1 28 PALEONTOLOGY. 





decreases. From the oblique and apparently spiral disposition 

 of the rows of teeth, their symmetrical arrangement on the 

 opposite sides of the jaw, and their graduated diversity of 

 form, they constitute the most elegant tesselated covering to 

 the jaws which is to be met with in the whole class of fishes. 

 The modifications of the form of the teeth above described, 

 by which the anterior ones are adapted for seizing and re- 

 taining, and the posterior for cracking and crushing alimentary 

 substances, are frequently repeated, with various modifications 

 and under different conditions, in the osseous fishes. They 

 indicate, in the present cartilaginous species, a diet of a lower 

 organised character than in the true sharks ; and a correspond- 

 ing difference of habit and disposition is associated therewith. 

 The testaceous and crustaceous invertebrate animals constitute 

 most probably the principal food of the Cestracion, as they 

 appear, by their abundant remains in secondary rocks, to have 

 done in regard to the extinct Cestracionts, with whose fossil 

 teeth they are associated. 



From their mode of attachment, these teeth would become 

 detached from the jaws of the dead fish, and dispersed in the 

 way above described ; and it is by such detached fossil teeth 

 that we first get dental evidence of the Cestraciont family in 

 former periods of the earth's history. 



If fig. 42 be compared with fig. 41, it would seem as if 

 the several teeth of each oblique row in 

 Cestracion had been welded into a single 

 dental mass in Cochliodus, the propor- 

 tions and direction of the rows being 

 closely analogous. Whether in Coch- 

 Fig. 42. lioclus there were any small anterior 



Cochliodus contortus, Ag. prehensile teeth, is hypothetical ; the 



(Carboniferous.) -, •. . -, , , , . . , 



large crushing dental plates must have 

 been admirably adapted to crack and bruise the shells of 

 mollusks and crustaceans. The Cochliochis contortus (Ag.) 



