2. In the prismatic type (Fig. 139, C, D) the lenses are prismatic and plano-convex, and the entire surface of the eye is covered by a smooth cuticle. The lenses are close together and usually hexagonal, but occasionally rhombic or square. Near the margin of the eye the lenses may become irregular, giving rise to a border in which the prismatic structure is more or less indistinct. The prismatic type of eye is found in the genera _Asaphus_, _Nileus_, _Illaenus_, etc. 3. The schizochroal eye occurs only in the family Phacopidae (Fig. 139, E, F). The lenses are biconvex and are separated by portions of the cephalic shield, so that each lens appears to rest in a separate socket, and the cornea is not continuous for the entire eye. The lenses are circular in outline, but owing to the upward and inward growth of the interstitial test they may appear, on the surface, to be hexagonal. The diameter of a lens may be as much as 0·5 mm. The crystalline cones have not been preserved. In specimens of _Phacops rana_, in which the inner face of the lens is more convex than the outer, J. M. Clarke[185] has obtained evidence of a posterior spheroidal cavity in addition to the anterior corneal cavity. The complete separation of the lenses in this type of eye has led to the suggestion that the schizochroal eye is an aggregate rather than a compound eye. But the difference between this and the holochroal eye is probably less than appears at first sight if the statement made by Clarke is confirmed, namely, that in young specimens of _Calymene senaria_ the lenses are relatively large and similar to those of _Phacops_, whereas in the adult the eye is holochroal.