RELATIONSHIP OF AJf.VOCCETES TO OSTRACODERMS 345 



This method of looking at the problem seems to me to he more 

 in consonance with the facts than the reverse; for, as pointed out by 

 Jaekel, the fishes with large plates are the oldest, and in Cyathaspis, 

 the very oldest of all, the size of the plates is most conspicuous; he 

 considers, therefore, this preconceived view that large plates are 

 formed by the fusion of small ones must give way to the opposite 

 belief. 



So also Eohon, as quoted by Traquair, who, in his first paper 

 accepted Lankester's view that the ridges of the pteraspidian shield 



Fig. 139.— Drepanaspis. Ventral and Dorsal Aspects. (After Lankesteb.) 



.4., anus; E., lateral eyes. 



were formed by the fusion of a linear arrangement of numbers of 

 placoid scales, suggests in his second paper that these ridges may 

 have been the most primitive condition of the dermal skeleton of tbe 

 vertebrate, out of which, by differentiation, the dermal denticles 

 (placoid scales) of the selachian, as well as their modifications in the 

 ganoids, teleosteans, and amphibians, have arisen. 



( hie thing is agreed upon on all sides ; no sign of bone-corpuscles 

 is to be found in this dermal covering of Pteraspis. In the deeper 

 layers are large spaces, the so-called pulp-cavities leading into 

 narrow canaliculi, the so-called dentine canals. The structure is 



