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.