vni EVES 229 



2. In the prismatic type (Fig. 139, C, 1)) 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 Phacopidat 1 

 (Fig. 139, E, F). The lenses are bi-convex and are separated liy 

 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 t> the 

 upward and inward growth of the interstitial test they may 

 appear, on the surface, to be hexagonal. The diameter (if a 

 lens may be as much as 0'5 mm. The crystalline cones have 

 not been preserved. In specimens of Phacofjs rant, in which 

 the inner face of the lens is more convex than the outer, 

 J. M. Clarke ' lias obtained evidence of a posterior spheroidal 

 cavity in addition to the anterior cornea! 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 specimen-. 

 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 Lindstroiii. lia\e 

 appeared successively in chronological order: the prismatic 

 occurring first in the Oleim* beds (Upper Cambrian), tin- holo- 

 chroal first in the Ceratopyge Limestone i I'ppennost Cambrian . 

 and the schizochroal first in the Ordovician. The number of 

 lenses in the eye \aries greatly. For example, in Trimerocephcdus 

 rnllmrtlii there are 14 only, whilst in Remopleurides i-mlnm* 

 there are as many as 15,000. Even in dili'ereni, s] of the 



.same genus there may be considerable differences. Thus //////,//* 

 Irongniarti possesses 1000, Ji. j//i/'<'/' 4000, lenses in eai h eye. 

 The number increases from the younir up tu the adult, but 

 decreases in old age. The lenses are usually anan-ed in 



1 Jin/in. .I/O/-////-//, ii., 1SS9, \>. 'JiV!, ]>1. -M. 



