VIIt EYES 22, 
fixed cheek is small in Phacops, Cheirurus, and Iilaenus ; 
relatively large in Remopleurides, Phillipsia, and Stygina. It 
was suggested by M‘Coy' that the free cheek represents the 
pleura 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 Olenellus, 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, h, 138). The position of the eye is con- 
sequently determined by the position of 
the facial suture; it may be near the 
glabella or near the lateral margin of 
the head, and either as far forward as the 
first segment of the glabella or nearly as 
far back as the neck-furrow. In many 
Trilobites the eye is more or less conical, 
J : i Fig. 138—Phacops latifrons, 
with its summit truncated or rounded, but Bronn, x 1. Devonian. 
Showing large compound 
in some genera It 18 ovoid, or ecrescentie. yee atiesi7attel) 
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 case (Lnerinurus intercostatus) 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 Acidaspis and Hncrinurus. 
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 Asaphus cornigerus, A. howalewskii, and Hneri- 
nurus punctatus, they are on the summits of prominent stalks, 
yet those stalks are immovable. 
1 Ann. Mag. Nat. Hist. (2) iv., 1849, p. 396. 
