Of these genera Eurycare is the oldest (see table p. 22). Amongst the many free and detached cheeks only a single, very little one has been found with the eye ball fixed. It seems to be of the same structure as in Sphærophthalmus. In Sphærophthalmus and Ctenopyge the eye globes are enormous, considering the size of the cheek in which they are set and occupy more than a third of the length of the free cheek (Pl. III f. 26, 31). They are hemispheric, blackish and glossy, more so in the former genus. The spheroidal lenses, projecting on the surface, are in Ctenopyge larger near the facial suture and small at the opposite side where the eye is fixed in the free cheek. For the rest, in both genera (Pl. III fig. 34) the lenses form a thin stratum, where they in a vertical section lie elongated, flattened and biconvex, slightly joined with each other at the point of contact. The fine form which they exhibit reminds of the lenses of Sphæroma.[29] They are in diameter thrice as long as they are high. Seen in a horizontal section passing through the point of contact they show hexaeders with a curiously jagged outline (Pl. III fig. 33). [Footnote 29: BELLONCI Atti dei Lincei. Memorie, vol. X, 1881, Sphæroma, pl. II fig. 11.] Peltura which is coeval with these, has a narrow semiglobose visual field (Pl. III figs 35-41), the superior surface of which is quite smooth and evenly rounded. On its interior side there stand out, somewhat distantiated, in a low relief semiglobular facets, quite as regular incrassations of the cornea, thus not forming free lenses, but rather reminding of the for the rest differently formed quasi-lenses of Limulus. In a vertical section they appear as the inferior moiety of real ovate lenses (Pl. III f. 40-41). The much younger Acerocare has a similar cornea. A very little specimen, the head of scarcely more than one millim. in length, retains both eyes, of which one shows the slightly convex lenses and the other a cast of the interior side as in Peltura. These both genera should in consequence of their peculiar limuloid cornea be ranged for themselves apart from the real lenticulate genera, but any material sufficient for doing this properly, is at present not at hand. II. =Aggregate eyes.=