SQUIRRELS, MARMOTS, AND BEAVERS 1239 



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 

 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 backward-and- 

 forward motion of the lower upon the 

 tipper jaw, which is so noticeable when 

 we watch a rabbit feeding; this char- 

 acteristic serving to distinguish Rodents 



SIDE VIEW OF THE SKULI, OF THE PRAIRIE 

 alike from Ungulates and from Carni- MARMOT 



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, 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 backward in the jaws, so that in the up- 

 per 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 a 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 onto 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 frequently of an orange or reddish color), 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 in- 

 cisor teeth are continually growing, they always present the same chisel-like edges, 

 which are worn away by use at a rate commensurate with that of the growth. It fol- 

 lows from this that if one of these teeth be broken away during life, the correspond- 

 ing tooth in the opposite jaw, having nothing to check its growth by wear, will 



