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.
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.