ORDER RODENTIA ; GENERAL CHARACTERS. 247 



we observe an interesting gradation between these two divisions 

 of the class. Although the animals comprising it are chiefly 

 adapted to a vegetable diet, their feet are endowed with a greater 

 variety of function, than are those of the Ruminant quadrupeds. 

 They are still, like those of the Carnivora, unguiculated, or fur- 

 nished with claws ; their toes remain separate, as do also, for the 

 most part, the bones of the fore-arm ; and in many species the 

 clavicles or collar-bones are present, which serve to keep apart 

 the shoulders, and thus to give to the upper extremities a greater 

 freedom of motion. 



216. The chief peculiarity of this order is seen in the remark- 

 able conformation of the teeth. All the animals which it includes 

 may be at once recognised by the presence of two large cutting 

 teeth, which project from the front of each jaw. These teeth 

 have been usually regarded as inci- 

 sors ; but they are really canines, 

 the direction of which has been 

 changed by the absence of the inci- 

 sors ; this is proved by the fact that, 

 in the Hare, and a few others of the 

 order, small true incisors may be 

 seen behind the two large gnawing 



Fro. 125.-SKULL OF RODENT ANIMAI-. ^^ g^ approacll to this struc . 



ture may be seen in the Lemur tribe, especially in the Aye-aye, 

 which was placed by Cuvier among the Rodentia ( 164). 

 The conformation of the gnawing teeth is beautifully adapted to 

 the purpose they have to fulfil. In order to make their way 

 through tough vegetable substances, a sharp edge is required for 

 them ; and they must at the same time possess great strength 

 and firmness. For this purpose, the tooth is principally com- 

 posed of very tough ivory, with a plate of hard enamel in front 

 only (See ANIM. PHYSIOL., 177-180). By the friction 

 of the teeth against each other, their substance is continually 

 being worn down ; but the hard enamel of course wears most 

 slowly, and is always left as a sharp projecting edge. These 

 teeth differ also from any which exist in the Mammalia previously 

 described, in the circumstance that their growth is continually 



