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.