ELASMOBRANCHII. 



341 



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

 six inches in length). In Otodus (fig. 302) the teeth are not 

 denticulated at their edges, and they have a secondary tooth 



Fig. 301. Oxyrhinaxipho- Fig 302. Otodus obliqitus. Fig. 303. Carcharodon pro- 

 don. Miocene. Eocene. ductus. Miocene. 



at each side of the base. The teeth of Oxyrhina (fig. 301), 

 lastly, are large and compressed, differing from those of Otodus 

 chiefly in wanting the lateral projections at the base. Upon 

 the whole, the deposits which have yielded the greatest abund- 

 ance of the teeth of these extinct Sharks, are the Upper Green- 

 sand (Cretaceous) and the London Clay (Eocene Tertiary). 



c. Batides. This group 

 includes the Rays and 

 Skates, and is distinguished 

 by the fact that the bran- 

 chial apertures are placed 

 on the under surface of 

 the body, forming two 

 rows of openings a little 

 behind the mouth. In 

 the typical members of 

 the group, the body is 

 flattened out so as to form 

 a kind of disc (fig. 304), 

 the greater part of which 

 is made up of the enor- 

 mously developed pectoral 

 fins. Upon the upper sur- 

 face of the disc are the 

 eyes and spiracles ; upon 

 the lower surface are the 

 nostrils, mouth, and bran- 

 chial apertures. The flat- 

 tened bodies of the Rays, however, must be carefully dis- 

 tinguished from those of the Flat-fishes (Pleuronectidce). In 



Fig. 304. Batides. Raia marginata, one 

 of the Skates. Reduced one-sixth. (After 

 Gosse.) 



