Vill EYES 229 
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, ete. 
3. The schizochroal eye occurs only in the family Phacopidae 
(Fig. 139, E, F). The lenses are bi-convex 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’ 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. 
These three types of eye, according to Lindstrom, have 
appeared successively in chronological order: the prismatic 
oceurring first in the Olenus beds (Upper Cambrian), the holo- 
chroal first in the Ceratopyge Limestone (Uppermost Cambrian), 
and the schizochroal first in the Ordovician. The number of 
lenses in the eye varies greatly. For example, in 7’rimerocephalus 
volborthi there are 14 only, whilst in Remoplewrides radians 
there are as many as 15,000. Even in different species of the 
same genus there may be considerable differences. Thus Bronteus 
brongniarti possesses 1000, B. palifer 4000, lenses in each eye. 
‘The number increases from the young up to the adult, but 
decreases in old age. The lenses are usually arranged in 
1 Journ. Morphol. ii., 1889, p. 253, pl. 21. 
