NATURAL SCIENCES OF PHILADELPHIA. 317 



and subcircular. It is much the same in the lower jaw. The 

 crowns show simply the brim of enamel, with a depressed island 

 of dentine. In the unworn state, however such as may be 

 observed in specimens with the milk tooth still in position there 

 are some decided differences. The outer border of the two anterior 

 teeth shows a deep nick, where there is a reentrant fold of the 

 enamel ; and the back molar has a similar indentation of the inner 

 side. This diminishes regularly with the continuous growth of 

 the incisors, until the crowns are ground down beyond the extent 

 of the infolding, when it ceases to appear and the plain elliptical 

 form of the crown is assumed. 



The incisors are small and delicate, in both jaws, contrasting 

 with the stout scalpels of Geomyidae. The superior pair are much 

 compressed, being narrower than deep, and strongly curved. Their 

 face is marked by a deep median groove, and the outer portion is 

 rabbeted away, so that the groove is visible in a profile view. The 

 teeth emerge from the sockets some distance apart, separated by 

 an intervening alveolar plate, but they are convergent, and their 

 tips are in close contact. The under incisors, no larger than the 

 upper ones, are of much the same general character, but are not 

 grooved, the smooth faces being simply rounded off. Their roots 

 make a slight protuberance at the outside of the base of the con- 

 dyloid ramus. 



D. External Characters of Dipodomys. 



The general configuration of this animal is lithe and graceful, 

 indicating agility and incessant activity. The bod} r is slender, 

 the neck distinct; the head large with tapering muzzle; the eyes 

 and ears are prominent; the fore limbs small and neat, indicating 

 predominance of prehensile over merely gressorial faculties ; the 

 hinder limbs are of great size, as perfectly saltatorial as those of 

 a kangaroo or jerboa; and the tail is longer than the body. Not- 

 withstanding the saltatorial nature of the animal, there is none of 

 that preponderance of organization of the hinder parts witnessed 

 in the kangaroo, with its massive haunches and enormous tail ; 

 the whole body is equally slender, the leaping power being mani- 

 fested in the enlargement of the hind limbs alone ; the tail, too, is 

 slender throughout. 



The head is distinguished from the body by a well-defined 

 cervical constriction. The broad high occipital region clips sud- 



