MASTICATION. 



249 



in each jaw and a small tooth on the nasal bones ; or, in many fishes, as in 

 the sturgeon and amphyoxus, they may be entirely absent. Their shape 

 is also subject to great variation. In the lowest forms of fish they are 

 short and blunt, and well fitted for grinding sea-weed and crushing shell- 

 fish ; such teeth are seen in the ray -fish. But in most fish the teeth are 



Fig. 99.— Diagram of Premolar Tooth op Cat, with Alveolus, after 

 Waldeyer (Magnified Thirty Diameters). (Thanhoffer.) 

 A, bony wall of alveolus ; zo, enamel prisms ; zh, enamel coating ; Ji, spaces in the base of the enamel 

 prisms; D, dentine; dc, dentinal tubules: ///, gum, with alveolar periosteum below it; C, cement; ck, 

 cement spaces ; fb, tooth-pulp ; i, nerve entering pulp ; and j', pulp blood-vessels. 



usually small and conical, generally cylindrical, but sometimes flattened, 

 straight, curved, bent sidewise, or even barbed ; or their edge may be ser- 

 rated, as in sharks generally ; or the base may be broader than the apex, 

 as in the sharks. The teeth of fishes are by no means limited to the free 



