THE SKELETON IN FISHES. THE TEETH. 109 



placoid scales. They are developed from a layer of dental germs 

 which occurs all over the surface of the skin, except in the 

 region of the lips. At this point the layer of tooth-producing 

 germs extends back into the mouth, being projected by a fold 

 of the mucous membrane (fig. 14, 7). Here new teeth are 

 successively formed, and as they grow each is gradually 

 brought into a position to take the place of its predecessor 

 by the shifting outwards of the gum over the jaw. Owing to 

 this arrangement sharks have practically an unlimited supply 

 of teeth (figs. 14 and 15). 



Two principal types of teeth are found in ELASMOBRANCHS. 

 In Sharks and Dogfish, on the one hand, the teeth are very 

 numerous, simple, and sharp-pointed, and are with or without 

 serrations and lateral cusps. Many Rays and fossil Elasmo- 

 branchs, on the other hand, have broad flattened teeth adapted 

 for crushing shells. Intermediate conditions occur between 

 these two extremes. Thus in Cestracion and many extinct 

 sharks, such as Acrodus, while the median teeth are sharp, 

 the lateral teeth are more or less flattened and adapted for 

 crushing. In various species belonging to the genus Raia the 

 teeth of the male are sharp, while those of the female are blunt. 

 A very specialised dentition is met with in the Eagle-rays 

 (Myliobatidae), in which the jaws are armed with flattened 

 angular tooth-plates, arranged in seven rows, forming a com- 

 pact pavement ; the plates of the middle row are very wide 

 and rectangular, those of the other rows are much smaller and 

 hexagonal. Lastly, in Cochliodus the individual crushing 

 teeth are fused, forming two pairs of spirally-coiled dental 

 plates on each side of each jaw. Pristis, the Saw-fish, has a 

 long flat cartilaginous snout, bearing a double row of persist- 

 ently-growing teeth planted in sockets along its sides. Each 

 tooth consists of a number of parallel dentinal columns, united 

 at the base, but elsewhere distinct. 



In the HOLOCEPHALI Ckimaera, Hariotta and Callorhyn- 

 c h us only three pairs of teeth or dental plates occur, two 



