532 MONOGRAPHS OF NORTH 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 subcireular. 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 Geomyide. he superior pair are much compressed, being 
narrower than deep, and strongly curved. Their face is marked by a deep 
median grooye, 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 Dipodomys—TVhe general configuration of this 
animal is hthe 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 
