522 MONOGRAPHS OF NORTII AMERICAN EODENTIA. 



occipital separated 1>_\ a continuous fissure from petrosals. Mandible small, 

 stout, with a very slight coronoid. (b Dental.) — Superior incisors sulcatc, con- 

 oiveht, pointing Strongly backward; deeper than wide. .Molars (pin. and m., 

 I | (simple, rootless, (c. External.) — General form Jerboa-like; hind legs very 

 long, sal tato rial. Tail rather longer than head and body, penicillate. Sides 

 densely lurry. Feet, with 1st digit rudimentary, but bearing a claw. Eyes 

 large and full Ears large, orbicular. Snout produced, acute, pilous, except 

 the small nasal pad. Whiskers halt' 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 Dipodumys. 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 ths petrosals with each other. In Geotnyidcc, 

 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 GeumyidcK 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. 



