XVI 
INTRODUCTION. 
HOLOCEPHALI. 
Of the evolution of the Chimseroids—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 Chimaeridae ; 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 \ the dentition of the 
Myriacanthidm (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 
Chimgeridge 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 Squalor aja and Myriacanthus exhibit a 
higher degree of specialization than the later Chimseroids are the 
extreme development of the vertebral rings in the former and the 
presence of extensive dermal plates in the latter. 
OSTEACODERMI. 
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 
Sir P. Egerton, Quart. Journ. Geol. Soc. vol. xxviii. (1872), p. 234. 
