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.
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.
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.