TEETH 21 



majority of the genera they are persistently growing teeth Jike 

 the incisors, in which development at their bases from ever-active 

 dentinal pulps and enamel organs compensates throughout life for 



Fig. 12. — Arvicola ampJiibius Linnseus. 



Skull and left mandibular ramus dissected to show the alveolar 

 courses of the teeth. 



the continuous loss of substance which takes place at the wearing 

 surface of each tooth. 



The worn surfaces of these teeth display a peculiar pattern of 

 triangles and transverse loops, produced by the truncation by 

 wear of the ends of the tall columns or prisms of which each 



