ALIMENTATION 



91 



the jaws in sharks the scales pass by gradations into the teeth. 

 Presumably teeth originated as placoid scales covering the sur- 

 face of the body as well as the margins of the jaws, a condition 

 that has persisted in sharks, accompanied by a certain amount of 

 specialization in the jaw scales; while in higher forms the scales 



Fig. 57. — Side-view of the upper teeth of the sea-lion showing the peg-like char- 

 acter of all of them. 



have become purely oral structures that have gradually taken 

 on the form of typical vertebrate teeth. 



Primitive vertebrate teeth, if one is to judge from their struc- 

 ture and from the manner in which they are used by the shark, 

 were prehensile in function. That is to say, they were used for 

 grasping, tearing and holding, and not for chewing. In the case 

 of the frog the teeth are poorly adapted for anything but holding. 

 The similarity in form and structure of the frog's teeth illustrates 



C p M 



Fig. 58. — Side view of the upper teeth of the horse, z, incisors; c, canine; p, pre- 

 molars; m, molars. 



what is known as homodont dentition. The teeth of mammals, on 

 the other hand, are with few exceptions heterodont, and may be 

 differentiated or specialized in four ways: (1) as incisors, located 

 at the front of the jaws and used for cutting or grasping; (2) as 

 canines for grasping or tearing; (3) as premolars for shearing; 

 or (4) as molars for grinding (Figs. 56, 57 and 58). 



