DEVONIC FISHES OF THE NEW YORK FORMATIONS 65 



as dependence can be placed upon analogy of this nature, forms allied to 

 the modern Port Jackson shark (Cestracion or Heterodontus, text fig. 15) 

 were present during the Devonic, and extremely abundant during the 

 Lower Carbonic. Tamiobatis, known by a unique skull from the Devonic 

 of Kentucky, is interesting in that, although probably a shark, its form 

 was decidedly raylike. No true rays, however, appear earlier than the 

 Mesozoic. 



Entire skeletons of undoubted Selachii are known first from the Lower 

 Lias. As for the detached dermal spines and tubercles occurring in various 

 Paleozoic horizons, we are without means for determining their precise 

 relations, hence they are conveniently grouped together under the head 

 of " Ichthyodorulites." These are treated in the present contribution 

 immediately after the Holocephali. 



Subclass HOLOCEPHALI 



The following general remarks on the Holocephali and Chimaeroids, 

 taken chiefly from Smith Woodward, may profitably find a place here : 



Dental plates essentially similar to those of the existing Chimaeroid 

 fishes are met with in rocks as early as the Middle Devonic ; but there is 

 still no evidence of any member of the Holocephali which can not be 

 included in the surviving order of Chimaeroidei. Some of the early forms 

 were certainly armed with dermal plates ; but paleontology as yet lends no 

 support tor the theory that the Chimaeridae are degenerate descendants 

 of fishes once possessed of membrane bones. The earliest known complete 

 skeletons are unfortunately only Liassic. 



