SACCOMYIDA—DIPODOMYIN 4I—DIPODOMYS. 525 
Description of the skull of Dipodomys.—As in other cases, it will be found 
most convenient to consider the skull as a whole first, and afterward to exam- 
ine its individual bones. An immature specimen is preferable for the latter 
purpose, though many or most of the suttires persist to extreme old age. 
In many respects, the skull of Perognathus approaches or closely resem- 
bles that of Dipodomys, but the family peculiarities reach their extreme 
development only in the latter. Comparative expressions used in the following 
paragraphs are to be considered exclusive of Perognathus, unless the contrary 
is stated. 
The skull is much depressed; elongated and acuminate in front; very 
broad behind, where the width is nearly two-thirds of the total length ; and, 
viewed from above, presents in general a triangular shape, with the lateral 
angles completely rounded off, and a deep emargination in the middle of the 
base. Zygomatic arches scarcely come into this view at all; the width of 
the skull midway being much less than the intermastoid diameter. The out- 
line of the zygomata is perfectly straight; between the turgid mastoid region 
and the expanded plate-like zygomatic process of the maxillary there stretches 
the thread-like malar, depressed to the level of the palate. The outline of 
the orbits is a quadrate notch between the saliences just mentioned. There 
is no interorbital constriction; were it not for the laminar zygomatic expan- 
sion of the maxillaries and the bullous mastoids, the space between the orbits 
would be the broadest part of the skull. The attenuate acuminate rostrum 
springs directly opposite the broad zygomatic part of the maxillaries, and 
extends beyond the incisors; it is at least one-third of the total length of the 
skull. The postero-lateral aspects of the skull present enormous bulging 
masses rounded and somewhat ovate—the extraordinarily developed mastoids. 
The same swellings constitute also nearly all the occipital region, the median 
line of which is a deep emargination. This character is perhaps unique ; 
nothing like it is seen even in Perognathus ; its peculiarity is on a par with 
the immense rostral development in a walrus for example. The resulting 
figure, as one author has aptly remarked, bears a ludicrously close resem- 
blance to the buttocks of the squatting human figure, the mastoids being the 
nates, the emargination being the cleft between, and the foramen magnum 
having an obvious suggestiveness. The whole surface of the skull is smooth, 
and gently convex in all directions. There are no ridges or roughnesses; the 
