RODENTS. 165 



unfortunate owner. Although in many cases the enamel on the 

 front of the incisor teeth of Rodents is of the ordinary white 

 colour, in others it is stained some shade of yellow, brown, red, 

 or even black. 



Behind these incisors there occurs a long tooth-less gap, 

 owing to the total want of canines. Then follow the cheek- 

 teeth, which are generally only four in number on each side 

 of both the upper and lower jaws, owing to the loss of the 

 anterior pre-molars, there being never more than three pairs of 

 the latter teeth, while in some cases they are totally wanting. 

 The cheek-teeth have flattened crowns adapted for grinding ; 

 and while in some cases these are surmounted by blunt tuber- 

 cles, they are more generally inter-penetrated by in-folds of the 

 enamel from the sides or summits, or both, by which in the 

 worn state they are divided into laminae, or have islands of 

 enamel on the grinding surface. 



The lower jaw is articulated to the skull by a knob, or con- 

 dyle, elongated longitudinally, and thus permitting of the back- 

 wards-and-forwards " munching " movement so characteristic of 

 these animals when eating. 



The feet are either completely, or almost completely, planti- 

 grade, and are usually furnished with five toes, generally armed 

 with sharp claws, although in a few instances these terminal 

 appendages partake more of the nature of hoofs. In nearly 

 all cases collar-bones are present, although these may be incom- 

 plete, or even rudimentary. In no case is the socket for the 

 eye in the skull surrounded by a ring of bone. 



In number the Rodents exceed any other of the Mammalian 

 Orders; and they have likewise a wider geographical distribution, 

 being practically cosmopolitan and represented even in Aus- 

 .tralia. They are, however, by no means evenly spread over 

 the globe, and attain their greatest development in South 

 America. In accordance with this general numerical superi- 



