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.