viii EYES 231 



found oil 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. hucklandi) 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, H. viacrocephaliis,^ 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. verneuili, 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 Trinucleus (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 Trinucleus (Fig. 140, B) and Ampyx 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, Gonocoryphe, and some 

 species of Illaenus 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 

 Ampyx, 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, pi. 33, fig. id. 



