522 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
occipital separated by a continuous fissure from petrosals. Mandible small, 
stout, with avery slight coronoid. (& Dental.)—Superior incisors sulcate, con- 
nivent, pointing strongly backward; deeper than wide. Molars (pm. and m., 
4—4) simple, rootless. (¢. External.)—General form Jerboa-like; hind legs very 
long, saltatorial. Tail rather longer than head and body, pemicillate. Soles 
densely furry. Feet with Ist digit rudimentary, but bearing a claw. Eyes 
large and full. Ears large, orbicular. Snout produced, acute, pilous, except 
the small nasal pad. Whiskers half as long as the whole body. Upper lip not 
cleft. Cheek-pouches ample. Pelage long and very soft. Pictura of body 
and tail bicolor. Size of a half-grown rat (Mus decumanus). 
The skull of Dipodomys, whether taken as a whole or considered in 
several of its details, is of extraordinary characters not nearly matched out- 
side the family to which it belongs. Many of its features are shared, to a 
greater or less extent, by Perognathus ; but the unusual characters are pushed 
to an extreme in Dipodomys. The foregoing paragraph merely indicates the 
more salient peculiarities; the skull is described in full beyond. The enor- 
mous development of certain elements of the temporal bone, and the results 
of this inflation upon the connections of the bone, and general configuration 
of the skull, are the leading characteristics. With this is co-ordinated the 
reduction of the squamosal and occipital, and the curious shape of the latter, 
as well as the anomalous abutment of the thread-like zygoma against the 
tympanic, and the contact of the petrosals with each other. In Geomyide, 
the temporals are of great size, but there is much less distortion of the topog- 
raphy of the parts, both squamosal and occipital maintaining ordinary charac- 
ters. The temporal sinuses together are scarcely less capacious than the 
brain-cavity itself; the sense of hearing must be exquisitely acute, if co-ordi- 
nated with the osseous state of the parts. 
Notwithstanding the singular condition of the skull of Dipodomys, result- 
ing from the hypertrophy of certain parts and the reduction of others, the 
relations with that of Geomyide are both close and clear; while Perognathus 
constitutes, in many respects, an excellent connecting link. Numerous coin- 
cidences could be pointed cut showing how the hint afforded by the presence 
in these two families of ample external cheek-pouches is borne out in more 
essential features, notwithstanding the all but complete difference in general 
outward appearance. 
