CRUSTACEA. TRILOBITA 295 



The eyes (fig. 123, e) are on the upper surface of 

 the head, one on each free cheek in 

 the angle made by the facial suture ; 

 they are more or less conical with 

 the summit truncated or rounded, 

 and with the visual surface on the 

 external part. The eyes are usually 

 compound, and in most cases con- 

 sist of a large number of lenses — in Fi S- 12 ^- Mglina bi- 

 P . nodosa, Arenig Beds. 



Kemopieundes the number is stated Natural size, 

 to be 15,000. Usually the lenses 



are biconvex or globular and adjacent to one another, but 

 in Phacops and its sub-genera the eyes are more highly 

 developed, the lenses being separated by portions of the 

 cephalic shield so that each appears to rest in a separate 

 socket. The eye is entirely on the free cheek, but rests 

 on a lobe or buttress on the adjacent part of the fixed 

 cheek. In a few Trilobites the eyes are said to be simple ; 

 for example, in Harpes each eye usually consists of two or 

 three lenses only; but it is probable that in such cases the 

 eye is a degenerate form of compound eye rather than a 

 simple eye. In a few genera (Agnostics, Microdiscus, 

 Ampyx) eyes are absent; in JEglina (fig. 124) they are 

 unusually large, occupying the greater part of the free 

 cheeks. Many of the Cambrian Trilobites which have 

 usually been regarded as possessing eyes (Paradoxides, 

 Olenus, Sao) are considered by Lindstrom to have been 

 blind, since the eye-like lobe, which is present, shows no 

 indication of structure and differs in development from 

 that of the eyes, which always appear in close connexion 

 with the facial suture. In many Cambrian and some few 

 later Trilobites a thread-like ridge, called the eye-line, 

 extends from the eye to the glabella. 



