GENERAL CONCLUSIONS 473 



other closely related forms of trilobites, it is evident that the distinction 

 between median eyes and lateral eyes had occurred at a very early date 

 and that the time required to produce the differentiation of the complex 

 faceted eyes must place the actual origin of the lateral eyes of arthropods 

 at a still earlier period. 



The degree of differentiation of the lateral eyes of invertebrates varies 

 greatly in different classes, and the divergence from the simpler types is 

 greater in the adult animal than in the larva and in the more highly 

 organized types of animal than in the more primitive. 



Now the earliest known fossil vertebrates, the ostracoderms, agree 

 with the invertebrate Eurypteridae in possessing both lateral and 

 median eyes, and they were contemporary with each other, living in 

 the sea under much the same conditions and in the same geological 

 period. Comparisons were therefore made between the ostracoderms 

 and the eurypterids, and between the living representatives of these 

 two extinct classes, the cyclostomes, which are the direct descendants 

 of the ostracoderms and certain of the more primitive types of 

 cartilaginous fishes on the one hand, and the land scorpions, spiders, 

 Limulus, and certain of the Crustacea on the other ; all of which 

 resemble each other in possessing lateral and median eyes in the same 

 relative positions with regard to each other and other organs in the 

 head. 



It will be unnecessary to refer to more than two or three of the more 

 salient points which have recently been settled by a critical examination 

 of the alleged similarities between the ostracoderm fishes and the 

 eurypterids. One of these apparent similarities was the possession in 

 Cephalaspid fishes of an exoskeleton which seemed to closely resemble 

 the chitinous exoskeleton of eurypterids and Xiphosura. Now the 

 exoskeleton of the fishes, whether it consists of denticles, scales, scutes, 

 or " armour plating," consists of an outer layer of epidermal bone or of 

 enamel, which covers a dermal bony stratum, or osteodentine ; and it 

 will be recalled that in the development of a tooth the formation of the 

 enamel is at the inner or deep end of the enamel cells or ameloblasts ; 

 further, the increase in thickness of the enamel is by the laying down of 

 new layers on the superficial surface of those which have already been 

 deposited ; and also that the dentine which is formed on the surface of 

 the dermal papilla by the odontoblasts is layed down in the reverse direc- 

 tion to the enamel, namely, from without inwards, the increase in thickness 

 of the dentine being due to its formation at the outer or superficial ends 

 of the odontoblasts. The shields or plates forming the armour plating 

 of the ostracoderm fishes are of the nature of a vaso-dentine, and the 



