§ 14. 



TEETH OF ELEPHANTS. 



143 



species (Jig. 3) the enamel forms narrow transverse bands ; 

 and the tooth of the Mammoth, or fossil elephant. (Jig. 2,) 

 has an analogous, but somewhat different distribution. 



1 2 3 



Lign. 21. — Grinding- surface of teeth of elephants. 



Fig. 1. The worn surface of the crown of a tooth of the African Elephant. Fig. 2. 

 Grinding surface of a tooth of the Mammoth, or fossil Elephant. Fig. 3. Grind- 

 ing surface of the Asiatic Elephant. 



The appearances shown in these horizontal sections of 

 the crown of the teeth, arise from the dentine being- 

 disposed in parallel and vertical plates which have an im- 

 mediate investment of enamel, and are bound together by 

 an external layer of cement that envelopes the entire series. 

 which often consists of from twenty to thirty plates. By this 

 peculiar modification of structure, a rough grinding surface, 

 adapted for the mastication of vegetable food, is always 

 maintained, from the unequal wearing away of substances 

 of such different degrees of hardness. 



14. Dental organs of the rodentia, or gnawers. — 

 In the jaws of an intermediate order of animals, we find 

 new modifications of the same apparatus. Thus the Ro- 

 dentia. or gnawers, have long sharp cutting teeth, like 

 nippers ; it is by these instruments that the rat can speedily 

 gnaw a hole through a board, and the squirrel in a nut. in 



