TEETH AND THEIR VARIATIONS. 203 



Before going any further, we must, however, notice 

 some of the points by which the teeth of Mammals are 

 distinguished from those of Eeptiles. Exclusive of 

 certain peculiar types like the Porpoises and Dolphins, 

 the teeth of Mammals are not similar to one another 

 throughout the jaws, but are arranged in groups of 

 different sizes and series. Thus in the upper jaw 

 (Fig. 64), there are commonly three simple front, or 

 incisor, teeth with chisel-like crowns ; then comes the 

 single large tusk, behind which are the cheek-teeth, of 

 which the number is very frequently seven, although 



FIG. 64. The teeth of the left side of the upper jaw of the Indian Civet. 



there are only six in the figured jaw. In those 

 Mammals which live on an animal diet, the three or 

 four foremost of the cheek-teeth (Fig. 64) retain a 

 more or less close approximation to the original com- 

 i pressed conical form, only the hinder ones acquiring 

 broad and flattened crowns adapted for masticating 

 or grinding. But in some of the herbivorous species, 

 on the contrary, as the Horse, all the teeth behind 

 the tusk may acquire these greatly developed and 

 flattened crowns. 



In the division of the teeth into incisors, tusks, and 



