SACCOMYID^— DIPODOMYlNiE-DirODOMYS. 



531 



CC68 oftlic frontal separates it from the nmxillury. Tlie alvcolur imrtiun is 

 remarkably curved backward, to suit the trend of the incisors, and a strong 

 alveolar plute separates the teeth for about iiulf tlicir length. 



The nasals are chiefly notable for their length and tenuity; they reach 

 fur beyond the incisors ; the buck half is linear and superficiul ; anterior to 

 this, wiiere the intermaxillaries bond down, the nasals become sonicwiuit 

 volute, prolonging a semi-tubular snout. Within, delicate turbinal scrolls arc 

 seen extending Ut the orifice. The nasals are supported, nearly to their ends, 

 by small intermaxillary spurs. 



The mandible remains for consideration. This bone is remarkably smoll, 

 considering the size of the rest of the skull, and is further notable for its 

 slight elevation posteriorly, its short incomplete symphysis, and the thickness 

 of the body of the bone. The coronoid process is remarkably small, not 

 nearly attaining the level of the condyle, at the root of which it appears os a 

 minute, backward sloping, prickle-like spur. Tiic condyloid ramus itself is 

 small, compressed, and oblique. The principal feature of the bone is an im- 

 mense flaring lamina, which arises upon the back part of the lower border of 

 the body of the bone, and expands obliquely outward and upward, with a 

 peculiar twist. This plate-like process is longer than the condyloid ramus 

 itself, ond ends in an acute point, so that the back of the jaw appears two- 

 pronged. There is a deep pit between the alveolus and the root of the coro- 

 noid plate. The mental foramen is conspicuous upon the outside of the jaw 

 close to the incisors. The small size and lowness of the jaw is seen in the 

 peculiarly retreating chin of the species ; and it is probable, to judge especially 

 from the condition of the coronoid, that the biting power is comparatively 

 slight. 



The vertebral formula of Dipodomys ordii is given by Baird as 7 cervical 

 (with anchyloses of ?d, ?a, and 4th), 12 dorsal, 9 lumbar, 4 sacral, and 28 

 caudal, = 60 ; tk'^re is doubtless an individual variability of several of the 

 caudal segments. There are five metacarpals and metatarsals, though the 

 inner one of each is reduced. There are perfect clavicles. The tibia and fibula 

 are united below. 



Dentition of Dipodomyt. — The dentition of D^ndomya is simple. Of 

 the four grinding-teeth above and below, the anterior one is a premolar, being 

 preceded by a deciduous tooth, which, however, is long persistent. This one 

 is rooted and with a more complicated crown thnn the rest ; the molurs 



