152 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



dentine ;* as has already been said, these two tissues 

 differ greatly in hardness ; they will, therefore, wear 

 down unequally and so give rise to a roughened sur- 

 face well adapted for grinding. In addition to the 



Fig. 67. Teeth of Wolf. 



i 1 to i 3, Incisors ; c, canine ; p 1 to u 4, premolars ; m I to m 3, molars ; h, heel 

 of the first molar or lower " carnassial" tooth. 



outer investment of cement, we find it also filling up 

 the interspaces (Fig. 68). In the Carnivora the 

 lower jaw is so articulated to the skull as to be 

 able to work from below upwards ; in the herbivorous 

 forms the jaw works from side to side. 



Some of the Cetacea (dolphins, whales) are dis- 

 tinguished by the total absence of teeth, and the 



* For an account of the minute structure of the teeth, see 

 Klein's "Elements of Histology," chap. xxi. 



