MAMMALIA. 



651 



Fig. 304. 



long arms, it swings from bough to bough with a facility little 

 to be expected from its appearance. 



(731.) The herbage that covers the plain, or the foliage of the 

 trees, are not, however, the only vegetable materials that have 

 been made available for the support of Mammiferous quadrupeds. 

 The RODENTIA are furnished with teeth adapted to gnaw even 

 the wood and the bark, or to crack nuts and other hard fruits, 

 from which they derive nourishment. 



This order of Mammals is, therefore, distinguished by the 

 possession of two incisor teeth in each jaw, so constructed as 

 to erode hard 

 substances, and 

 which moreover 

 by a peculiar 

 mechanism, to 

 be described in 

 another place, 

 are always kept 

 sharp and tren- 

 chant: such are 

 the incisor teeth 

 of the Beaver 

 or of the Hare 

 (fig. 304). 



The skeletons of the RODENTIA are slight and feeble, adapted 

 to the bird-like activity of their habits. Their fingers and toes 

 are well developed, and the bones of the leg and fore-arm free 

 throughout their whole length, although the movements of pro- 

 nation and supination are as yet very limited. In many genera, 

 more especially in such as climb trees like the Squirrels, the 

 clavicles are very perfectly formed, so that the fore legs can 

 be employed to a certain extent as hands, for conveying food to 

 the mouth. 



Very generally, the hind legs of the RODENTIA are consider- 

 ably longer than their anterior extremities ; hence such genera 

 run by bounds or leaps, and their course is very rapid. In the 

 Jerboa (Dipus) (Jig. 305) this disproportionate size of the hind 

 legs is excessive, insomuch that the creature moves by leaps, like 

 a Kangaroo ; and, the metatarsal bones of the three middle toes 

 being consolidated into one bone, the whole limb resembles more 

 that of a bird than of a quadruped. 



