VERTEBRATES: MAMMALS. 



grow at the base as fast as they wear 

 away at the summit. The lower jaw is 

 articulated with the skull in such a man- 

 ner that the jaws have no horizontal mo- 

 tion, except backwards and forwards, as 

 Rodent. is requisite in the act of gnawing. The 

 enamelled ridges of the molars are transverse, thus in 

 opposition to the horizontal forward and backward mo- 

 tion of the jaw, and exactly adapted to the process of 

 trituration. The form of Rodentia is generally such 

 that the hind parts considerably exceed those of the 

 front ; they are thus adapted to leaping instead of walk- 

 ing. Their fore-arms have little or no power of rotation, 

 and the bones of the fore-arm are in many cases united. 

 Rodents have simple or but little divided stomachs, long 

 intestines, brain without convolutions, and eyes directed 

 laterally. The number of species is great, six hundred or 

 more having been described. Most of them are small, 

 the beaver, with one or two exceptions, being the largest. 

 Rodents are found in all parts of the world, and are espe- 

 cially numerous in America. They comprise at least 

 five families, the Sciuridae or Squirrel Family, Sacco- 

 myidae or Gopher Family, Muridae or Rat Family, Hystri- 

 cidae or Porcupine Family, and Leporidae or Hare Family. 

 SCIURID/E, OR SQUIRREL FAMILY. This Family com- 

 prises the Squirrels and their allies, which have the 

 tibia and fibula distinct, and the molars 5 5 or 4 ^. It 

 includes three sub-families. 



I. Sciurinae, characterized by a distinct post-orbital 

 process, and by molars, rooted, 5 ~. 



The Genus Sciurus True Squirrels is character- 

 ized by compressed incisors, long ears, divided snout and 

 upper lip, long tail, with the hairs arranged mainly on 

 the sides, absence of cheek pouches, and inner lines of 

 the upper molars parallel. Squirrels are lightly built, 



