162 ORDERS OF FISHES. 



S'phenodus. In the Cretaceous rocks are numerous teeth, re- 

 ferred to the genera Corax, Galcocerdo, Otodus, Lamna, Oxy- 

 rhina, and Odontaspis, all of which continue to be represented 

 in the Tertiary deposits. The teeth of CarcJiarodon (fig. 

 529) also occur in the Cretaceous series, but the genus is 

 mainly Tertiary. The teeth in this genus are triangular, 

 serrated on both sides, and sometimes of immense size (five 

 or six inches in length). Teeth, apparently undistinguishable 

 from Carcharodon, have been dredged from the bottom of 

 the ocean at great depths, in considerable numbers, by the 

 "Challenger Expedition." In Otodus (fig. 528) the teeth 

 are not denticulated at their edges, and they have a secondary 



Fig. 527. — Oxyrhina xipho- Fig. 528. — Otodus ohliqitvs. Fig. 529. — Carclmrodon jpro- 



don. Miocene. Eocene. ductus. Miocene. 



tooth at each side of the base. The teeth of Oxyrhina (fig. 

 527), lastly, are large and compressed, differing from those 

 of Otodns chiefly in wanting the lateral projections at the 

 base. Upon the whole, the deposits which have yielded the 

 greatest abundance of the teeth of these extinct Sharks, are 

 the Upper Greensand (Cretaceous) and the London Clay 

 (Eocene Tertiary). 



c. Batidcs. — This group includes the Eays and Skates, and 

 is distinguished by the fact that the branchial apertures are 

 placed on the under surface of the body, forming two rows 

 of openings a little behind the mouth. In the typical mem- 

 bers of the group, the body is flattened out so as to form a 

 kind of disc (fig. 530), the greater part of which is made up 

 of the enormously developed pectoral fins. Upon the upper 

 surface of the disc are the eyes and spiracles ; upon the lower 

 surface are the nostrils, mouth, and branchial apertures. The 



