COMPARATIVE ANATOMY OF THE TEETH. 283 



The mode in which the teeth are implanted in the jaws 

 varies according to the diet. Those, for example, like the 

 true rat, which live on a mixed food, do not require so 

 great a depth of the crown, and the teeth are hence not so 

 firmly fixed, nor so large as in those rodents whose food is 

 entirely vegetable, and where the friction is greater. 



The highest numher of teeth in this order is stated at 

 twenty-eight. The rahbit has six molars on each side, in 

 the upper jaw, and five in the lower. The squirrel has 

 five molars in the upper on either side, and four in the 

 lower. The rat has three molars on each side, in both 

 jaws, though the spring-rat, as it is called, has four molars 

 in both jaws, on each side. 



EDENTATA. It would be supposed from the name of this 

 order that all the genera composing it were without teeth. 

 Hence Mr. Owen very justly remarks, it is to be regretted 

 that such a term should have been applied, seeing that all 

 the species of this order, except two, have teeth, though 

 nearly all are without incisors. 



The ant-eater, armadillo, and sloth, are examples under 

 this head. 



The ant-eater has twenty-six teeth seven on each side 

 of the upper jaw, and six on each side of the lower. The 

 number in the armadillo is stated to vary from twenty-four 

 to twenty-six on each side of the upper jaw, and twenty- 

 two to twenty-four in the lower ; making, in the whole, 

 from ninety-four to one hundred teeth. The sloth has 

 eighteen teeth, all molars there being five on each side 

 of the upper jaw, and four on each side of the lower. 



The extinct gigantic animals by the name of megather- 

 oids, some of whose teeth are nearly a foot in length, 

 belong also to this order. 



PACHYDERMATA. This order includes the elephant, mas- 

 todon, hippopotamus, rhinoceros, hog, horse, &c. 



The elephant, distinguished by its two enormous tusks, 

 has 28 teeth. In the upper jaw, on each side, there are two 

 incisors and six molars. In the lower there are only six 

 molars on either side. The tusks are situated in the inter- 



