between the more or less free lying lenses as shown here in a new
figure taken from a specimen from Dudley, fig. 47. The granules of
the surface are larger and more rare between the ocelli than for
instance in Dalm. imbricatulus. Small lenses, undeveloped as SALTER
calls them, may in fact only be such granules of somewhat larger size
than usual. Lenses having the appearance as in SALTER'S fig. 6, as if
a covering was partially destroyed and the lens visible below it, I
have also found in Swedish specimens, as represented in fig. 47. In a
vertical section, fig. 50, a delicate covering integument is seen and
the oval lenses lie regularly with their frames around them quite as
in Dalmanites obtusus and in horizontal sections, figs. 48, 49, there
are also the same sort of black, irregular dots, being the sectioned
tubes of the surface between the lenses. But in another feature there
is great interest. Beneath each lens there are fascicules of tiny rods,
twice as long as the lenses. They are represented in a vertical section
in fig. 50 and in a tangential section in fig. 49.
As our imperfect knowledge of so delicate anatomical structures in
fossil crustaceans does not admit of secure comparisons with the visual
organs of recent crustaceans, no suggestion can be given to interpret
their nature. Probably they have no connection with the structure of
the eyes and it is not even certain that they are of organic origin.
=Dysplanus= BURM.
If there were no other characters to distinguish this genus from
Illænus its strangely deviating hypostoma must do it. We have examined
two species and in both the hypostoma is almost oval, evenly rounded
both anteriorly and posteriorly. Near the anterior border two narrow,
long hornlike wings project. They are flattened, thin and lamellar from
the basis and end in a hollow, acuminated point.
=D. centrotus= DALM.
Pl. III figs. 53-56.
This species has the maculæ placed near the posterior border, in a
semilunar flat, devoid of the terrace-lines. The maculæ are elongated
tubercles and do not show any structure when sectioned. The cephalic
eyes have a thin membrane covering the short cupshaped lenses, which,
closely packed, in the transverse section give the usual polygonal
mosaic of hexaeders. They have a radiate structure, when decayed.
_D. ladogensis_ HOLM.
Pl. III figs. 57, 58.
A specimen from Öland has the maculæ lying in the large field of
terrace lines above the smooth zone at the posterior margin of the
hypostoma. They are larger than in the former species, pearshaped,
smooth and no structure visible.
=Encrinurus= EMMR.
I cannot find any former figure indicating the maculæ of the hypostoma
in this genus than the only one of ANGELIN, Pal. Scand. pl. IV fig. 6.
As to the shape of the singularly formed hypostoma in the three Swedish
species Encr. punctatus, obtusus and lævis we have found the anterior
border so characteristic in them that it is sufficient to distinguish
them from each other and to annul the doubts concerning the distinction
of Encr. lævis from Encr. punctatus. We therefore in pl. IV figs.
12-17 give the anterior margins of these three species, along with the
profiles.
_Encrin. punctatus_ WAHLENBERG.
Pl. IV figs. 5-9, 12, 13.
The surface of the hypostoma is so finely and obscurely granulated that
it seems to be almost smooth. It is in the shallow groove above the
lamellar tongueshaped posterior border that the elongated tubercular
macula are situated. They are prominent and in horizontal sections
they have a surface mottled by irregular black and white specks, the
whole surrounded by a white border, fig. 6. This border and the other
surface is pierced by series of parallel slitlike pores, fig. 7, a
feature peculiar to this genus. The vertical sections indicate a chain
of indistinct prisms with a black, rounded central hollow between
thick strata of clear whitish shell substance, figs. 8, 9. The shell
substance on the sides is pierced by the straight, black tubes.
The cephalic eyes have the prisms indistinctly developed or badly
preserved with a diameter of 0,04 millim. and height of 0,07 millim. In
Encr. lævis the prisms are more distinct and larger with a diameter of
0,00 millim. (pl. IV figs. 10-11).
The maculæ are well developed, ovate, in E. lævis and obtusus and in
the same position as in E. punctatus.
=Griffithides= PORTLOCK.
The hypostoma of Griff. globiceps PHILL. shows according to the figure
given in H. WOODWARD'S work pl. VI fig. 5 two somewhat obscure maculæ,
but no mention is made of them in the description.
=Harpes= GOLDF.
_Harpes d'Orbignyanus_ BARR. NOVÁK II Taf. I fig. 4, with small black
maculæ. Harpes venulosus CORDA. NOVÁK II, Taf. I figs. 1, 2. Two
narrow, horizontally placed maculæ.
=Harpina= BARR.
_H. prima_ BARR. NOVÁK II Taf. I fig. 5. Two maculæ in continuation of
a narrow groove, at the base of the central globosity.
=Holmia= WALCOTT.
_Holmia Lundgreni_ MOBERG, Sveriges äldsta trilobiter, Tafl. 14, fig.
10, 11, 12. The maculæ are much prominent and tubercular, elongate
narrow, oblique but not mentioned in the description. Hypostoma
probably belonging to the terrace-line group. As well seen in original
specimens and in the figures in Dr. HOLM'S paper on Olenellus (Holmia)
Kjerulfi there are also in this species two distinct macula; near the
posterior margin of the hypostoma, above the marginal groove.
=Homalonotus= KÖNIG.
Maculæ have been previously observed in the following species.
_Hom. delphinocephalus_ GREEN, HALL Pal. N. York vol. II pl. 68 fig.
11, incomplete figure with two well marked tubercular maculæ. No
description.
=Hom. Knighti= KÖNIG. SALTER II, pl. 12 fig. 10. Maculæ well expressed
as oblique tubercles. On page 120 it is said »with a pair of lateral
tubercles well developed (as in Asaphus ...)».
This species is identical with ANGELIN'S Homalon. rhinotropis, Pal.
Sc., pl. XX fig. 1 e and his figure has two hollow maculæ seen from the
inside of the hypostoma.
=Homalonotus Knighti= KÖNIG (= Homal. rhinotropis ANGELIN).
Pl. IV figs. 20, 21.
The maculæ lie as oblong, smooth tubercles surrounded by the
irregularly grown granules of the surface, just below the central
globosity of the hypostoma. In a preparation from the interior surface
of the macula, as seen in transmitted light, fig. 21, this is white
with only a few grey spots and on the superior border black streaks,
lying obliquely. The substance of the shell around the macula is
perforated by the minutest pores amongst which a few larger are
intermingled. The granules are also perforated as those in Calymmene
and the tubes continue through the shell, visible by their black colour.
=Hysterolenus= MOBERG.
_Hyst. Törnquisti_ MBG., En trilobit från Dictyograptusskiffern p. 320.
In the descriptive letter-press, there is said, that in the exterior
(lateral) parts of the anterior groove of the hypostoma a distinct
tubercle is to be seen on each side. But none of the figures given, Pl.
17 figs. 6, 7, shows them or at least very indistinctly.
=Illænus= DALMAN.
It is remarkable, considering the great number of species in this
almost exclusively Lower Silurian species, that there is none but a
single species in which the maculæ hitherto have been delineated. This
is Illænus angustifrons var. depresses HOLM I, pl. VIII fig. 18. A
little, fragmentary hypostoma with globular maculæ.
=Illænus Chiron= HOLM.
Pl. IV figs. 22-25.
The hypostoma has the shape characteristic of all true Illæni, viz.
a straight anterior margin with two large flat rectangular or nearly
quadratic wings, a globular or spherical median field, forming the
chief portion of the exterior surface, which is covered by some rare,
fine, terrace lines. In some other Illæni the surface is smooth. At
the posterior base of the median field the two maculæ are placed,
tiny, oblong, but lying on the same level, at right angles to the
median longitudinal axis of the hypostoma. They are covered with
lenses of irregular and indistinct shape fig. 25. I do not think that
they in this and the other species of Tiberius are lenses of the same
conformation as in Bronteus, they are rather the tops of the subjacent
lenses which are of the prismatic form. The cephalic eyes do not
show any lenses on the outside. They have a narrow, opaque exterior
integument, hiding hexaedral, thickly packed, straight prisms, forming
a stratum in thickness of 0,2 millim.
The hypostoma consists of several thin layers of superposed calcite,
which easily peal off, so that it is seldom that the true exterior
sculptured surface remains.
=Illænus Esmarki= SCHLOTH.
Pl. IV figs. 26-33.
This the most common of all the Illæni has in several specimens
shown the hypostoma with the maculæ in situ. The hypostoma is smooth
excepting on the marginal ridges where there are some terrace lines.
On the median axis of the central gibbosity (fig. 30) five faintly
visible transverse folds are situated, a curious feature which we
have not observed in any other trilobite. The maculæ are exceedingly
small, fig. 31, elongate, spindleshaped and acuminated in both ends.
On their surface there are accumulated several lenslike globules. As
the horizontal sections show there are, however, no regular lenses,
but a network of polygonal meshes with radii in the hollows, fig. 32.
The perfect identity of this structure with that of certain states
of the cephalic eyes will at once be perceived by comparing this
figure with figure 26 representing the same sort of polyedric meshes,
though a little more regular in the cephalic eye, where, however,
the irregularity also prevails near the periphery of the eye. If we
now compare the sections of the cephalic eye of Illænus Chiron and
the section of the regular and nearly unchanged eyes of Ill. Esmarki
with the second section of the latter, we cannot avoid the conclusion
that we in those of Ill. Chiron, figs. 22, 23, and the first of Ill.
Esmarki, fig. 27, see the primitive and intact state of the lenses and
in the last sections of Ill. Esmarki, figs. 26, 28, the changed and
deteriorated state of the same prismatic lenses.
The vertical section of the hypostomic eyes fig. 33 shows in their
present changed condition an evident longitudinal direction of the
chief elements, quite as the lenses of the eyes. But in how far
there ever has existed such prismatic lenses, now changed, we have no
evidence definitely to ascertain, but the probability seems to be great
that this was the case.
=Illænus gigas= HOLM.
Pl. IV figs. 34-37.
The maculæ in this as well as the other Illæni are so faintly elevated
and so inconspicuous that the greatest attention is needed to find them.
The maculæ are elongated, narrow, elliptic on the same level and
parallel with the straight anterior margin of the hypostoma. A cast
of the left macula is pitted by numerous marks of the lenticular
globules of the surface, fig. 37. In the cephalic eye the prisms are
seen translucent beneath the cornea. Near the superior border they are
larger, fig. 34, almost the double size of those near the inferior
border, fig. 35.
=Illænus sphæricus= HOLM.
Pl. IV figs. 42, 43.
Comes near to the previous species. The globules on the surface of
the elliptic maculæ are not, however, spread over the entire macula,
leaving only a narrow border all around free.
The lenses in the cephalic eyes fig. 42 are globular as seen on the
surface, alike those of the maculæ.
=Illænus Roemeri= VOLLBORTH.
Pl. IV figs. 38-41.
The maculæ lie on elevated tubercles and are of a strange shape, being
sharpely pointed outwards, rounded inwards. That in fig. 39 is still
more peculiar with a narrow stripe on the interior and upper margin.
The lenses are larger and more distinct than in any other species of
this genus.
Beside the species now mentioned we have also observed hypostomas
with maculæ in Illænus laticlavius EICHWALD from Estland, in Illænus
Linnarssoni HOLM, and in an unnamed species from Dalecarlia figured
without macula by Dr. HOLM in his memoir on the Illænidæ pl. III fig.
22. We have not seen any lenses on the maculæ of these three lenses.
=Lichas= DALMAN.
We have not been able to find any notice about the exceptional maculæ
of this genus. There is only a figure of the hypostoma of Lichas hylæus
HALL Pal. of N. York vol. VII pl. XXV fig. 5 which would seem to show a
pair of narrow, oblique maculæ, but these are quite without resemblance
with the maculæ of the true Lichas.
The hypostoma of Lichas is of a characteristic type, broad, with the
incised posterior margin and its two lappets reminding of Asaphus,
with its large rounded, faintly elevated median part near the anterior
margin reminding of Illænus, as also do the lenticles of the maculæ.
The maculæ are of a minimal size, scarcely 1 millim. in diameter and
thence easily overlooked. Pl. IV figs. 46, 48.
As the hypostomas in this genus generally are found detached it is
often difficult to decide to which species they have belonged. So it is
with that delineated in Pl. IV fig. 44 natural size. The two maculæ are
situated near the centre of the hypostoma, just below the grooves which
surround the large, globular disc. They are sunk in a little cavity,
rounded and entirely covered by globular lenses (pl. IV fig. 46).
In another detached hypostoma of an unknown species the macula is
smaller and the few lenticular globules larger than in the other, fig.
48.
=Lichas latifrons= ANGELIN.
Pl. IV fig. 49.
The maculæ, seen in a thin section from the inside, are reticulate,
fig. 49, or of a pattern exactly like that in the changed eyes or
maculæ of Illænus and even Asaphus, and thus indicating that it was
composed of short prismatic lenses. The vertical section of a macula
of a specimen from another locality rather suggests the presence of
large sphærical lenses than elongated prisms. The great thinness of the
macula in contrast to the excessive thickness of the shell around the
eye is seldom so evident as here.
A specimen of an unknown species, found detached, has the macula?
unlike the other species, oblique with the inferior end directed
inwards, placed on the top of a sort of a little elevation, fig. 47.
=Megalaspides= BRÖGGER.
This author has a figure of M. dalecarlicus in II pl. 1 fig. 19 and
also of an unnamed species fig. 20, both showing globular maculæ. In
HOLM'S original description of his Megalaspis dalecarlicus (Trilobiten
des Phyllograptusschiefers Dalecarliens figs. 8, 9) the same hypostoma
is also figured. This genus seems to have a closer affinity with
Ptychopyge than with Megalaspis.
=Megalaspis= ANG.
The following figures of hypostomas with maculæ have been published.
_Megal. limbata_ by BRÖGGER I, tab. XII, fig. 10, and in II pl. 2, fig.
22, somewhat differing from the first figure.
_Meg. planilimbata_ BRÖGGER II tab. 2 figs. 21, 21 a. Both figures
incomplete and incorrect.
The hypostoma has a large central ovate gibbosity with the maculæ near
its basis, surrounded by the winglike expanded limbus.
=Megal. attenuata= WAHLENBERG.
Pl. V figs. 1-6.
The long and narrow maculæ are crescent-shaped and project beyond the
hypostoma supported on the superior surface of a sharp extenuated
edge where they rest as on a shelf. They consist, as seen in vertical
sections, figs. 5, 6, of alternating rows of light and black streaks,
and in a horizontal section the spongious network appears, as it is
found in Asaphus and others and thus indicating the tendency of this
organ to assume a prismatic structure though undeveloped.
The cephalic eyes in M. attenuata form the most regular hexaedral
prisms fig. 1 a, short and covered by a thin cornea. The border zone of
the eyes has a structure exactly like that of the maculæ, the same as
in the spongious zone of Asaphus.
=Megal. limbata= BOECK.
Pl. V fig. 7.
We give a new figure of BRÖGGER'S original specimens, as his figure 23,
Tab. II is incomplete and incorrect. The maculæ are situated a little
higher up on the sides of the central gibbosity.
=Megal. planilimbata= ANG.
Pl. V fig. 8.
The maculæ are crescent-shaped and not so prominent as in Meg.
attenuata and the visual spot in the same position as there.
To observe is that the general structure of the hypostomic shell in
this and many other species is prismatic.
=Nileus= DALMAN.
Hypostoma with maculæ figured in N. armadillo by ANGELIN pl. XVI fig. 5
c, and BR. II pl. 3 fig. 40.
The hypostoma in this genus is transversal with large lateral
expansions, the central convexity only faintly elevated and almost
coherent with the posterior part. The maculæ lie exactly on the
horizontal median line of the hypostoma. The anterior wings are well
developed like slightly bent acuminated horns and the posterior wings
are conical points. The terrace lines are transverse and a little wavy.
=Nileus armadillo= DALM.
Pl. V figs. 10-15.
There is some variability in the hypostoma of specimens from different
localities. The most known and numerous specimens are from the
renowned localities of Östergötland. They are decidedly transverse,
the proportion of the hight to breadth is as 3 to 5 and the narrow
border runs all round. In specimens from Kongslena, Vestergötland, the
proportion is as 15 to 22, in specimens from Sandvik, Öland, again, we
have 13 to 23 or 6 to 11. The maculæ, fig. 14, are oblong, inwardly
pointed, quite smooth, with a little concavity in their centre,
and their surface is slanting obliquely towards the surface of the
hypostoma, almost as in Niobe. There is no trace of any structure.
The cephalic eyes are covered by a comparatively thick membrane, a
direct continuation of the general test, fig. 12, hiding a row of
prismatic lenses. In some specimens elongate, clear, crystalline rods
continue downwards from the bases of the lenses and are probably
nothing but inorganic crystals of calcareous spar, fig. 11.--The eyes
are larger comparatively than in other species.
=Nileus (Symphysurus) palpebrosus= DALMAN.
Pl. V figs. 16-18.
As to its general form the hypostoma resembles that of the preceding
species, but there is no border line, which is so characteristic to the
former. The maculæ have the same form and are likewise situated on the
median line. The cephalic eyes have prismatic lenses with much convex
bases, in a transverse section they are hexaedral and of internal
radiate structure.
=Niobe= ANGELIN.
The following figures have formerly been given of its hypostoma with
maculæ.
_N. emarginula_ BR. II tab. 2 fig. 33.
_N. explanata_ BR. II tab. 2 fig. 35.
_N. frontalis_ BR. II fig. 37.
_N. insignis_ BR. I tab. IV fig. 1 d, 11 tab. 2 fig. 28.
_N. læviceps_ BR. II pl. 2 fig. 34.
The maculæ are distinguished from all other sorts of maculæ, excepting
those of Megalaspis, in being, as it were, shelved on a sort of
support, formed through the bulging of the hypostomic surface just
below them, one such console for each. Their surface lies consequently
almost rectangularly to the adjoining surface of the hypostoma. They
are white and smooth.
=Niobe frontalis= ANG.
Pl. V figs. 19-21.
The pocketlike consoles are shorter and broader than in N. læviceps
and the maculæ also larger and of an elliptic shape. In a horizontal
section near the surface the macula has a speckled appearance of
black dots, indicating an undeveloped prismatic structure, much more
primitive than in the Asaphidæ.
=Niobe læviceps= ANG.
Pl. V figs. 22-26.
It differs from the preceding through longer consoles below the maculæ,
which are oval and in a horizontal section exhibit the same spongious
structure, pl. V fig. 25. The cephalic eyes consist of polyedric
prisms, which through deterioration show a central cylindrical core
surrounded by darkish matter like Asaphus. The passage from solid,
homogenous prisms to changed ones is most evident in the figure 22.
=Ogygia= BRONGN.
Figures of the maculæ are found in
_O. Buchi_ SALTER, Mem. Geol. Surv., Dec. II pl. VI fig. 3 p. 2, where
it is stated that »there are two transverse furrows near the apex, with
compressed tubercles between them» There are two narrow crescentic
maculæ and between them a little lower a longer, crescentic ridge,
which may be the ridge that in other species connects the maculæ.
Moreover, the shape of the hypostoma is not concordant with that of the
other species. In Monogr. Brit Trilob. pl. 15 figs. 2, 3 there is only
a reproduction of the former figure.
_O. corndensis_ MURCH. SALTER in Monogr. pl. 16 fig. 10 interior side
of the hypostoma with two lateral maculæ. In the description two pairs
of furrows the uppermost is the maculæ. H. WYATT-EDGELL »On the Genera
of Trilobites Asaphus and Ogygia and the Subgenus Ptychopyge» in
Geol. Magaz. 1867, p. 14, 15 fig. 2 probably a cast with two oblique
impressions of maculæ.
_O. dilatata_ (Asaphus) SARS var. Sarsi ANG. SARS in Isis 1835 p. 342,
pl. IX fig. 11. The figure has two small maculæ near the posterior
margin and the author says »mit einer starken Vertiefung oder
Einschnitt an jeder Seite des Endes». ANGELIN pl. XLII fig. 1 _b_.
BRÖGGER'S fig. 38 pl. 3, in II, collies near to the specimen, which is
described below, also belonging to Dr. G. HOLM.
=O. dilatata= var. =Sarsi= ANG.
Pl. V figs. 27, 28.
The hypostoma is in so far deviating that the terrace lines have a
nearly vertical direction and in this respect much resemble those of
the Phillipsiæ and the Proeti. The maculæ, near the exterior angles
of the border groove are pear- or spoon-shaped with the apex directed
downwards and outwards, and in consequence the longitudinal axis quite
opposite to that of the usual direction. The surface is smooth, a
little concave. No structure has been discovered. From the pointed
apex a slightly elevated ridge runs along the posterior border of the
hypostoma and joins the apex of the other macula.
=Paradoxides= BRONGN.
The oldest record of any hypostoma at all amongst the trilobites is
that given by WAHLENBERG in his Petref. Suecana (1818) p. 37 Tab. 1,
fig. 6, which he, however, considered as the head of the trilobite
which he named Entomostracites bucephalus. Figures with maculæ have
been given of
_Par. Davidis_ SALTER Mem. Geol. Survey Dec. XI pl. X, fig. 3, a fine
figure of a hypostoma with two large oblique maculæ. LINNARSSON in
»de undre Paradoxideslagren vid Andrarum» pl. II fig. 2 delineates a
gigantic hypostoma with two crescentic tubercular maculæ, having 10
mms. in length.
_Par. Forchhammeri_ ANGELIN Pl. II fig. 3. BRÖGGER, Paradoxidesskiffr.
vid Krækling tab. II fig. 10 two marks somewhat different from
ANGELIN'S figure. LINNARSSON 1. c. pl. I figs. 9, 10 two hypostomas
with maculæ.
_Par. Tessini_, var. _Wahlenbergi_ ANGEL. Pl. I a, fig. 1 b.
_Par. Tessini_, var. _oelandicus_ ANG. Tab. I a, fig. 2 b. ANGELIN'S
figure of this hypostoma is incomplete in so far that he had not found
the peculiar falciform horns on both sides of the posterior border. Pl.
V, fig. 33.
_Par. rugulosus_ CORDA has, according to BRÖGGER, Krækling, pl. II fig.
2 two long, straight callosities, probably corresponding to the maculæ
in other Paradoxidæ.
CHR. BOECK gives in Mag. for Naturvidenskaberne Bd 8, 1828, in his
paper on the Trilobites on the plate fig. 16 the hypostoma of a
Bohemian species with well developed concave maculæ, probably a cast.
He compares it with the Entom. bucephalus of WAHLENBERG, but says he
cannot explain its nature.
The maculæ of P. oelandicus, pl. V, fig. 34, are tubercles, oblong,
smooth, and have invariably an oblong scar along the centre, where
probably once a thinner membrane, as in the cephalic eyes of Calymmene,
contained the lenses.
=Peltura= M. EDW.
=Peltura scarabæoides= WAHLENB.
Pl. III fig. 42.
We have given a figure of the hypostoma, showing two large, elongated
tubercular maculæ or rather resemblances of such, as they are covered
with terrace lines as well as the rest of the surface. They have,
however, the same position as real maculæ might have, but are directed
in a line parallel with the longitudinal axis of the hypostoma. The
cephalic eyes have been described above at page 29.
=Phacops= EMMRICH.
Maculæ figured of Phac. (Acaste) Downingiæ. SALTER Monogr. pl. 2 fig.
34 b, p. 25 »high up on each side a small tubercle».
_Phac. rana_ HALL Pal. N. Y. vol. VII, pl. VIII A, fig. 18. Hypostoma
with the large commalike maculæ high up.
Of the following species Ph. cephalotes and Ph. macrophthalmus belong
to the genus Phacops proper, while Phac. quadrilineata, as well as
Ph. Downingiæ are of a quite different generic type. The name Acaste
proposed for them is not good as LEACH in 1817 named a Cirrhiped as
Acasta.
=Ph. cephalotes= BARR.
Pl. V figs. 35, 36.
The hypostoma is elongate, almost triangular, tricuspidate at the
posterior border, a little above this there is a crescent-shaped ridge,
and still a little higher above this and the median horizontal line,
the two macula, as diminutive tubercles. Seen with magnifying powers
they are slightly oblong or reniform.
=Ph. macrophthalmus= BURM.
Pl. VI figs. 1-9.