RODENTIA. 287 



claws, although they sometimes have broad nails. Either 

 the whole or the greater part of the sole of the foot is 

 applied to the ground in walking, so that these animals 

 may be described as entirely or partially plantigrade. 

 Their skulls are characterized by the condyle of the lower' 

 jaw being elongated from front to back instead of from 

 side to side, and thus permitting of that backwards-and- 

 forwards motion of the lower upon the upper jaw, which 

 is so noticeable when we watch a rabbit feeding ; this char- 

 acter serving to distinguish Rodents alike from Ungulates 

 and from Carnivores. 



"The Rodents present a peculiarity in the structure 

 of the mouth, which is quite unknown in any other mam- 

 mals. In examining the mouth of any one of these animals 

 say a rabbit it will be found that behind the upper 

 front teeth the outer hairy skin of the face is continued 

 inwards into the sides of the mouth, which by this means 

 is divided into two distinct chambers, communicating with 

 one another through a comparatively narrow orifice; the 

 first chamber containing only the front teeth, while the 

 cheek teeth are included in the second chamber. It ap- 

 pears that this arrangement is designed to prevent the en- 

 trance of extraneous substances into the true cavity of the 

 mouth when the creatures are engaged in their character- 

 istic operation of gnawing. In addition to this peculiarity, 

 the whole of the inside of the cheeks in the hares and rab- 

 bits is covered with hair; while the pouched rats and 

 hamsters have large pouches inside the cheeks, which are 

 also lined with hairy skin. On the other hand, the gophers 

 have pouches in the cheeks which open externally instead 

 of internally. 



"Rodents are almost entirely herbivorous in their 

 habits; and they all of them obtain their food by gnaw- 

 ing. The hares are among the fleetest runners of all mam- 

 mals, while the jerboas and chinchillas are distinguished 

 by their leaping powers. Rodents are mostly harmless 

 and inoffensive creatures, fleeing with the greatest terror 

 and precipitancy from the smallest of foes ; but a few, like 

 the common rat, when driven to bay, will defend them- 

 selves desperately, and will then inflict comparatively se- 

 vere bites with their powerful front-teeth. 



