HOW ANIMALS EAT 273 



Enamel is sometimes wanting, as in the molars of the 

 sloth and the tusks of the elephant. 



In fishes and reptiles, there is an almost unlimited 

 succession of teeth ; but mammalian teeth are cast and 

 renewed but once in life. 



Vertebrates use their teeth for the prehension of food, 

 as weapons of offense or defense, as aids in locomotion, 

 and as instruments for uprooting or cutting down trees. 

 But in the higher class they are principally adapted for 

 dividing or grinding the food. 84 While in nearly all 



FlG. 234. Upper Molar Tooth of Indian Elephant (Elephas indicus}, showing trans- 

 verse arrangement of dentine, d, with festooned border of enamel plates, e; c, 

 cement; one-third natural size. 



other vertebrates the food is bolted entire, mammals 

 masticate it before swallowing. Mastication is more 

 essential in the digestion of vegetable than of animal 

 food ; and hence we find the dental apparatus most effi- 

 cient in the herbivorous quadrupeds. The food is most 

 perfectly reduced by the rodents. 



Teeth, as we shall see, are appendages of the skin, 

 not of the skeleton, and, like other superficial organs, 

 are especially liable to be modified in accordance with 

 the habits of the creature. They are, therefore, of great 

 zoological value ; for such is the harmony between them 

 and their uses, the naturalist can predict the food and 

 general structure of an animal from a sight of the teeth 

 alone. For the same reason, they form important 

 DODGE'S GEN. ZOOL. 18 



