EYES 



227 



fixed cheek is small in Phacops, Cheirur^is, and lUaenus ; 

 relatively large in liemojdeurides, FhillijJsia, and Stygiyia. It 

 was suggested by M'Coy ^ that the free cheek represents the 

 pleiu'a of an anterior segment which has not become fused with 

 the other cephalic pleurae. The fixed cheek appears to be 

 formed of the coalesced pleurae of the other cephalic segments, 

 but of those pleurae the only indication seen in adult specimens 

 is in the neck-ring ; in young specimens of Olenelhis, however, 

 the presence of other pleurae is indicated by furrows on the 

 cheeks in front of the neck-furrow. 



A pair of compound eyes are present in the majority of 

 Trilobites. Each eye is situated on the free cheek, at that part 

 of its inner margin where the facial suture bends to form an 

 angle (Figs. 137, A, li, 138). The position of the eye is con- 

 sequently determined by the position of 

 the facial suture ; it may be near the 

 glabella or uear the lateral margin of 

 the head, and either as far forward as tlie 

 first segment of the glabella or nearly as 

 far back as the neck-furrow. In many 

 Trilobites the eye is more or less conical, 

 with its summit truncated or rounded, but 

 in some genera it is ovoid, or crescentic. 

 In Aeglina (Fig. 150, H) the eye is 

 flattened and scarcely raised above the general level of the cheek. 

 The eye of a Trilobite is oriented so that its longer axis is 

 parallel or nearly parallel to the axis of the body (Fig. 150, G) ; 

 but in one Q,a,^e {Encrinurus intercostatvs) it is placed at right 

 angles to this axis. The size of the eye varies considerably ; it 

 is largest in Aeglina, in which it covers nearly the whole of the 

 free cheek ; it is small in AcLdus2ns and Encrinurus. 



Though the eye is always entirely on the free cheek, the 

 adjoining part of the fixed cheek is raised to form a buttress on 

 which the eye rests ; this buttress, which is known as the 

 " palpebral lobe," is seen clearly when the fixed cheek is removed. 

 The eyes of Trilobites are always sessile ; for although in some 

 species, such as AsapJrus cornigerus, A. howaleivsMi, and Encri- 

 nurus punctatus, they are on the summits of prominent stalks, 

 yet those stalks are immovable. 



1 Ann. MiKj. Xat. Hist. ('J) iv., 1849, p. ^96. 



Fig. 138 — Pltacops lutifrons, 

 Bronii, X 1. Devonian. 

 Showing large conipoimd 

 eye. (After Zittel.) 



