126 



PALEONTOLOGY. 



Iii the tertiary formations, the fossil spines present for the 

 most part the generic characters of those of existing Plagio- 

 stomes — e.g., Spinax, Trygon, and Myliobates ; but one form, 

 found in the eocene beds near Paris, is the type of the extinct 

 genus Aulacanthus of Agassiz. 



The teeth of the plagiostomous fishes — viz. sharks (Squa- 

 lidce), rays (Raiidce), and Cestracionts, are very numerous, and, 

 being attached only by ligament to the membrane of the 

 mouth, they soon fall off in the decomposition of the dead 

 fish, become scattered abroad by the movements of the body 

 through the action of the waters, and sink into the sediment. 



Family 1. — Cestraciontid/E. 



{Port- J a ckson Sha rk. ) 



The existing genus which has thrown most light upon the 

 fossil teeth which have thus become imbedded in the oceanic 

 deposits of the palaeozoic and mezozoic periods, is the Cestracion, 

 now restricted to the Australian and Chinese seas, where it 



is represented by two or three 

 species, and suggests the idea 

 of a form verging towards 

 extinction. It formerly flou- 

 rished under a great number 

 of varied generic or family 

 modifications, represented by 

 species, some of which at- 

 tained dimensions far exceed- 

 ing the largest known living 

 Cestracions. The dentition 

 of these fishes is adapted to the prehension and masti- 

 cation of crustaceous and testaceous animals ; they are of a 

 harmless, timid character ; and have the before -described 

 denticulate dorsal spines given to them as defensive weapons. 



<^2^3 



Fig. 40. 

 Cestracion Philipi (recent). 



