f>:!2 monographs of nortii American rodentia. 



proper are rootless and perennial. In both jaws, the set of the molars is very 

 oblique; in the upper, the anterior tooth bends strongly backward, and the 

 posterior one somewhat forward, thus bringing their crowns in close apposi- 

 tion, though their roots are divergent. It is the same in the lower jaw, though 

 the greatest obliquity there is in the strong forward set of the posterior 

 tooth. The teeth are all simple, compressed prisms, broader in the transverse 

 than in the fore and aft direction. In the worn state, the crowns of the two 

 intermediate molars are simply elliptical; that of the anterior molar is rather 

 a spherical triangle, with convex posterior and two concave anterior sides ; 

 the back molar is small 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 spec- 

 imens with the milk-tooth still in position — there are some decided differ- 

 ences. The outer border of the two anterior teeth shows a deep nick, where 

 there is a re-entrant 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 Geomyida. 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 condyloid ramus. 



External characters of D'rpodomys. — The general configuration of this 

 animal is lithe and graceful, indicating agility and incessant activity. The 

 body is slender, the neck distinct ; the head large, with tapering muzzle ; the 

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

 dominance 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. Notwithstanding the saltatorial nature of 



