Part 15

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.