286 



AN INTRODUCTION TO ZOOLOGY. 



setj i.e. towards the articular part of the jaw-bone. 

 The teeth, of mammals are divided into incisors or 

 front teeth, canines, premolars, and molars, i.e. 

 cheek teeth or "grinders"; but all these are not 

 always present together. 



The incisors, which occupy the most central region 

 of the jaw (premaxillary bone in the upper jaw, and 

 the opposite part of the lower one), are, as a rule, dis- 

 tinguished from the canines, which lie next, by their 



Fis. 109.— Teeth of Man. Dental formula, ii^ I ^ 



2 1 2 I 3' 

 1 = l9t incisor. 4= 1st premolar. 7=2nd molar 



2=2nameisor. 6= 2nd premolar. 8=3rd molar or Tviadom 



3=oanine. 6=lst molar. tooth. 



shape; premolars are, as a rule, distinguished from 

 molars by the fact that the former have predecessors 

 among the milk teeth, while the latter only appear in 

 the second set. All these may be looked upon as 

 modifications of a common type, much as in vege- 

 table morphology foliage leaves, bracts, sepals, petals, 

 stamens, and carpels, are all looked upon as modifica- 

 tions of a common leaf type — or as the walking legs, 

 claws, antennae, and jaws of a lobster are looked upon 



