RELATIONSHIP OF AMMOCGTES TO OSTRACODERMS 345 
This method of looking at the problem seems to me to be 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 Rohon, as quoted by Traquair, who, in his first paper 
accepted Lankester’s view that the ridges of the pteraspidian shield 
K 
A ee 
6 aS CA 
<LI EN S a 
Fe eg teesh cata’ Bork . 
f CESS : 
Fic. 139.—DREPANASPIS. VENTRAL AND Dorsan Aspucts. (After LANKESTER.) 
A,, anus; F., 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 the 
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. 
One 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 Jeading into 
narrow canaliculi, the so-called dentine canals. The structure is 
