460 



INCISOR AND MOLAR TEETH 



grow to a very great length, extending back within the jaw to 

 near the hinder part of the skull. These teeth are reinforced in 

 tlie upper jaw by a small second pair in the Lagomorpha only. 

 The incisors are chisel-shaped, and often brown or yellow upon 

 the outer face, as is the case also with some Insectivores. This 

 peculiar shape, and their strength, renders them especially capable 

 of the gnawing action which characterises the Eodents. It has 

 been pointed out that where the incisors are wider than thick, 

 the gnawing powers are feebly developed ; and that on the con- 

 trary, where these teeth are thicker than wide, the animals are 

 good gnawers. The incisors have often an anterior groove, or it 

 may be grooves. 



Fig. 232.— Molar teeth of Rodents. A, of C'lipybara (Hijdrochoerus) ; B, of Squirrel 

 (Scivnis) ■ C, of riciiii,/„cfi//iii (After TiiUberg.) 



The cheek teeth vary in number from two {Hydromys) to .six 

 (Eabbit) on each side of the two jaws. Four is the prevailing 

 number outside the large division of the Itat-like Eodents. 

 They are often set at an angle to the horizontal plane of the jaw, 

 looking outwards and obliquely to its longitudinal axis ; the in- 

 dividual teeth too are not unfrequently bowed in form, remind- 

 ing us of those of TiKCdilvn. This of course only occurs in those 

 genera which have hypselodont teeth. The pattern of the teetli 

 varies much, and the different forms recall the teeth of more 

 than one other group of mammals. They are either bunodont 

 or lophodont. In many cases the tooth is encircled with a ridge 

 of enamel, which is either almost simple or has a more com- 

 plicated contour ; such teeth are )jy no means unsuggestive of the 

 Toxodonts. Some of the lophodont molars are by no means 

 unlike those of the Proboscidea. In ,S'ciurns vulgaris the 



