 6 I cannot understand. There are
weathered or spoilt lenses figured.

There is no doubt that the granulated surface of the head continues
between the more or less free lying lenses as shown here in a new
figure taken from a specimen from Dudley, fig. 47. The granules of
the surface are larger and more rare between the ocelli than for
instance in Dalm. imbricatulus. Small lenses, undeveloped as SALTER
calls them, may in fact only be such granules of somewhat larger size
than usual. Lenses having the appearance as in SALTER'S fig. 6, as if
a covering was partially destroyed and the lens visible below it, I
have also found in Swedish specimens, as represented in fig. 47. In a
vertical section, fig. 50, a delicate covering integument is seen and
the oval lenses lie regularly with their frames around them quite as
in Dalmanites obtusus and in horizontal sections, figs. 48, 49, there
are also the same sort of black, irregular dots, being the sectioned
tubes of the surface between the lenses. But in another feature there
is great interest. Beneath each lens there are fascicules of tiny rods,
twice as long as the lenses. They are represented in a vertical section
in fig. 50 and in a tangential section in fig. 49.

As our imperfect knowledge of so delicate anatomical structures in
fossil crustaceans does not admit of secure comparisons with the visual
organs of recent crustaceans, no suggestion can be given to interpret
their nature. Probably they have no connection with the structure of
the eyes and it is not even certain that they are of organic origin.


=Dysplanus= BURM.

If there were no other characters to distinguish this genus from
Illænus its strangely deviating hypostoma must do it. We have examined
two species and in both the hypostoma is almost oval, evenly rounded
both anteriorly and posteriorly. Near the anterior border two narrow,
long hornlike wings project. They are flattened, thin and lamellar from
the basis and end in a hollow, acuminated point.


=D. centrotus= DALM.

Pl. III figs. 53-56.

This species has the maculæ placed near the posterior border, in a
semilunar flat, devoid of the terrace-lines. The maculæ are elongated
tubercles and do not show any structure when sectioned. The cephalic
eyes have a thin membrane covering the short cupshaped lenses, which,
closely packed, in the transverse section give the usual polygonal
mosaic of hexaeders. They have a radiate structure, when decayed.


_D. ladogensis_ HOLM.

Pl. III figs. 57, 58.

A specimen from Öland has the maculæ lying in the large field of
terrace lines above the smooth zone at the posterior margin of the
hypostoma. They are larger than in the former species, pearshaped,
smooth and no structure visible.


=Encrinurus= EMMR.

I cannot find any former figure indicating the maculæ of the hypostoma
in this genus than the only one of ANGELIN, Pal. Scand. pl. IV fig. 6.

As to the shape of the singularly formed hypostoma in the three Swedish
species Encr. punctatus, obtusus and lævis we have found the anterior
border so characteristic in them that it is sufficient to distinguish
them from each other and to annul the doubts concerning the distinction
of Encr. lævis from Encr. punctatus. We therefore in pl. IV figs.
12-17 give the anterior margins of these three species, along with the
profiles.


_Encrin. punctatus_ WAHLENBERG.

Pl. IV figs. 5-9, 12, 13.

The surface of the hypostoma is so finely and obscurely granulated that
it seems to be almost smooth. It is in the shallow groove above the
lamellar tongueshaped posterior border that the elongated tubercular
macula are situated. They are prominent and in horizontal sections
they have a surface mottled by irregular black and white specks, the
whole surrounded by a white border, fig. 6. This border and the other
surface is pierced by series of parallel slitlike pores, fig. 7, a
feature peculiar to this genus. The vertical sections indicate a chain
of indistinct prisms with a black, rounded central hollow between
thick strata of clear whitish shell substance, figs. 8, 9. The shell
substance on the sides is pierced by the straight, black tubes.

The cephalic eyes have the prisms indistinctly developed or badly
preserved with a diameter of 0,04 millim. and height of 0,07 millim. In
Encr. lævis the prisms are more distinct and larger with a diameter of
0,00 millim. (pl. IV figs. 10-11).

The maculæ are well developed, ovate, in E. lævis and obtusus and in
the same position as in E. punctatus.


=Griffithides= PORTLOCK.

The hypostoma of Griff. globiceps PHILL. shows according to the figure
given in H. WOODWARD'S work pl. VI fig. 5 two somewhat obscure maculæ,
but no mention is made of them in the description.


=Harpes= GOLDF.

_Harpes d'Orbignyanus_ BARR. NOVÁK II Taf. I fig. 4, with small black
maculæ. Harpes venulosus CORDA. NOVÁK II, Taf. I figs. 1, 2. Two
narrow, horizontally placed maculæ.


=Harpina= BARR.

_H. prima_ BARR. NOVÁK II Taf. I fig. 5. Two maculæ in continuation of
a narrow groove, at the base of the central globosity.


=Holmia= WALCOTT.

_Holmia Lundgreni_ MOBERG, Sveriges äldsta trilobiter, Tafl. 14, fig.
10, 11, 12. The maculæ are much prominent and tubercular, elongate
narrow, oblique but not mentioned in the description. Hypostoma
probably belonging to the terrace-line group. As well seen in original
specimens and in the figures in Dr. HOLM'S paper on Olenellus (Holmia)
Kjerulfi there are also in this species two distinct macula; near the
posterior margin of the hypostoma, above the marginal groove.


=Homalonotus= KÖNIG.

Maculæ have been previously observed in the following species.

_Hom. delphinocephalus_ GREEN, HALL Pal. N. York vol. II pl. 68 fig.
11, incomplete figure with two well marked tubercular maculæ. No
description.

=Hom. Knighti= KÖNIG. SALTER II, pl. 12 fig. 10. Maculæ well expressed
as oblique tubercles. On page 120 it is said »with a pair of lateral
tubercles well developed (as in Asaphus ...)».

This species is identical with ANGELIN'S Homalon. rhinotropis, Pal.
Sc., pl. XX fig. 1 e and his figure has two hollow maculæ seen from the
inside of the hypostoma.


=Homalonotus Knighti= KÖNIG (= Homal. rhinotropis ANGELIN).

Pl. IV figs. 20, 21.

The maculæ lie as oblong, smooth tubercles surrounded by the
irregularly grown granules of the surface, just below the central
globosity of the hypostoma. In a preparation from the interior surface
of the macula, as seen in transmitted light, fig. 21, this is white
with only a few grey spots and on the superior border black streaks,
lying obliquely. The substance of the shell around the macula is
perforated by the minutest pores amongst which a few larger are
intermingled. The granules are also perforated as those in Calymmene
and the tubes continue through the shell, visible by their black colour.


=Hysterolenus= MOBERG.

_Hyst. Törnquisti_ MBG., En trilobit från Dictyograptusskiffern p. 320.
In the descriptive letter-press, there is said, that in the exterior
(lateral) parts of the anterior groove of the hypostoma a distinct
tubercle is to be seen on each side. But none of the figures given, Pl.
17 figs. 6, 7, shows them or at least very indistinctly.


=Illænus= DALMAN.

It is remarkable, considering the great number of species in this
almost exclusively Lower Silurian species, that there is none but a
single species in which the maculæ hitherto have been delineated. This
is Illænus angustifrons var. depresses HOLM I, pl. VIII fig. 18. A
little, fragmentary hypostoma with globular maculæ.


=Illænus Chiron= HOLM.

Pl. IV figs. 22-25.

The hypostoma has the shape characteristic of all true Illæni, viz.
a straight anterior margin with two large flat rectangular or nearly
quadratic wings, a globular or spherical median field, forming the
chief portion of the exterior surface, which is covered by some rare,
fine, terrace lines. In some other Illæni the surface is smooth. At
the posterior base of the median field the two maculæ are placed,
tiny, oblong, but lying on the same level, at right angles to the
median longitudinal axis of the hypostoma. They are covered with
lenses of irregular and indistinct shape fig. 25. I do not think that
they in this and the other species of Tiberius are lenses of the same
conformation as in Bronteus, they are rather the tops of the subjacent
lenses which are of the prismatic form. The cephalic eyes do not
show any lenses on the outside. They have a narrow, opaque exterior
integument, hiding hexaedral, thickly packed, straight prisms, forming
a stratum in thickness of 0,2 millim.

The hypostoma consists of several thin layers of superposed calcite,
which easily peal off, so that it is seldom that the true exterior
sculptured surface remains.


=Illænus Esmarki= SCHLOTH.

Pl. IV figs. 26-33.

This the most common of all the Illæni has in several specimens
shown the hypostoma with the maculæ in situ. The hypostoma is smooth
excepting on the marginal ridges where there are some terrace lines.
On the median axis of the central gibbosity (fig. 30) five faintly
visible transverse folds are situated, a curious feature which we
have not observed in any other trilobite. The maculæ are exceedingly
small, fig. 31, elongate, spindleshaped and acuminated in both ends.
On their surface there are accumulated several lenslike globules. As
the horizontal sections show there are, however, no regular lenses,
but a network of polygonal meshes with radii in the hollows, fig. 32.
The perfect identity of this structure with that of certain states
of the cephalic eyes will at once be perceived by comparing this
figure with figure 26 representing the same sort of polyedric meshes,
though a little more regular in the cephalic eye, where, however,
the irregularity also prevails near the periphery of the eye. If we
now compare the sections of the cephalic eye of Illænus Chiron and
the section of the regular and nearly unchanged eyes of Ill. Esmarki
with the second section of the latter, we cannot avoid the conclusion
that we in those of Ill. Chiron, figs. 22, 23, and the first of Ill.
Esmarki, fig. 27, see the primitive and intact state of the lenses and
in the last sections of Ill. Esmarki, figs. 26, 28, the changed and
deteriorated state of the same prismatic lenses.

The vertical section of the hypostomic eyes fig. 33 shows in their
present changed condition an evident longitudinal direction of the
chief elements, quite as the lenses of the eyes. But in how far
there ever has existed such prismatic lenses, now changed, we have no
evidence definitely to ascertain, but the probability seems to be great
that this was the case.


=Illænus gigas= HOLM.

Pl. IV figs. 34-37.

The maculæ in this as well as the other Illæni are so faintly elevated
and so inconspicuous that the greatest attention is needed to find them.

The maculæ are elongated, narrow, elliptic on the same level and
parallel with the straight anterior margin of the hypostoma. A cast
of the left macula is pitted by numerous marks of the lenticular
globules of the surface, fig. 37. In the cephalic eye the prisms are
seen translucent beneath the cornea. Near the superior border they are
larger, fig. 34, almost the double size of those near the inferior
border, fig. 35.


=Illænus sphæricus= HOLM.

Pl. IV figs. 42, 43.

Comes near to the previous species. The globules on the surface of
the elliptic maculæ are not, however, spread over the entire macula,
leaving only a narrow border all around free.

The lenses in the cephalic eyes fig. 42 are globular as seen on the
surface, alike those of the maculæ.


=Illænus Roemeri= VOLLBORTH.

Pl. IV figs. 38-41.

The maculæ lie on elevated tubercles and are of a strange shape, being
sharpely pointed outwards, rounded inwards. That in fig. 39 is still
more peculiar with a narrow stripe on the interior and upper margin.
The lenses are larger and more distinct than in any other species of
this genus.

Beside the species now mentioned we have also observed hypostomas
with maculæ in Illænus laticlavius EICHWALD from Estland, in Illænus
Linnarssoni HOLM, and in an unnamed species from Dalecarlia figured
without macula by Dr. HOLM in his memoir on the Illænidæ pl. III fig.
22. We have not seen any lenses on the maculæ of these three lenses.


=Lichas= DALMAN.

We have not been able to find any notice about the exceptional maculæ
of this genus. There is only a figure of the hypostoma of Lichas hylæus
HALL Pal. of N. York vol. VII pl. XXV fig. 5 which would seem to show a
pair of narrow, oblique maculæ, but these are quite without resemblance
with the maculæ of the true Lichas.

The hypostoma of Lichas is of a characteristic type, broad, with the
incised posterior margin and its two lappets reminding of Asaphus,
with its large rounded, faintly elevated median part near the anterior
margin reminding of Illænus, as also do the lenticles of the maculæ.
The maculæ are of a minimal size, scarcely 1 millim. in diameter and
thence easily overlooked. Pl. IV figs. 46, 48.

As the hypostomas in this genus generally are found detached it is
often difficult to decide to which species they have belonged. So it is
with that delineated in Pl. IV fig. 44 natural size. The two maculæ are
situated near the centre of the hypostoma, just below the grooves which
surround the large, globular disc. They are sunk in a little cavity,
rounded and entirely covered by globular lenses (pl. IV fig. 46).

In another detached hypostoma of an unknown species the macula is
smaller and the few lenticular globules larger than in the other, fig.
48.


=Lichas latifrons= ANGELIN.

Pl. IV fig. 49.

The maculæ, seen in a thin section from the inside, are reticulate,
fig. 49, or of a pattern exactly like that in the changed eyes or
maculæ of Illænus and even Asaphus, and thus indicating that it was
composed of short prismatic lenses. The vertical section of a macula
of a specimen from another locality rather suggests the presence of
large sphærical lenses than elongated prisms. The great thinness of the
macula in contrast to the excessive thickness of the shell around the
eye is seldom so evident as here.

A specimen of an unknown species, found detached, has the macula?
unlike the other species, oblique with the inferior end directed
inwards, placed on the top of a sort of a little elevation, fig. 47.


=Megalaspides= BRÖGGER.

This author has a figure of M. dalecarlicus in II pl. 1 fig. 19 and
also of an unnamed species fig. 20, both showing globular maculæ. In
HOLM'S original description of his Megalaspis dalecarlicus (Trilobiten
des Phyllograptusschiefers Dalecarliens figs. 8, 9) the same hypostoma
is also figured. This genus seems to have a closer affinity with
Ptychopyge than with Megalaspis.


=Megalaspis= ANG.

The following figures of hypostomas with maculæ have been published.

_Megal. limbata_ by BRÖGGER I, tab. XII, fig. 10, and in II pl. 2, fig.
22, somewhat differing from the first figure.

_Meg. planilimbata_ BRÖGGER II tab. 2 figs. 21, 21 a. Both figures
incomplete and incorrect.

The hypostoma has a large central ovate gibbosity with the maculæ near
its basis, surrounded by the winglike expanded limbus.


=Megal. attenuata= WAHLENBERG.

Pl. V figs. 1-6.

The long and narrow maculæ are crescent-shaped and project beyond the
hypostoma supported on the superior surface of a sharp extenuated
edge where they rest as on a shelf. They consist, as seen in vertical
sections, figs. 5, 6, of alternating rows of light and black streaks,
and in a horizontal section the spongious network appears, as it is
found in Asaphus and others and thus indicating the tendency of this
organ to assume a prismatic structure though undeveloped.

The cephalic eyes in M. attenuata form the most regular hexaedral
prisms fig. 1 a, short and covered by a thin cornea. The border zone of
the eyes has a structure exactly like that of the maculæ, the same as
in the spongious zone of Asaphus.


=Megal. limbata= BOECK.

Pl. V fig. 7.

We give a new figure of BRÖGGER'S original specimens, as his figure 23,
Tab. II is incomplete and incorrect. The maculæ are situated a little
higher up on the sides of the central gibbosity.


=Megal. planilimbata= ANG.

Pl. V fig. 8.

The maculæ are crescent-shaped and not so prominent as in Meg.
attenuata and the visual spot in the same position as there.

To observe is that the general structure of the hypostomic shell in
this and many other species is prismatic.


=Nileus= DALMAN.

Hypostoma with maculæ figured in N. armadillo by ANGELIN pl. XVI fig. 5
c, and BR. II pl. 3 fig. 40.

The hypostoma in this genus is transversal with large lateral
expansions, the central convexity only faintly elevated and almost
coherent with the posterior part. The maculæ lie exactly on the
horizontal median line of the hypostoma. The anterior wings are well
developed like slightly bent acuminated horns and the posterior wings
are conical points. The terrace lines are transverse and a little wavy.


=Nileus armadillo= DALM.

Pl. V figs. 10-15.

There is some variability in the hypostoma of specimens from different
localities. The most known and numerous specimens are from the
renowned localities of Östergötland. They are decidedly transverse,
the proportion of the hight to breadth is as 3 to 5 and the narrow
border runs all round. In specimens from Kongslena, Vestergötland, the
proportion is as 15 to 22, in specimens from Sandvik, Öland, again, we
have 13 to 23 or 6 to 11. The maculæ, fig. 14, are oblong, inwardly
pointed, quite smooth, with a little concavity in their centre,
and their surface is slanting obliquely towards the surface of the
hypostoma, almost as in Niobe. There is no trace of any structure.

The cephalic eyes are covered by a comparatively thick membrane, a
direct continuation of the general test, fig. 12, hiding a row of
prismatic lenses. In some specimens elongate, clear, crystalline rods
continue downwards from the bases of the lenses and are probably
nothing but inorganic crystals of calcareous spar, fig. 11.--The eyes
are larger comparatively than in other species.


=Nileus (Symphysurus) palpebrosus= DALMAN.

Pl. V figs. 16-18.

As to its general form the hypostoma resembles that of the preceding
species, but there is no border line, which is so characteristic to the
former. The maculæ have the same form and are likewise situated on the
median line. The cephalic eyes have prismatic lenses with much convex
bases, in a transverse section they are hexaedral and of internal
radiate structure.


=Niobe= ANGELIN.

The following figures have formerly been given of its hypostoma with
maculæ.

_N. emarginula_ BR. II tab. 2 fig. 33.

_N. explanata_ BR. II tab. 2 fig. 35.

_N. frontalis_ BR. II fig. 37.

_N. insignis_ BR. I tab. IV fig. 1 d, 11 tab. 2 fig. 28.

_N. læviceps_ BR. II pl. 2 fig. 34.

The maculæ are distinguished from all other sorts of maculæ, excepting
those of Megalaspis, in being, as it were, shelved on a sort of
support, formed through the bulging of the hypostomic surface just
below them, one such console for each. Their surface lies consequently
almost rectangularly to the adjoining surface of the hypostoma. They
are white and smooth.


=Niobe frontalis= ANG.

Pl. V figs. 19-21.

The pocketlike consoles are shorter and broader than in N. læviceps
and the maculæ also larger and of an elliptic shape. In a horizontal
section near the surface the macula has a speckled appearance of
black dots, indicating an undeveloped prismatic structure, much more
primitive than in the Asaphidæ.


=Niobe læviceps= ANG.

Pl. V figs. 22-26.

It differs from the preceding through longer consoles below the maculæ,
which are oval and in a horizontal section exhibit the same spongious
structure, pl. V fig. 25. The cephalic eyes consist of polyedric
prisms, which through deterioration show a central cylindrical core
surrounded by darkish matter like Asaphus. The passage from solid,
homogenous prisms to changed ones is most evident in the figure 22.


=Ogygia= BRONGN.

Figures of the maculæ are found in

_O. Buchi_ SALTER, Mem. Geol. Surv., Dec. II pl. VI fig. 3 p. 2, where
it is stated that »there are two transverse furrows near the apex, with
compressed tubercles between them» There are two narrow crescentic
maculæ and between them a little lower a longer, crescentic ridge,
which may be the ridge that in other species connects the maculæ.
Moreover, the shape of the hypostoma is not concordant with that of the
other species. In Monogr. Brit Trilob. pl. 15 figs. 2, 3 there is only
a reproduction of the former figure.

_O. corndensis_ MURCH. SALTER in Monogr. pl. 16 fig. 10 interior side
of the hypostoma with two lateral maculæ. In the description two pairs
of furrows the uppermost is the maculæ. H. WYATT-EDGELL »On the Genera
of Trilobites Asaphus and Ogygia and the Subgenus Ptychopyge» in
Geol. Magaz. 1867, p. 14, 15 fig. 2 probably a cast with two oblique
impressions of maculæ.

_O. dilatata_ (Asaphus) SARS var. Sarsi ANG. SARS in Isis 1835 p. 342,
pl. IX fig. 11. The figure has two small maculæ near the posterior
margin and the author says »mit einer starken Vertiefung oder
Einschnitt an jeder Seite des Endes». ANGELIN pl. XLII fig. 1 _b_.
BRÖGGER'S fig. 38 pl. 3, in II, collies near to the specimen, which is
described below, also belonging to Dr. G. HOLM.


=O. dilatata= var. =Sarsi= ANG.

Pl. V figs. 27, 28.

The hypostoma is in so far deviating that the terrace lines have a
nearly vertical direction and in this respect much resemble those of
the Phillipsiæ and the Proeti. The maculæ, near the exterior angles
of the border groove are pear- or spoon-shaped with the apex directed
downwards and outwards, and in consequence the longitudinal axis quite
opposite to that of the usual direction. The surface is smooth, a
little concave. No structure has been discovered. From the pointed
apex a slightly elevated ridge runs along the posterior border of the
hypostoma and joins the apex of the other macula.


=Paradoxides= BRONGN.

The oldest record of any hypostoma at all amongst the trilobites is
that given by WAHLENBERG in his Petref. Suecana (1818) p. 37 Tab. 1,
fig. 6, which he, however, considered as the head of the trilobite
which he named Entomostracites bucephalus. Figures with maculæ have
been given of

_Par. Davidis_ SALTER Mem. Geol. Survey Dec. XI pl. X, fig. 3, a fine
figure of a hypostoma with two large oblique maculæ. LINNARSSON in
»de undre Paradoxideslagren vid Andrarum» pl. II fig. 2 delineates a
gigantic hypostoma with two crescentic tubercular maculæ, having 10
mms. in length.

_Par. Forchhammeri_ ANGELIN Pl. II fig. 3. BRÖGGER, Paradoxidesskiffr.
vid Krækling tab. II fig. 10 two marks somewhat different from
ANGELIN'S figure. LINNARSSON 1. c. pl. I figs. 9, 10 two hypostomas
with maculæ.

_Par. Tessini_, var. _Wahlenbergi_ ANGEL. Pl. I a, fig. 1 b.

_Par. Tessini_, var. _oelandicus_ ANG. Tab. I a, fig. 2 b. ANGELIN'S
figure of this hypostoma is incomplete in so far that he had not found
the peculiar falciform horns on both sides of the posterior border. Pl.
V, fig. 33.

_Par. rugulosus_ CORDA has, according to BRÖGGER, Krækling, pl. II fig.
2 two long, straight callosities, probably corresponding to the maculæ
in other Paradoxidæ.

CHR. BOECK gives in Mag. for Naturvidenskaberne Bd 8, 1828, in his
paper on the Trilobites on the plate fig. 16 the hypostoma of a
Bohemian species with well developed concave maculæ, probably a cast.
He compares it with the Entom. bucephalus of WAHLENBERG, but says he
cannot explain its nature.

The maculæ of P. oelandicus, pl. V, fig. 34, are tubercles, oblong,
smooth, and have invariably an oblong scar along the centre, where
probably once a thinner membrane, as in the cephalic eyes of Calymmene,
contained the lenses.


=Peltura= M. EDW.

=Peltura scarabæoides= WAHLENB.

Pl. III fig. 42.

We have given a figure of the hypostoma, showing two large, elongated
tubercular maculæ or rather resemblances of such, as they are covered
with terrace lines as well as the rest of the surface. They have,
however, the same position as real maculæ might have, but are directed
in a line parallel with the longitudinal axis of the hypostoma. The
cephalic eyes have been described above at page 29.


=Phacops= EMMRICH.

Maculæ figured of Phac. (Acaste) Downingiæ. SALTER Monogr. pl. 2 fig.
34 b, p. 25 »high up on each side a small tubercle».

_Phac. rana_ HALL Pal. N. Y. vol. VII, pl. VIII A, fig. 18. Hypostoma
with the large commalike maculæ high up.

Of the following species Ph. cephalotes and Ph. macrophthalmus belong
to the genus Phacops proper, while Phac. quadrilineata, as well as
Ph. Downingiæ are of a quite different generic type. The name Acaste
proposed for them is not good as LEACH in 1817 named a Cirrhiped as
Acasta.


=Ph. cephalotes= BARR.

Pl. V figs. 35, 36.

The hypostoma is elongate, almost triangular, tricuspidate at the
posterior border, a little above this there is a crescent-shaped ridge,
and still a little higher above this and the median horizontal line,
the two macula, as diminutive tubercles. Seen with magnifying powers
they are slightly oblong or reniform.


=Ph. macrophthalmus= BURM.

Pl. VI figs. 1-9.

The triangular hypostoma has a little below the median horizontal
diameter two shallow grooves, and a little above the same diameter the
extraordinary minute maculæ may with some care be observed, figs. 8,
9. They are visible as a small bare, smooth spot amongst the curious
elevated and incoherent terrace lines of the surface. The exceedingly
small size of these maculæ naturally does not allow any research of
their structure. They are rather in this species and many of the
congeneric to be regarded as small ocelli. In some specimens these
spots are quite rudimentary and are reduced to the smallest area
possible. The anterior wings are of a peculiar earlike shape, finely
striate, enlarged and flat near the hypostoma and ending in a little
hollow horn (figs. 6, 7). A little below it, the posterior wing emerges
as a small angular knob. The aggregate cephalic eyes are more distinct.

As the vertical section learn us the regular ovate lenses consist
generally of clear, crystalline calcareous spar and are on the surface
covered by an extremely thin membrane, not a common one for all, but
a separate cornea for each lens, which envelopes its superior moiety
and continues down as a thickened appendix between the lens and the
interstitial test (figs. 3-5). In the horizontal sections it surrounds
the lenses as an annular wall. Around some lenses there is as if still
another ring lay outside this, but more indistinct, probably only
clearer portions of the test surrounding them. The cheeks close to the
lenses are indistinctly perforated by longitudinal tubes.


=Phacops (Acaste) quadrilineata= ANGELIN.

Pl. V figs. 37, 38.

We have not succeeded in finding any well preserved hypostoma.

The lenses of the cephalic eyes resemble much those of the preceding
species though more elongate and ovate. They are covered by a membrane
which surrounds their upper moiety and between the lenses joins the
interstitial test. In the horizontal section these membranes surround
the lenses as an annular wall.


=Phaëtonides= BARR.

=Ph. Stokesi= MURCH.

Pl. VI fig. 11.

The hypostoma somewhat reminds of that of Calymmene, has projecting
angles and a prominent knob below the straight anterior border. The two
oblique and oblong macula are tubercular and placed near the lateral
borders. This hypostoma is upon the whole much related to that of
Phillipsia and Proetus.


=Phillipsia= PORTLOCK.

DE KONINCK figured a hypostoma of Phillipsia in his »Description des
Animaux Foss. de Belgique» as Cyclus Brongniartianus pl. LII fig. J
and NOVÁK in III figs. 6, 7 copied it and corrected the error. It is
provided with two globular maculæ united by a curved ridge.

_Phillipsia Eichwaldi_ WOODW. NOVÁK III fig. 5 a hypostoma with
globular maculæ. We have been able to examine three different, unnamed
or undeterminated species, which we distinguish by numbering them.

_Phill. No. 1_ from the Keokuk group of Crawfordsville, N. America. The
hypostoma is broader than in the other species. The nearly horizontal
elongated maculæ placed near the median line, close to the lateral
margins.

_Phill. No. 2_ (pl. VI, figs. 15-18) probably from the Carboniferous
formation of Belgium is of a lengthened form. The elliptic maculæ in
the groove, below the central elevation. They are smooth and surrounded
by a flat border.

The cephalic eyes are much destroyed, but seem to have consisted of
somewhat semi-prismatic lenses or prisms of the same type as Dysplanus.

_Phillipsia No. 3_ (pl. VI figs. 12-14) from the Carboniferous strata
of Beeren Eiland communicated by Hr J. G. ANDERSSON. It comes near the
preceding, is lengthened, but has broader and shorter anterior wings
and the inferior surface just above the posterior border peculiarly
pitted. The macula lying above this pitted field are ovate. The lenses
of the cephalic eye are hemispheric on the surface.


=Platymetopus= ANGELIN.

=Pl. planifrons= ANGELIN.

Pl. IV figs. 50, 51.

This genus may well be kept distinguished from Lichas in consequence
of its peculiar hypostoma and its still more peculiar maculæ, not to
remind of the other deviating features. The maculæ lie at the end of
narrow, sigmoid grooves of their own above the long transverse groove,
separating the posterior border from the anterior elevated moiety.
They are elongated, curved and entirely covered with a great number of
irregular lenticles, translucent through the thin covering membrane.


=Pliomera= ANG.

_Pliomera Törnquisti_ HOLM in Trilobit. Dalecarliens p. 5 fig. 2.

While Pl. Fischeri and Pl. actinura not have any hypostomic maculæ,
this species, which Dr. HOLM with some doubt joins in the same genus,
has a hypostoma, which as to its general shape resembles that of both
the mentioned species, evenly rounded at its posterior border, with
a shallow groove encircling the central field. A little below the
horizontal diameter of this the maculæ stretch obliquely inwards and
downwards. They rest on a little elevation and are elongate, anteriorly
acuminated. The figure on HOLM'S plate does not show the maculæ.


=Proetus= STEININGER.

There are some indistinct indications of maculæ in several of the
hypostomas figured in BARRANDE'S Système pl. 15.

_Proet. signatus_ LINDSTR., Gotl. Trilob. Pl. XV fig. 17, shows very
distinctly the two maculæ.


=Proet. concinnus= DALM.

Pl. VI figs. 19-23.

In this, as well as in the few other species of Proetus which we have
studied, there is that peculiarity of the surface of the hypostoma that
two more or less distinct ridges form an acute angle just below the
faintly arcuated anterior border. The terrace lines are exactly alike
those of the Phillipsiæ, only visible on the central field and ceasing
just above the maculæ, where there is a transverse smooth field. The
posterior margin has two pair of very short points, one on each side.
The macula are lengthened tubercles, somewhat bent and have an oblique
position. They are surrounded by tiny prickles and as seen in fig.
23 the macula proper forms a white, elliptic spot on the tubercle.
The cephalic eyes are lengthened prisms covered by a thick membrane.
They are convex at their base and in a transverse section they show a
radiate structure of their interior.


_Proet. conspersus_ ANGELIN.

Pl. VI figs. 24-26.

We have figured a decorticated hypostoma with impressions of two
reniform maculæ. The vertical sections of the cephalic eyes show a thin
stratum where the lenses are much shorter than in the previous species
and covered by a thin film of the common test. The same is also the
case in a species which probably is Proet. verrucosus.


_Proet. signatus_ LINDSTRÖM.

Pl. VI figs. 27-30.

On the lateral borders of the hypostoma there are short spines, two
on each side, but they are placed differently on the specimens and
even wanting in some. The elongated, elliptical maculæ bear in their
lower end a white spot on which is seen a little cluster of a varying
number of small, segregated ocelli, from three to five, according to
the different specimens, like so many black points. The maculæ attain
the exceptional length of 4,7 millim. in some specimens, but commonly
only 2 millim. We have as yet not been able to detect these remarkable
ocelli in any other species of Proetus.


Ptychoparia CORDA.

WALCOTT in »Palæontology of the Eureka District», gives on plate X fig.
21 a representation of a hypostoma which near the posterior margin
shows two maculæ as narrow, crescentlike ridges and above them two
longer and thicker ridges directed obliquely toward the central axis of
the hypostoma.


=Ptychopyge= ANG.

Previous figures of hypostomas with maculæ are the following:

_Pt. aciculata_ BR. II, pl. 1 fig. 13.

_Pt. glabrata_ BR. II, pl. 1 fig. 14.


=Pt. aciculata= ANG.

Pl. VI figs. 40, 41

The broad hypostoma bears the elongated, acuminated maculæ in an
oblique direction, contrary to what is seen in most other species. The
anterior end slopes inwards and the lower end outwards, the surface,
which in the cast drawn is surrounded by a marginal line, is directed
sidewards against the broad lateral wings and is obscured by the
declivity formed by the groove. The same disposition of the mature
also occurs in Ptych. glabrata. These two species consequently in
this respect differ essentially from the other four species which we
have examined, but there is for the rest no reason to make these form
different genera or subgenera.


=Pt. angustifrons= DALM.

Pl. VI figs. 31-36.

There are two varieties of hypostomas: one broad fig. 32 and one more
elongated fig. 33. The maculæ having a convex surface lie at the
inferior end of the lateral grooves prominent on a semicylindrical
support, slightly inclining outwards. In the broad variety they are
oblique (fig. 34) and in the elongated variety they are horizontal
(fig. 35). In a vertical section fig. 36 we have the same indications
of a prismatic arrangement in the maculæ as in the Asaphi. The
horizontal section does not represent so clear and distinct a
reticulate structure as in the Asaphi, the prisms are much smaller.

The cephalic eyes are composed of closely packed hexaedral prisms, the
gradual change from solid homogenous ones to those which are interiorly
destroyed, only with the exterior form preserved is easily seen. The
figure of a horizontal section (fig. 31) gives a good idea of the
peculiar destruction of the interior of these prisms. In the vertical
section the prisms are not discernible.


=Pt. cincta= BRÖGGER? according to FRIEDR. SCHMIDT in Mus. Holm.

Pl. VI figs. 37-39.

The hypostoma which BRÖGGER figures (II, pl. I fig. 2 _a_) as belonging
to this species is completely identical with that of Pt. angustifrons,
while the specimens which FR. SCHMIDT has marked out as being the
true Pt. cincta have a different hypostoma. The maculæ vary in the
specimens, being in some placed horizontally or in a right angle to
the longitudinal axis of the hypostoma, in others, again, a little
obliquely outwards. They form prominent, free lying tubercles. It is
from the superior face that the rudimentary lenses have left their
traces, as seen in a longitudinal section (fig. 39). It is consequently
this superior surface which is the macula proper.


=Pt. glabrata= ANG.

Pl. VI fig. 42.

We have given a figure of the elliptic acuminate maculæ as seen on a
cast of the inside of a hypostoma, the same which is the original to
fig. 14 in BR. II.


=Schmidtia= MARCOU.

MOBERG has in his paper on »Sveriges äldsta Trilobiter», pl. 15, fig.
6, drawn a hypostoma of Schmidtia Torelli with scars of maculæ, placed
like that of the Paradoxidæ. There is no mention made of them in the
descriptions.


=Trochurus= BEYRICH.

=Tr. pusillus= ANG.

Pl. IV figs. 52, 53.

The strange transverse, bipartite hypostoma carries the small, round,
tuberculose maculæ on the superior moiety. As seen from the interior
(fig. 53) they are obscurely maculate, but it is doubtful whether these
black dots are derived from lenses, as the surrounding hypostomic
surface also has the same mottled aspect.


=Trochurus= sp.

Pl. IV figs. 54, 55.

This hypostoma belongs to an unknown species of Trochurus from the
Southern Gotland (Burs). Its macula are placed almost as in the
preceding species, but are much larger. They are surrounded by
prominent margins. No lenses have been discovered on them.




CONCLUSIONS.


In reviewing the genera and species enumerated above, as to their
number in which the structure of the macula has been observed, we
arrive at the following results.

  ---+----------------+-----------+---------------------+
     |                | Number of | Number of species in|
     |     Genera.    |  species  |  the maculæ of which|
     |                |   with    |  the structure has  |
     |                |  maculæ.  |    been examined.   |
  ---+----------------+-----------+---------------------+
   1 | Acaste         |     1     |         --          |
   2 | Acidaspis      |     1 ?   |         --          |
   3 | Asaphus        |    18     |          7          |
   4 | Barrandia      |     1     |         --          |
   5 | Bronteus       |     9     |          5          |
   6 | Bumastus       |     7     |          1          |
   7 | Calymmene      |     4     |          2          |
   8 | Centropleura   |     1     |         --          |
   9 | Chasmops       |     1     |         --          |
  10 | Chirurus       |     9     |          2          |
  11 | Ctenopyge?     |     1     |         --          |
  12 | Dalmanites     |    10     |          2          |
  13 | Dysplanus      |     2     |          2          |
  14 | Encrinurus     |     3     |          1          |
  15 | Griffithides   |     1     |         --          |
  16 | Herpes         |     2     |         --          |
  17 | Harpina        |     1     |         --          |
  18 | Holmia         |     2     |         --          |
  19 | Homalonotus    |     2     |          1          |
  20 | Hysterolenus   |     1     |         --          |
  21 | Illænus        |     9     |          5          |
  22 | Lichas         |     4     |          1          |
  23 | Megalaspides   |     2     |         --          |
  24 | Megalaspis     |     3     |          1          |
  25 | Nileus         |     2     |          2          |
  26 | Niobe          |     5     |          2          |
  27 | Ogygia         |     3     |         --          |
  28 | Paradoxides[42]|     8     |         --          |
  29 | Peltura        |     1     |         --          |
  30 | Phacops        |     3     |         --          |
  31 | Phaëtonides    |     1     |         --          |
  32 | Phillipsia     |     5     |         --          |
  33 | Platymetopus   |     1     |         --          |
  34 | Pliomera       |     1     |         --          |
  35 | Proetus        |     3     |         --          |
  36 | Ptychoparia    |     1     |         --          |
  37 | Ptychopyge     |     4     |          2          |
  38 | Schmidtia      |     1     |         --          |
  39 | Trochurus      |     2     |         --          |
     |                +===========+=====================+
     |          Total |   136     |         36          |

[Footnote 42: To these have here been added two Bohemian species P.
bohemicus and P. spinosus omitted above at page 64.]

We have thus 136 species of 39 genera in which hypostomic maculæ have
been found and only 36 species, in which it has been possible to study
the structure of the maculæ through sections.

Common for a great number of maculæ in various groups, whether they
show any organic structure or not, is the excessive thinness of their
shell in comparison with that of the surrounding hypostoma. This is
also in accordance with the tenuity of the cephalic eyes in relation to
the test of the cheeks.

The structure which characterizes the macula as a visual organ,
although often so rudimentary, is not in all instances spread through
the whole substance of the macula. This structure has in several
species been restricted to a narrow circumference of the body of the
macula, to its interior apex as in Bronteus and Proetus. Only in the
Asaphidæ, in Illænus and Lichas the entire macula shows this structure.
Perhaps, to judge by certain indications in Bronteus, once in a larval
or preceding stage of evolution the whole surface of the macula was
also in that genus covered with lenses, which have been reduced.

The different groups in which the examined genera may be divided are
the following seven, showing the remarkably great diversity of these
organs. There is even in the same genus so great a variability that
species with structure in the macula occur along with those devoid of
any structure or also, as in Lichas, with a different structure. It
must, however, be remembered, that the species of such genera may not
be coeval.

_Group 1._ In the sectioned macula there is no trace whatever of any
structure. The test of the macula considerably thinner than that
of the hypostoma. According to the affinities of the genera they
may be subdivided as follows. _a_ Bumastus, Dysplanus. _b_ Nileus,
Symphysurus, ? Ogygia. _c_ Calymmene, Homalonotus. _d_ Chirurus pro
parte.

_Group 2._ The whole macula is of a spongious or irregularly polyedric
structure, showing prisms in vertical sections. Its concordance with
the structure of the deteriorated cephalic eyes or with the so called
border zone is complete. If a supposition may be hazarded, I think that
the spongious or reticulate structure in the maculæ is their real and
original state, a lower stage of development of the visual organs, out
of which the prisms in the cephalic eyes have been formed, and that the
prismatic lenses in their decay, as seen in the Asaphidæ, reveal to us
their original state and structure, and thus, as it were, return to the
primary stage in the maculæ.

Asaphus, Isotelus, Megalaspis, Ptychopyge, Niobe, ? Megalaspides, ?
Barrandia.

Illænus, Lichas, Trochurus, ? Platymetopus.

Encrinurus also probably belongs to this group.

_Group 3._ Maculæ with well developed globular lenses on the interior
third of the macula. The blank part of the macula without any
structure. In one instance (Chirurus spinulosus) the whole macula
covered with lenses.

Bronteus, Chirurus spinulosus.

_Group 4._ The macula form sunk pits with smooth bottom. Structure
unknown. In a few species there seems to be no less than four maculæ in
two pair above each other.

Dalmanites.

_Group 6._ The elongate straight maculæ carrying on their innermost
point from three to five diminutive, segregate ocelli situated on a
clear, white patch. Proetus.

As yet these ocelli have not been observed on the related genera
Cyphaspis, Phaëtonides, Phillipsia and Griffithides, but they may be
preliminarily ranked here in consequence of their close conformity.

_Group 7._ The maculæ have in this group been reduced to a pair of tiny
ocelli situated high up on the hypostoma, near its anterior margin.

Phacops, Acaste.

The position of the hypostomic eyes on the ventral surface of the
trilobites is not quite so abnormous, nor so isolated a feature amongst
the crustaceans as might at first be supposed. Amongst the recent
crustacea there is probably none, which as to the conformation of its
labrum or hypostoma resembles the trilobites so closely as the species
of the genus Apus, however different they may else be in all other
respects. We have studied chiefly the hypostoma of Apus cancriformis.
In its shape as seen from the outside and as to its outlines it reminds
strongly of the trilobite hypostoma. It is nearly square (pl. VI figs.
46, 47) with rounded corners. The anterior margin is arched and in
the middle it has a broad tonguelike prolongation. On the sides of
the anterior margin there are two short, sharply pointed wings. The
lateral margins are thick, almost as doubled, and bent forward so that
a groove is formed alongside them and the large, evenly vaulted median
surface of the hypostoma. These grooves are continued in a short groove
parallel with the posterior margin. From this margin a narrow oblique
surface is slanting backwards, as it were, doubling that margin. On
its surface there are two small, resplendent, white or clear spots,
almost of the shape of a crescent, though more irregular when seen
in higher enlargement (pl. VI fig. 49). They are translucent as to
be visible on the interior side of the hypostoma. In Apus glacialis
there are also similar spots in the same position, but they form short
narrow, straight stripes, parallel with the border of the posterior
margin. I have no opinion as to the nature or function of these white
spots. I do not think that they at all are homologous with the macula;
of the trilobites. On the other hand we see a little higher up on the
exterior side of the hypostoma two faintly elevated elliptic tubercles
which as to their position and shape highly remind of the maculæ of the
trilobites. They are hollow on the inside and we have not been able to
find any peculiar structure in them. That they have any visual function
is precluded through the circumstance that the whole exterior surface
is densely covered with microscopic bristles (pl. VI fig. 49) which are
especially remarkable along the posterior margin.

In the class of the Cirrhipedia a sufficient number of facts is known
to ascertain us of the presence of eye spots on the hypostomas of the
larva. The researches of SPENCE BATE[43] show in the larva of Balanus
balanoides, on the hypostoma or labrum, which he calls proboscis,
an eye, a little from the posterior margin of the labrum. It seems
during the various moults of the animal to have changed its place,
being in the two first moults situated near the anterior margin of the
hypostoma. In Clitia Strömi it lies at the anterior part of the labrum
(figs. 9, 10). SPENCE BATE does not accept the opinion of their nature
as eyes, but the following authors as DARWIN and CLAPARÈDE agree in
considering these spots as eyes. DARWIN[44] describes two such eyes
in Lepas australis and, if I catch his meaning rightly, one on each
side of the labrum. CLAPARÈDE has given a figure of the larva of Lepas
anatifera[45] in the hypostoma of which (by him called »Rüssell»,
proboscis) a single eye is seen, having at the sides two round
problematic organs, that he at first considered as auditory cells,
but later as eyes. HESSE[46] has also in a species of Scalpellum seen
several coloured spots on the inferior surface of the carapace, below
the antennæ and he expressly says that they are eyes and a special pair
of supplemental eyes that are becoming functional after the first moult.

[Footnote 43: On the development of the Cirripedia, Ann. Mag. Nat.
list. 1851, p. 324, pl. VI figs. 1, 2, 3, pl. VII fig. 10.]

[Footnote 44: Monogr. of Cirripedia I p. 17.]

[Footnote 45: Beobachtungen ... über wirbellose Thiere, 1863, pl. XVII
fig. 22.]

[Footnote 46: Mémoire sur les métamorphoses que subissent les Anatifes
appelés Scalpels obliques. Annales des Sciences nat. 1859. Tome XI p.
163.]

These sternal eyes in the larva are previous to the eyes in the dorsal
side of the later stages and whenever such are discovered in other
groups of adult animals as the trilobites they must of course be
considered as remnants from the larval stage. As a rule, in several
tribes of the crustacea it has been observed that the eyes originate
on the ventral or sternal side of the body and in the advancement of
growth move to the superior side. So according to PACKARD and others
the embryo of Limulus has originally the median eyes on the ventral
side. In the same way the cephalic eyes of Dalmanites move, as BARRANDE
has shown, from the ventral side to their permanent place on the
superior side of the head shield. But while in the Cirrhipedia the
larval or hypostomic eyes are lost in the adult they remain on the
hypostoma of a great number of the adult trilobites and have probably
left their mark behind in A pus. No wonder then that we find these
maculæ developed as precursors of the hypostomic eyes in a number of
the oldest and in other respects blind trilobites as the Olenellidæ and
Paradoxidæ to which they may have been of some use as a sort of visual
organs, though probably very imperfect.

We find the maculæ of the trilobites present from the oldest Cambrian
times and we find also in them a progressive evolution, in some to a
high degree, lenses and facets, perfectly identical with those of the
eyes on the head shield, converting them into true eyes. It was in the
Lower Silurian times, in the Lower grey Orthoceratite limestone of
Sweden, that Illænus Esmarki appeared provided with actual prismatic
lenses on its maculæ and probably also Lichas. Still higher at the top
of the Lower Silurians, in the Leptæna limestone of Dalecarlia the
genus Bronteus has appeared with those highly developed hypostomic eyes.

But there are, no doubt, still more facts to adduce for filling up
extant lacunæ in the knowledge of these matters. The material now at
hand has, however, not been sufficient for doing this. At present I
must leave this interesting field of investigation and be contented if
I have succeeded in showing where rich harvests await every one who
will make a search for them.


Explanation of the plates.


  All specimens figured belong to the Swedish State Museum,
  unless otherwise stated.


Pl. I.


=Acidaspis crenata= EMMR. p. 85.

  Fig.  1 & 2.  Vertical sections of the eye 60/1.
   »    3.  The outside of the hypostoma 6/1.
   »    4.  The anterior margin of the hypostoma, showing
              one of the clubshaped spots in transmitted
              light 20/1.
   »    5.  The granulations of the hypostoma in transmitted
              light 80/1.
   »    6.  Vertical section of the hypostoma with granules 60/1.
            Specimens from Djupvik, Eksta.


=Agnostus glandiformis= ANGELIN p. 35.

  Fig. 7. Vertical section of a part of a rolled up specimen 8/1.
            Specimen signed CQ. Andrarum, Scania.


=Asaphus expansus= L. p. 39.

  Fig.  8.  Horizontal section of the eye near the upper margin.
              _a._ The surrounding spongious zone,
              _b._ The surface of the eye with lenses 60/1.
              Spec. CG.
   »    9.  Horizontal section from the centrum of the eye 60/1. Spec. CF.
   »   10.  Horizontal section near the inferior margin,
              _a_ the eye, _b_ marginal zone, _c_ a portion
              of the cheek 60/1. Specim. CF.
   »   11.  Vertical section near the inferior margin 60/1,
              same specimen as in fig. 10.
   »   12.  Eye fractured vertically showing regular prisms
              in the eye (_a_) and irregular ones in the marginal
              zone (_b_) 30/1.
   »   13.  Part of the hypostoma with the macula 12/1.
   »   14.  The right macula from the inside 12/1.
   »   15.  A vertical section across the hypostoma,
              _aa_ the maculæ 8/1.
   »   16.  A similar one of the macula at left in the
              preceding figure 60/1.
   »   17.  The right macula from the outside in transmitted
              light 60/1. Sp. BQ.


=Asaphus fallax= DALM.? p. 41.

  Fig. 18.  Horizontal section of the eye 60/1. Spec. CD.
   »   19.  Vertical section of the same passing obliquely
              through two rows of prisms 60/1.
   »   20.  Fragment of the same eye with entire prisms 30/1.
   »   21.  The hypostoma seen from the inside 3/1 from
              Sandvik, Öland.
   »   22.  The left macula 12/1. Spec. BO. From Vestanå, Östergötland.


=Asaphus raniceps= DALM. p. 40.

  Fig. 23.  The left macula 12/1. Spec. BV.
   »   24.  Horizontal section of right macula 60/1. Spec. BY.
   »   25.  Vertical section of a macula 60/1.
   »   26.  A similar section of a macula (_a_) in the
              longitudinal axis of the hypostoma 12/1.
              Exterior surface at right.
            All from Östergötland.


=Asaphus= sp. p. 41.

  Fig. 27.  A portion of the eye near the superior margin 30/1.
   »   28.  Horizontal section of the eye 60/1.
   »   29.  Horizontal section near the inferior margin 60/1.
   »   30.  Vertical section near the superior margin 60/1.
            All figures from specimen CE. From Gotska Sandön.

K. Sv. Vet. Akad. Handl., Bd 34, No 8.

Lindström: Trilobites. Pl. I.

[Illustration]

G. Liljevall del.

Ljustryck af Chr. Westphal Stockholm.


Pl. II.


=Asaphus= (Isotelus) =gigas= J. HALL p. 42.

  Fig.  1.  The right macula surrounded by terrace lines 3/1.
   »    2.  Horizontal section of a macula with a part of the test 60/1.
   »    3.  Vertical section of the same, the surface of
              the outside upwards 60/1. All specimens from
              Cincinnati, Ohio, Hudson River group. Spec. BU.


=Bronteus irradians= LINDSTRÖM. p. 43.

  Fig.  4.  The hypostoma 4/1.
   »    5.  The left macula 12/1. Specimen from Gotland, Löjsta.


=Bronteus laticauda= ANG. p. 44.

  Fig.  6.  Horizontal section of the cephalic eye 20/1.
   »    7.  The same highly magnified 250/1.
   »    8 & 9.  Vertical sections of the cephalic eyes 20/1.
              In fig. 8 with integument, in fig. 9 decorticated.
   »   10.  Hypostoma. Specim. N. 2/1.
   »   11.  The right macula, from the outside. Spec. O. 12/1.
   »   12 & 13.  Left maculæ, casts of the interior surface 12/1.
              Specimens from Dalecarlia.


_Bronteus platyactin_ ANG. p. 43.

  Fig.  14.  Vertical section of cephalic eye, _a_ the eye.
              _b_, cheek 20/1.
   »    15.  Hypostoma 2/1.
   »    16.  The same from the interior surface. Sockets
               of the maculæ sunk in the shell 2/1.
   »    17.  The right macula 12/1.
   »    18.  Portion of the eye on the macula in transmitted light 20/1.
   »    19.  The same more magnified 60/1. Specimens
               from the stratum _b_. Wisby, Norderstrand.


=Bronteus polyactin= ANG. p. 8.

  Fig.  20.  Horizontal section of cephalic eye 20/1.
   »    21.  Vertical section of the same 60/1.
   »    22.  Hypostoma 4/1.
   »    23-25.  Samples of right maculæ 20/1.
   »    26.  Horizontal section of macula, the portion
               bearing the lenses 20/1.
   »    27.  Vertical section of the same 60/1, _a_ the
               lenses. Specimen from Stora Carlsö.
   »    28.  Hypostoma 4/1.
   »    29.  The right macula 12/1.
   »    30.  A portion of the lenses on the macula in reflected
               light 30/1. The lenses are only faintly convex and
               encased in a white frame. The diameter of the largest
               lenses 0,06 millim. Specimen from Lansa, Fårö.


=Bumastus barriensis= MURCH. p. 46.

  Fig.  31.  Hypostoma 4/1.
   »    32.  The right macula 12/1. Specim. CU from Wisby.


=Bumastus sulcatus= LINDST. p. 46.

  Fig.  33.  Horizontal Section of a cephalic eye 20/1.
   »    34.  The same magnified 60/1.
   »    35.  A similar, near the inferior margin and close
               to the surface, _a_ the eye, _b_ cheek 60/1.
               Compare fig. 38.
   »    36.  Vertical section of the cephalic eye near the
               suture 20/1.
   »    37.  The same, more magnified 60/1.
   »    38.  A similar, near the inferior margin, _a_ eye,
               _b_ cheek 60/1.
   »    39.  Cast of the interior surface of a hypostoma 2/1.
   »    40.  The same seen from the side.
               Originals to figs. 33-38 from Samsugu,
               Othem, Gotland, figs. 39 & 40 from Stora
               Vede, Follingbo, Gotland.


=Bumastus= sp. p. 47.

  Fig.  41.  Horizontal section of cephalic eye, near the inferior margin,
               _a_ eye, _b_ cheek, 60/1.
   »    42.  Vertical section of the same 60/1.
   »    43.  A similar near the inferior margin, _a_ eye,
               _b_ cheek, 60/1.
   »    44.  Hypostoma 4/1.
   »    45.  Side view of the same.
   »    46.  The right macula 12/1. Specimens from Korpklint near Wisby.

=Bumastus= sp. p. 46.

  Fig.  47.  Hypostoma 4/1.
   »      48.  The right macula 12/1. Specimen from Klints, Othem, Gotland.

K. Sv. Vet. Akad. Handl., Bd 34, No 8.

Lindström: Trilobites. Pl. II.

[Illustration]

G. Liljevall del.

Ljustryck af Chr. Westphal Stockholm.


Pl. III.


=Bumastus= n. sp. p. 47.

  Fig.   1. Hypostoma 4/1. Specimen AY. From Wisby, Norderstrand.
   »     2. Its left macula 12/1.


=Bumastus= n. sp. (different from the preceding) p. 47.

  Fig.  3.  Hypostoma 4/1. Spec. AT from Wisby, Norderstrand.
   »    4.  Vertical section of the macula _a_ in its longest axis.
   »    5.  A portion of the cephalic eye, the facets very low.


=Calymmene intermedia= LINDSTR. p. 48.

  Fig.  6.  Hypostoma 4/1. Spec. BG from Gotland.
   »    7.  The left macula 12/1.


=Calymmene tuberculata= BRÜNNICH p. 48

  Fig.  8.  The inside of the hypostoma 3/1. Spec. BA from Gotland.
   »    9.  Horizontal section near the surface, seen from the
              inside 20/1.


=Chasmops macroura= (?) p. 49.

  Fig.  10.  The left macula, cast of the inside 12/1. Rostock.


=Chirurus glaber= ANG. p. 50.

  Fig.  11.  A portion of the surface of the cephalic eye 12/1.
               From Dalecarlia.


=Chirurus ornatus= DALM. p. 50.

  Fig.  12.  Horizontal section of a Cephalic eye near the surface 60/1.
   »    13.  Vertical section of the same, 20/1, at the inferior
               margin. A line added to show the supposed size of the
               fragmentary lenses.
   »    14.  The left macula of the hypostoma 12/1.
               From Östergötland, Westanå.


=Chirurus spinulosus= NIECZKOWSKI p. 51.

  Fig.  15.  The right macula of the hypostoma from the
               inside 12/1. Belongs to Dr. G. HOLM, from
               Esthonia, Kuckers C^2.


=Chirurus= sp. p. 51.

  Fig.  16.  The left hypostomic macula 12/1. Öland, Brunsby kanal.


=Cyrtometopus clavifrons= DALM. p. 49.

  Fig.  17 & 18.  Horizontal sections of cephalic eyes 60/1.
   »    19.  Vertical sections of cephalic eyes and a part
               of the cheek 20/1.
   »    20.  A highly magnified portion of the eye in vertical section
               60/1.
   »    21.  Hypostoma, 3/1 the anterior wings completed after another
               specimen. From Östergötland.


=Cyphaspis elegantula= ANGELIN p. 52.

  Fig.  22 & 23.  Horizontal and Vertical sections of cephalic eyes 60/1.
                    Diameter of facets 0,03 mm.
   »    24 & 25.  Hypostomas from the front side and
                    in side view 8/1. Upper Silurian, Gotland.


=Ctenopyge pecten= SALTER p. 29.

  Fig.  26.  The head shield, the free cheeks added from other
               specimens 4/1.
   »    27.  A free cheek, viewed without foreshortening, as in its
               natural position 4/1. Alumschists of Andrarum, Scania


=Ctenopyge= n. sp. (LINNARSSON, Geol. För. Förhandl. 1880, pl. VI,
figs. 14, 15) p. 29.

  Fig.  28.  A free cheek 6/1.
   »    29.  Surface of the eyes, near the suture 30/1.
   »    30.  The same next the free cheek 30/1. Andrarum.


=Sphærophthalmus alatus= ANG. p. 29.

  Fig.  31.  The head shield; the free cheeks have been added 8/1.
   »    32.  A free cheek without foreshortening 8/1.
   »    33 & 34.  Horizontal and Vertical sections of cephalic eyes
               100/1. The cornea nearly 0,01 mm. in thickness. Andrarum.


=Peltura scarabæoides= WAHLENB. p. 29.

  Fig.  35.  The head shield; the free cheeks added 2/1.
   »    36.  A free cheek, not foreshortened 4/1.
   »    37.  A cast of the inside of the cephalic eye 20/1.
   »    38.  A portion of the cephalic eye from the inside 60/1.
   »    39.  A portion of the same in transmitted light 100/1.
   »    40.  Vertical section of an eye 100/1. The rock on the inside.
   »    41.  A similar one passing through two rows of lenses, the one
               behind the other. The rock on the outside 100/1.
   »    42.  A hypostoma 8/1. Andrarum.


=Dalmanites imbricatulus= ANGELIN p. 53.

  Fig.  43.  The cephalic eye near the inferior margin 20/1.
   »    44.  Vertical section of the same and a portion of the free
               cheek 60/1. The lenses are almost contiguous and the
               interspace between them is formed by the common test of
               the body, which is perforated by longitudinal tubes.


=Dalmanites obtusus= LINDSTR. p. 53.

  Fig.  45.  Horizontal section of cephalic eye near the inferior
               margin 30/1.
   »    46.  Vertical section, the interspaces being in this wider than
               in the preceding 60/1. From Gotland.


=Dalmanites vulgaris= SALTER P. 54.

  Fig.  47.  Part of the cephalic eye near the inferior margin 20/1.
   »    48.  Horizontal section near the surface at the inferior
               margin 30/1.
   »    49.  A tangential section, so that the inferior part is below
               the lenses where the peculiar radiating structure is seen
               in horizontal section 30/1.
   »    50.  Vertical section of a small Specimen, fibrous appendices
               below the lenses 30/1.
   »    51.  Hypostoma 2/1.
   »    52.  The right macula 12/1. English specimens from Dudley,
               Shropshire.

=Dysplanus centrotus= DALMAN p. 55.

  Fig.  53 & 54.  Horizontal and vertical sections of an eye 60/1.
   »    55.  Hypostoma 3/1.
   »    56.  The anterior wing of the hypostoma seen in foreshortening
               6/1. Specimen AX from Westanå, Östergötland.


=Dysplanus ladogensis= HOLM p. 55.

  Fig.  57.  Hypostoma 3/1.
   »    58.  The same, seen from the side 3/1. Specimen AI from
               Östergötland.

K. Sv. Vet. Akad. Handl., Bd 34, No 8.

Lindström: Trilobites. Pl. III.

[Illustration]

G. Liljevall del.

Ljustryck af Chr. Westphal Stockholm.


Pl. IV.


=Chirurus speciosus= HIS.

  Fig.   1.  The right macula, cast of the interior side 12/1.
               Specimen from Grumpevik, Vamlingbo, Gotland.


=Cybele bellatula= DALM. p. 52.

  Fig.   2.  The hypostoma 3/1. Vestanå, Östergötland.


=Dalmanites sclerops= DALMAN p. 54.

  Fig.   3.  The hypostoma 2/1.


=Encrinurus punctatus= WAHLENB. p. 56.

  Fig.   4.  Horizontal Section of an eye 69/1. Diameter
               of the prisms 0,04 mill., their length 0,07 millim.
   »     5.  The hypostoma 3/1.
   »     6.  The left macula. Reflected light 12/1.
   »     7.  Part of the left macula in transmitted light 20/1.
               The black streaks are sectioned pores. Spec. BH.
   »     8.  Vertical section of the right macula, a sagittal
               section 20/1. BH.
   »     9.  A similar one _a_ along the shorter diameter 20/1.
               Specimens 5, 6 from Vikers Boge, 4, 9 Medebys, Hall,
               7, 8 Svarfvarehuk. All from Gotland.


=Encrinurus lævis= ANG. p. 56.

  Fig.  10.  Horizontal section of the cephalic eye.
   »    11.  Vertical section of the same. Both 60/1.
               Prisms 0,05 in breadth, 0,06 in length. Specimen
               from Wisby.
   »    12-17.  Longitudinal profiles and anterior margins of the
               hypostomas of Encr. punctatus (figs. 12, 13),
               E. obtusus (figs. 14, 15) and E. lævis (figs. 16, 17).


=Harpes vittatus= BARR. p. 31.

  Fig.  18.  The two ocelli at right 8/1.
   »    19.  Sagittal Section of two ocelli 60/1. Specimen from Lochkow
               E, e^2, Bohemia.


=Homalonotus Knighti= KÖNIG p. 57.

  Fig.  20.  The hypostoma 3/2.
   »    21.  The right macula, from the interior side in transmitted
               light 20/1. From Scania.


=Illænus chiron= HOLM p. 58.

  Fig.  22.  Horizontal Section of an eye.
   »    23.  Vertical section of the same, both 60/1. Spec. AG.
   »    24.  Hypostoma 2/1. Spec. AF.
   »    25.  The right macula seen in reflected light. The lenses
               translucent 12/1. Ex. AA. All from Persnäs, Öland.


=Illænus Esmarki= SCHLOTH. p. 511.

  Fig.  26 & 27.  Horizontal sections of the eyes in two different
               specimens 60/1.
   »    28.  Vertical section of an eye 60/1.
   »    29.  Likewise, _a_ the eye, _b_ a portion of the
               cheek 12/1.
   »    30.  Hypostoma 2/1.
   »    31.  The  inside of  the  right  macula 12/1. Spec. AR.
   »    32.  The outside of the left macula in transmitted light  60/1.
   »    33.  Sagittal section of the macula (_a_) 60/1.
               All specimens from Vestanå, Östergötland, excepting
               fig. 30 belonging to Dr. HOLM, from
               Norway, Bjerkaas.


=Illænus gigas= HOLM P. 59.

  Fig.  34 & 35.  Surface views of the prisms of the eyes, fig. 34 at
                    the superior margin and 35 at the inferior one 12/1.
   »    36.  Cast of the inside of a hypostoma 1/1.  Same original as
               HOLM'S fig. 6 Tafl. I, Illænider.
   »    37.  Cast of the inside of the left macula. Same specimen 12/1.


=Illænus Roemeri= VOLLBORTH (Ill. vivax HOLM) p. 59.

  Fig.  38.  Hypostoma 1/1. Spec. AO.
   »    39.  The right macula; the lenses translucent in reflected light
               12/1. Spec. AO.
   »    40, 41.  Both  maculæ of another, larger specimen 12/1. Sp. AP.
               From the Leptæna limestone of Dalecarlia.


=Illænus sphæricus= HOLM p. 59.

  Fig.  42.  The prisms of cephalic eye 12/1.    }
   »    43.  The right maculæ, transparent in    }
               transmitted light 12/1. Sollerön, }  Spec. AH.
               Dalecarlia.                       }


=Lichas= sp. p. 60.

  Fig.  44.  The hypostoma 1/1.
   »    45.  Cast of the inside of the right macula 12/1. Spec. T.
   »    46.  Cast in wax of the preceding 12/1.
   »    47.  Cast of the inside of the hypostoma. The central portion
               with the maculæ 3/1. Spec. V.
   »    48.  Inside of the right macula 12/1. Spec. Y.
               From a quarry north of the gallows near Wisby.


=Lichas latifrons= ANG. p. 60.

  Fig.  49.  The right macula, transmitted light, seen from the inside
               60/1. From Kyrkberget, Wisby.


=Platymetopus planifrons= ANG. p. 67.

  Fig.  50.  The hypostoma 1/1.
   »    51.  The left macula. Transparent lenses? 12/1.
               From the  Leptæna limestone of  Dalecarlia.


=Trochurus pusillus= ANG. p. 70.

  Fig.  52.  The hypostoma 5/1.
   »    53.  The right macula in transmitted light. Seen from the inside
               60/1. From Fröjel.


=Trochurus?= p. 70.

  Fig.  54.  The hypostoma 3/1.
   »    55.  The right macula from the inside 12/1. From När Gotland.

K. Sv. Vet. Akad. Handl., Bd 34, No 8.

Lindström: Trilobites. IV.

[Illustration]

G. Liljevall del.

Ljustryck af Chr. Westphal Stockholm.


Pl. V.


=Megalaspis attenuata= WAHLENB.

  Fig.   1.  Horizontal section of cephalic eye near the inferior margin
               60/1. _a_ surface of the eye, _b_ frame with irregular or
               reserve prisms, _c_ part of the cheek with normal
               structure. Spec. DP.
   »     2.  Hypostoma 2/1. Spec. BM.
   »     3.  The right macula 12/1.
   »     4.  Part of the same in transmitted light 60/1. Spec. DR.
   »     5.  Vertical (sagittal) section of the inferior part of the
               hypostoma, crossing the macula _a_ 12/1.
   »     6.  Vertical section of the same macula 60/1. All spec. from
                Westanå, Östergötland.


=Megalaspis limbata= BOECK p. 61.

  Fig.   7.  Hypostoma, mostly decorticated 2/1. Specimen from Fogelsång,
               Scania, the same as BRÖGGER'S original to his
               fig. 23.


=Megalaspis planilimbata= ANGEL. p. 61.

  Fig.   8.  Hypostoma, Öland, Äleklinta 2/1.


=Megalaspis= sp.

  Fig.   9.  Vertical section of macula (_a_) 12/1. Öland, Lower
               grey Orthoc. limestone.


=Nileus armadillo= DALM. p. 62.

  Fig.  10.  Horizontal section of cephalic eye 60/1.
   »    11.  Vertical section of the same 60/1.
   »    12.  A similar section 60/1. _a_ the prisms of clear
               calcareous spar, _b_ the surface of the eye and
               the free cheek of yellow spar.
   »    13.  The hypostoma 2/1.
   »    14.  The right macula 12/1.
   »    15.  Vertical section of macula (_a_) 12/1. All specimens
               from Persnäs, Öland, excepting fig. 13 from Westanå,
               Östergötland.


=Symphysurus palpebrosus= DALMAN p. 62.

  Fig.  16.  Horizontal section of cephalic eye 60/1.
   »    17.  Vertical section of the same 60/1.
   »    18.  Hypostoma 2/1. From Westanå, Östergötland.

=Niobe frontalis= ANG. p. 63.

  Fig.  19.  The hypostoma 2/1.
   »    20.  The right macula 10/1.
   »    21.  Part of the same in transmitted light 60/1.
               Specimen  from Kungs Norrby, Östergötland.


=Niobe læviceps= ANG. p. 63.

  Fig.  22.  Horizontal section of cephalic eye near the inferior margin,
               _a_ cheek 60/1.
   »    23.  Hypostoma 2/1.
   »    24.  The right macula 10/1.
   »    23.  Part of the same in transmitted light 60/1.
   »    26.  Vertical section of macula _a_ 12/1.


=Ogygia dilatata= var. =Sarsi= ANGEL. p. 63.

  Fig.  27.  Hypostoma 2/1.
   »    28.  The right macula 10/1. From Christiania, Haakavik, Collection
               of Dr. G. HOLM.


=Olenus= sp. p. 33.

  Fig.  29.  A part of the free cheek with the scallop 6/1.
   »    30.  Profile of the scallop. Andrarum, Scania.


=Parabolina spinulosa= WAHLENB. p. 33.

  Fig.  31.  The free cheek with the scallop 3/1.
   »    32.  Profile of the scallop. From Jemtland.


=Paradoxides oelandicus= ANGEL. p. 64.

  Fig.  33.  Hypostoma 2/1.
   »    34.  The left macula 12/1. Öland, Torp.


=Phacops cephalotes= BARR. p. 65.

  Fig.  35.  The hypostoma; the shell mostly decorticated and the
               maculæ visible as casts on the inside.
   »    36.  Cast of the inside of the right macula 12/1. Spec. from
               Lochkow, Bohemia.


=Phacops quadrilineata= ANGEL. p. 66.

  Fig.  37.  Horizontal section of cephalic eye near the inferior margin
               30/1. The dark points lie in the outermost stratum of the
               cheek.
   »    38.  Vertical section of cephalic eye 20/1. An irregularly
               hexaedral crystal below each lens. Specimen from Wisby _a_.

K. Sv. Vet. Akad. Handl., Bd 34, No 8.

Lindström: Trilobites. Pl. V.

[Illustration]

G. Liljevall del.

Ljustryck af Chr. Westphal Stockholm.


Pl. VI.


=Phacops macrophthalmus= BURM. p. 65

  Fig.   1.  Horizontal section of the cephalic eye 30/1.
   »     2.  A similar one near the inferior margin, _a_ cheek 30/1.
   »     3.  Vertical section of a cephalic eye 30/1.
   »     4.  A similar, twice as much enlarged 60/1.
   »     5.  Also a similar, _a_ cheek 30/1.
   »     6.  Hypostoma 2/1. Spec. EA.
   »     7.  The wings of the hypostoma, seen without foreshortening
               4/1. EA.
   »     8.  The diminutive right macula 12/1, surrounded by incoherent
               terrace lines.
   »     9.  The left macula 12/1. Specimens from Gerolstein Eifel, Devon.


=Acaste= sp.? p. 51.

  Fig.  10.  The hypostoma 8/1. From Mulde, Fröjel, Gotland.


=Phaëtonides Stokesi= MURCH. p. 66.

  Fig.  11.  Hypostoma 4/1. Gotland, Eksta, Djupvik.


=Phillipsia= sp. p. 66.

  Fig.  12.  A portion of the cephalic eye 20/1.
   »    13.  The hypostoma 4/1.
   »    14.  The right macula 12/1. Specimen from Beeren Eiland, Oswalds
               förberg, communicated by Hr. J. G. ANDERSSON.


=Phillipsia= sp.? p. 66.

  Fig.  16.  The hypostoma 3/1.
   »    16.  The right macula 12/1.
   »    17.  Horizontal section of the cephalic eye 60/1.
   »    18.  Vertical section of the cephalic eye, _a_ cheek 60/1.
               Probably from the Carbon of Belgium.


=Proetus concinnus= DALM. p. 67.

  Fig.  19.  Horizontal section of the cephalic eye, a little oblique, at
               right the innermost stratum with the rock 60/1.
   »    20.  Vertical section of the cephalic eye. The diameter of the
               prisms 0,03 millim.
   »    21.  Hypostoma 4/1.
   »    22.  The right macula 12/1.
   »    23.  The same with a light, transparent spot 12/1. Specimen
               from Djupvik, Eksta.


=Proetus conspersus= ANG. p. 68.

  Fig.  24.  A portion of the cephalic eye 20/1.
   »    25.  The hypostoma. Cast of the interior side 4/1.
   »    26.  Cast of the interior side of the left macula 12/1.
               Fig. 24 from Öja, Gotland, the others from Kräklingbo.

=Proetus signatus= LDM. p. 68.

  Fig.  27.  Hypostoma 4/1.
   »    28-30.  Three pair of maculæ with dark ocelli in a white spot.


=Ptychopyge angustifrons= DALM. p. 69.

  Fig.  31.  Horizontal section of the cephalic eye 250/1.
   »    32.  Hypostoma, a broad variety 1/1.
   »    33.  Another, a narrow variety 1/1.
   »    34.  The left macula of the hypostoma fig. 32 4/1.
   »    35.  A similar of the narrow variety 4/1.
   »    36.  Vertical section of a macula. _a_ the macula 20/1.
               Specimens from Vestanå and Kungs Norrby, Östergötland.


=Ptychopyge cincta= BRÖGGER p. 69

(according to FR. SCHMIDT. The figure 12 _a_ of BRÖGGER resembles,
however, the hypostoma of Pt. angustifrons, but not the hypostomata
which have been prepared from the Ptych. cincta).

  Fig.  37.  A hypostoma; the maculæ are directed obliquely 2/1.
   »    38.  The right macula of another specimen 4/1.
   »    39.  Vertical section of a macula 60/1. Specimens from Kungs
               Norrby, Östergötland.


=Ptychopyge aciculata= ANG. p. 68.

(Compare BRÖGGER II Taf. I, fig. 13.)

  Fig.  40.  The hypostoma, cast of the interior surface 1/1.
   »    41.  Cast of the right macula 4/1. Specimen from Öland, Brunsby.


=Ptychopyge glabrata= ANG. p. 69.

  Fig.  42.  Original of BRÖGGER'S fig. 14. Cast of the inside of
               the right macula 4/1. Ǎlleberg.


=Elyx laticeps= ANG. p. 19.

  Fig.  43.  The interior surface of the right fixed cheek and the
               glabella. Magnified. Specimen from Andrarum. 4/1.


=Conocoryphe= p. 19.

  Fig.  44.  Cast of the interior surface of the left cheek. Magnified.
               From Andrarum. 4/1.


=Solenopleura brachymetopa= ANGEL. p. 19.

  Fig.  45.  The head shield, cast of the interior surface. Magnified.
               Specimen from Andrarum, Scania. 4/1.


=Apus cancriformis= SCHÄFFER p. 73.

  Fig.  46.  The hypostoma from the outside 8/1.
   »    47.  The same from the inside 8/1.
   »    48.  A side view of the same, 8/1. _a_ the duplicature of the
               head shield, _b_ posterior integument of the hypostoma,
               _c_ the ventricle, _d_ the mouth.
   »    49.  The right white spot at the posterior margin of the hypostoma,
               seen from the outside in transmitted light 150/1.

K. Sv. Vet. Akad. Handl., Bd 34, No 8.

Lindström: Trilobites. Pl. VI.

[Illustration]

G. Liljevall del.

Ljustryck af Chr. Westphal Stockholm.


       *       *       *       *       *


Transcriber Note


Minor Typos were corrected. Inconsistencies in capitalization and
abbreviations were left as printed.