io8 Vertebrata. 



molars are separated from the incisors by a long 

 interspace. The lower jaw is large, and its condyle 

 is so articulated as to permit it to slide backwards 

 and forwards in mastication, thus giving the power of 

 gnawing. 



FIG. 52. 



Skull of porcupine, showing v, the large infra-orbital cavity. 



Rodents have small, smooth brains, usually a 

 simple or saccular stomach, and a long caecum or 

 blind pouch from the intestine (except in dormice). 

 Some genera, like guinea-pigs, hares, and rabbits, have 

 no collar-bones, others, like squirrels and beavers, 

 have these bones well marked. The hares and rabbits 

 have a thin layer of enamel surrounding the backing 

 of dentine on the incisor teeth, and have two small 

 incisors behind the large ordinary pair in the upper 

 jaw. 



The squirrel family are usually long-tailed elegant 

 creatures, and in one genus, the flying squirrel (fig. 5 1 ), 

 there is a lateral parachute of skin stretching from the 

 fore to the hind limbs. The beavers have flat scaly- 

 tails and webbed hind feet. The rats and mice 



