﻿XVI INTRODUCTION. 



HOLOCEPHALI. 



Of the evolution of the Chimaeroids — the only known order of 

 this subclass — palaeontology at present reveals very few particulars. 

 In the Lower Devonian rocks there are dental plates essentially 

 similar in character to those of the still- existing Chiniaeridae ; and 

 in the earliest known Chimaeroid skeleton — that of Squaloraja from 

 the Lower Lias — the paired fins also differ in no particular from 

 those of its surviving congeners. The Squaloraiidae and Myria- 

 canthidae, however, exhibit some features in their dentition which 

 may be regarded as comparatively primitive ; and in other respects 

 both these early families display a few characters resulting from 

 specialization, such as have not been attained in the more per- 

 sistent and later types. 



As originally pointed out by Egerton x , the dentition of the 

 Hyriacanthidse (and we may add also that of the Squaloraiidae) 

 presents considerable superficial resemblance to that of certain 

 Cochliodont Elasmobranchs ; and it is thus easy to conceive how it 

 may have been developed, in a similar manner, from a dental arma- 

 ture such as was possessed by the earliest members of the last- 

 named subclass. In every respect the evolution has advanced 

 further than in the Cochliodonts, all anterior prehensile teeth 

 having disappeared ; and the growth of the dental plates, instead 

 of taking place exclusively at the inner border, seems to have 

 gradually extended to the whole of the attached surface. The 

 Chimaeridae exhibit an advance beyond the two families just con- 

 sidered, in the circumstance that all the dental plates are thickened, 

 while the hinder upper pair are both closely apposed in the median 

 line and much extended backwards. 



The characters in which Squaloraja and Myriacanthus exhibit a 

 higher degree of specialization than the later Chimaeroids are the 

 extreme development of the vertebral rings in the former and the 

 presence of extensive dermal plates in the latter. 



OSTKACODEKMI. 



At the conclusion of the sections on Elasmobranchii and Holo- 

 cephali, the numerous undetermined fragments of dermal armour, 

 chiefly consisting of vascular dentine, and hence probably referable 

 to one or other of the subclasses just discussed, are provisionally 

 arranged as Ichthyodorulites. A large number of these are still 

 1 Sir P. Egerton, Quart. Journ. Geol. Soc. vol. xxviii. (1872), p. 234. 



