VIII EYES 2 
Lop) 
= 
found on the middle of the cheek in the young of some species, 
and is sometimes connected with the glabella by an eye-line ; the 
latter disappears ‘before the adult state is reached, and in some 
species the tubercle also disappears, but in others (such as 
T. seticornis, T. bucklandi) it persists in the adult individuals. 
From the lateral position of these eyes they can hardly be 
compared with the median simple eye of other Crustacea. In 
Harpes it is more probable that, as suggested by J. M. Clarke, 
they are schizochroal eyes imperfectly developed. Their structure 
(Fig. 139, G, H) is somewhat similar to that of schizochroal 
eyes, and moreover, in one species, 1. macrocephalus,' there are, in 
addition to the three main tubercles, other smaller tubercles in 
regular rows. Further, the eye-line occupies the same position 
as in other Trilobites which have undoubted compound eyes. The 
absence of a facial suture cannot be taken as evidence against 
these eyes being of the ordinary type, since in some species of 
Acidaspis (e.g. A. vernewili, A. vesiculosa) which possess com- 
pound eyes there is, in consequence of the coalescence of the 
fixed and free cheeks, no suture. 
In some species of 7’rinucleus (Fig. 140, B) the simple eye is 
found in the same position as the eye in Harpes, and if, as some 
writers have maintained, there is evidence of the existence of a 
suture in that genus, then there is no reason for regarding the 
eye as other than a degenerate form of compound eye. The 
probability of its being such is supported by the existence of a 
compound eye in a similar position in the allied form Orometopus 
(Fig. 140, A) which possesses a facial suture. 
In some species of Zrinucleus (Fig. 140, B) and Ampya there 
is a small median tubercle on the front part of the glabella, which 
from its position may be a simple unpaired eye, but its structure 
appears to be unknown. : 
Some Trilobites possess no eyes. Well-known examples of 
such are <Agnostus, Microdiscus, Ampyx, Conocoryphe, and some 
species of Jllaenus and Trinucleus; such blind Trilobites are 
almost confined to the Cambrian and Ordovician periods. All 
the forms of later periods, with the exception of a species of 
Ampyz, and possibly one or two other species, possess eyes. In 
addition to those undoubtedly blind forms Lindstrom considers 
that most of the Olenidae and Paradoxidae were without eyes. 
1 Goldfuss, ‘‘ Beitr. zur Petrefaktenkunde,” 1839, p. 359, pl. 383, fig. 2d. 
