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.