66 RODENTS. 



mentioned The feet are usually furnished with five toes, which generally terminate 

 in sharp claws, although they sometimes have broad nails. In walking, either the 

 whole or the greater part of the sole of the foot is applied to the ground, so that 

 these animals may be described as entirely or partially plantigrade. Rodents are 

 nearly always furnished with collar-bones (clavicles), although these may be more 



or less imperfectly developed, and are 

 thereby broadly distinguished from all 

 living Ungulates. Their skulls are 

 characterised by the condyle of the 

 lower jaw being elongated from front 

 to 1 lack, 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 cbar- 



SIDE VIEW OF THE SKULL OF THE FRAIRIE-MARMOT. UCti T Serving tO distinguish RoddltS 



alike from Ungulates and from Carni- 

 vores. Another point in connection with the skull is that the cavity for the eye 

 is not separated behind by a bar of bone from the temporal fossa; this feature 

 serving to distinguish the Rodents from the aye-aye, in which the eye-socket is 

 surrounded by a bony ring. 



The teeth being so important in the definition of the Rodents 

 Teeth. . ... 



require somewhat fuller consideration. With regard to the incisors, 



it may be observed that these teeth are of great length, and curved nearly in the 



arc of a circle ; their inserted portion extending far backwards in the jaws, so that 



in the upper jaw it comes nearly in contact with the base of the first of the cheek- 

 teeth, while in the lower jaw it runs beneath the whole of the cheek-series. The 

 lower incisors form ! small segment of a very large circle (roughly speaking), while 

 the upper ones constitute a much greater segment of a far smaller circle. In the 

 great majority of Rodents the enamel on the incisor teeth is confined almost 

 exclusively to their front surface, and is generally thicker on one side than on the 

 other; but in the hares and rabbits it also extends somewhat on to the lateral 

 surfaces. In cross-section these teeth are somewhat triangular : the front enamel- 

 covered surface being broad and flattened, and the two lateral surfaces gradually 

 converging to a rounded posterior edge. Whereas, however, the inner surface, 

 which comes in contact with the tooth on the opposite side of the jaw, is nearly 

 flat, the outer surface is convex. As a natural result of the front surface of these 

 teeth being composed of the hard enamel (which is very frecmently of an orange or 

 reddish colour), whereas the remaining portion consists of much softer ivory, it 

 follows that the effect of wear is to produce a sharp chisel-edge at their summits. 

 Indeed, the structure of an incisor tooth of a Rodent is precisely analogous to a 

 chisel : the hard enamel corresponding to the steel with which the latter is faced, 

 and which forms the cutting edge, while the ivory represents the soft iron forming 

 the support to the thin plate of steel. As these incisor teeth are continually grow- 

 ing, they always present the same chisel-like edges, which are worn away by use 

 at a rate commensurate with that of the growth. It follows from this that if one 



