VIII EVES 229 



2. In the prismatic type (Fig. 139, C, D) the lenses are 

 prismatic and phmo-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 l)i-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 Fhacops 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 Fhacops, whereas in the adult the eye is holochroal. 



These three types of eye, according to Lindstrcim, have 

 appeared successively in chronological order : the prismatic 

 occurring 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 Trimerocephalus 

 volhorthi there are 14 only, whilst in Remopleurides radians 

 there are as many as 15,000. Even in different species of the 

 same genus there may be considerable differences. Thus Brontevs 

 brongniarti possesses 1000, B. ^xdifcr 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 



' Journ. Morphol. ii., 1889, p. 253, \A. -21. 



