134 COMPARATIVE DENTAL ANATOMY 



there is a corresponding antero-posterior move- 

 ment of the jaw. In some of the semi-carnivo- 

 rous species, like the rats and mice, the molars 

 are tubercnlate, and much wear renders them 

 cup-shaped like the much-worn molars of man. 

 The American Beaver (Fig". 44) is one of the larg- 



FlG. 44. — Teeth of Beaver (Castor canadensis). Occlusal view. 



est living members of the order. It has the 

 formula, — 



1_1 c ' 0-0 P 0-0 4-4 



It has exceedingly large, strong, scalpriform in- 

 cisors, with which it cuts down small trees for the 

 purpose of building the dams for which it is fa- 

 mous. The molars are strong, with the tissues 

 arranged like those of the molars of the Ungulates 

 for the purpose of reducing the resisting vege- 



