PLAGIOSTOMI 



115 



tertiary crag (probably miocene) of Norfolk, and in the mio- 

 cene mollasse of Switzerland. 



When the teeth form broad transverse undivided plates, as 

 in fig. 41, they characterize the genus JEtobates. Fossils of 

 this genus occur in the English eocenes and the Swis smol- 

 lasse. 



In the "crag" of Norfolk and Suffolk, and in marine plio- 

 cene beds, fossils have been found which closely resemble 



/ 



Fig. 41. 



JEtobates subarcuatus 

 (Eocene, Bracklesham). 



Fig. 41 a. 



Raia clavata 

 ^Dermal spines). 



the osseous and spinigerous plates that beset the skin of 

 the kind of ray called "tbornback" (fig. 41 a), and which 

 indicate the existence of a pliocene species allied to the Raia 

 clavata. 



Thus we obtain evidence of fishes of the plagiostomous 

 order in the marine deposits of every formation from the 

 upper Silurian beds to the present period. But none of the 

 palaeozoic fossils are referable to any existing genus. A 

 few only of the mezozoic Plagiostomes, and those chiefly 

 from the chalk, are so determinable : but most of them 

 belong or are allied to a family (Cestraciontidai), now nearly 

 extinct. The evidence of the generic forms of Plagiostomes 

 characteristic of the present time become common only in 

 the tertiary periods. No fossil species is the same with 

 any existing one. 



