406 U. S. P. R. R. EXP. AND SURVEYS—ZOOLOGY—GENERAL REPORT. 
DIPODOM YS, Gray. 
Dipodomys, Gray, Ann. and Mag. N. H, VII, Aug. 1840, 521. 
Aup. and Baca. N. A. Quad. III, 1853, 137. 
? Macrocolus, WaGner, Wiegm. Archiv, 1846, I, 176. 
The species of this genus are characterized by the large, broad, depressed head, large rounded ears, acute snout, ample 
cheek pouches, opening externally. Each foot with the first digit rudimentary, but bearing a short claw; the four 
remaining, moderately developed, with rather long claws ; those on the fore feet longest. ‘Tail as long or longer than the 
body ; covered with hair which is longer and brush-like towards the tip. The hind legs are very long, jerboa-like. The 
under surface of the hind feet densly furred to the claws. 
Upper incisors longitudinally grooved. Molars rootless. Occipital plane deeply emarginatéd. Zygoma very slender. 
The species are generally yellowish brown, or brownish above, with a white spot above the 
eye and behind the ears ; the under parts, a stripe from the abdomen-across the thighs to the 
tail; the base of the tail and a stripe on each side of it white. 
The skull of the genus Dipodomys is very remarkable in its general characteristics, differing 
measurably from that of all other American rodents, excepting its allies among the Saccomyinae. 
It is excessively broad, and yet much depressed, its greatest width more than twice its depth, 
and about equal to the length from occiput to nasal bones. The outline, viewed from above, is 
slightly oval, broadest behind, and well rounded at either end, with the narrow and tapering 
snout springing abruptly forward, and extending so far that the nasal bones occupy at least 
one-third of the total length. The posterior part of the oval shows a deep rounded notch, the 
bottom of which is constituted by the occipital bone, bounded on either side by the backward 
projection of the auditory bulle, which thus extend far posterior to the occipital bone. The 
sides of the oval are interrupted by a deep quadrate notch, forming the boundaries of the orbit. 
Viewed laterally, the skull is highest just above the orbits; the dorsal outline slopes 
anteriorly with a very gentle concavity to the end of the snout ; posteriorly, it is nearly straight 
to the occipital region, where it becomes rapidly convex. The lower outline is nearly straight. 
A striking peculiarity in the skull of Dipodomys is the entire absence of ridges or crests, and 
the rounding off of all the angles on the upper surface, and the posterior half generally. The 
only angular bends in the skull are the upper edge, anteriorly, of the orbit, and the anterior 
edge of the zygomatic process of the maxillary. 
The snout, as already remarked, is very narrow and tapering, as well as considerably 
elongated. The nasal bones project considerably beyond the incisors, applied, however, for 
nearly the whole distance, against a vertical plate of the intermaxillary, The edges of the nasal, 
and the nasal process of the intermaxillary, are bent downwards into a short cylinder, inter- 
rupted below by a slit twice the width of an incisor. There is also a narrow plate which 
projects forwards between the upper half of the incisors as exposed, formed by the junction of 
their inner alveolar walls. For the posterior half, the nasal bones are linear with parallel 
sides, and do not reach back nearly as far as opposite the orbit. 
The nasal process of the intermaxillary is also linear, rather narrower than the nasal bones, 
and extending a little posterior to these ; they first make their appearance on the upper surface 
of the skull along the middle of the nasals. 
The frontal bone extends considerably forward, and runs out to an acute point on eithér side 
between the nasal process of the intermaxillary and the zygomatic of the maxillary, reaching, 
