SACCOMYID.E—DIPODOMYINZ—DIPODOMYS. 531 
cess of the frontal separates it from the maxillary. The alveolar portion is 
remarkably curved backward, to suit the trend of the incisors, and a strong 
alveolar plate separates the teeth for about half their length. 
The nasals are chiefly notable for their length and tenuity; they reach 
fur beyond the incisors; the back half is linear and superficial ; anterior to 
this, where the intermaxillaries bend down, the nasals become somewhat 
volute, prolonging a semi-tubular snout. Within, delicate turbinal scrolls are 
seen extending to the orifice. The nasals are supported, nearly to their ends, 
by small intermaxillary spurs. 
The mandible remains for consideration. This bone is remarkably small, 
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 as a 
minute, backward sloping, prickle-like spur. The 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, and 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 2d, 3d,and 4th), 12 dorsal, 9 lumbar, 4 sacral, and 28 
caudal, = 60; there 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 Dipodomys.—The dentition of Dipodomys 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 than the rest; the molars 
