PEDIGREES. 173 



remembered we discussed on p. 39. They are 

 remarkable as well for their beauty — seen in situ — 

 as for the evidence of adaptation to function 

 which they have undergone, resulting in crush- 

 ing teeth of a very perfect description. The 

 Cestracionts reached the hey-day of their de- 

 velopment during the Mesozoic period. The 

 living species is but an isolated member of his 

 kind. Another very remarkable and ancient 

 type of shark, living at the present day in the 

 sea around Japan, is the Japanese frill-gilled 

 shark {ChlamijdoselacJie). Amongst its most 

 striking features are its teeth, resembling those 

 of the living Notidanus, to which it is related, 

 and certain fossil forms occurring as far back as 

 the Jurassic epoch. 



The Tectospondyli contains those dog-fishes 

 which have no anal fin, and the rays and devil- 

 fishes. Earlier naturalists, impressed by the 

 superficial characters only, grouped all the shark- 

 like fishes together, leaving the ray-like forms 

 together to form a separate sub-order. Recent 

 investigation has shown how dangerous are con- 

 clusions based on external appearances. We 

 now realise that adaptation to similar physical 

 conditions may result in the transformation of 

 animals not nearly related to an extraordinary 

 external likeness. This is exemplified in the 

 case just mentioned. More deep-seated char- 

 acters show that the spiny dog-fishes agree rather 

 with the rays than the sharks. It is significant 

 that many of the very oldest known rays 

 apparently difi'er but little from species now 

 living. 



