92 COMPAEATIVE DENTAL ANATOMY 



eral form of the teeth of sharks is that of a sub- 

 compressed triangle, but some forms have small 

 accessory tubercles on each side of the central 

 blade. Sometimes the blade is wide, short, and 

 stout ; again it is long and slender, or oval or leaf- 

 shaped; the flat faces are sometimes smooth, and 

 again grooved or wrinkled. 



The Sharks merge into the Uays, in forms 

 which have both cutting and crushing teeth. The 

 teeth of all Eays are in pavement-like arrange- 

 ment of various shapes, and are formed for crush- 

 ing purposes. They are quite contact so as to form 

 a more or less continuous sheath over the whole 

 surface of the jaw. The shapes of the plate-like 

 teeth are quite various, and sometimes present 

 beautiful forms, in both recent and fossil species. 

 Some are perfectly oval; some quadrangular 

 (which fit closely together) ; in some — as Mylio- 

 batis — the teeth are of horny or bony structure, 

 and are closely joined together; in some forms the 

 teeth are rounded eminences ; in all varieties they 

 are formed in a thecal fold imbedded in a fibrous 

 membrane, are carried up over the edge of the 

 jaw, and are replaced as lost, just as in the 

 Sharks. 



The Saw-fish is a Ray, and is remarkable for 

 the unique elongated snout, which is covered on 

 both edges with true teeth, set in bony sockets 

 and growing from persistent pulps. This fish 



