306 PROCEEDINGS OF THE ACADEMY OF 



the tympanies proportionally inflated, with large non-tubular ori- 

 fice of meatus; the petrosals bullous, their apices in contact across 

 the median line below the basisphenoid. Tympanies, mastoids, 

 and parietals entering orbits. Occipital singularly reduced and 

 narrowed, bent into three planes at right angles; supra-occipital 

 bifurcate to inclose an interparietal ; paroccipitals narrow, flange- 

 like ; basioccipital separated by a continuous fissure from petro- 

 sals. Mandible small, stout, with a very slight coronoid. (b. den- 

 tal.) Superior incisors sulcate, connivent, pointing strongly back- 

 ward ; deeper than wide. Molars (| f ) simple, rootless, (c. ex- 

 ternal.) General form jerboa-like ; hind legs very long, saltatorial. 

 Tail rather longer than head and body, penicillate. Soles densely 

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

 large and full; and ears large, orbicular. Snout produced, acute, 

 pilous except a small nasal pad. Whiskers half as long as the 

 whole body. Upper lip not cleft. Cheek pouches ample. Pelage 

 long, and veiy 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 con- 

 sidered in several of its details, is of extraordinary characters 

 not nearly matched outside the family to which it belongs. Many 

 of its features are shared, to a greater or less extent, by Perogna- 

 thus ; but the unusual characters are pushed to an extreme in 

 Dipodomys. The foregoing paragraphs merely indicate the more 

 salient peculiarities ; the skull is described in full beyond. The 

 enormal 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 character- 

 istics. 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 tym- 

 panic, and the contact of the petrosals with each other. In 

 Geomyidee, the temporals are of great size, but there is much less 

 distortion of the topography of the parts, both squamosal and 

 occipital maintaining ordinary characters. The temporal sinuses 

 together are scarcely less capacious than the brain-cavity itself; 

 the sense of hearing must be exquisitely acute, if co-ordinated with 

 the osseous state of the parts. 



Notwithstanding the singular condition of the skull of Dipodo- 

 mys, resulting from the hypertrophy of certain parts, and the 



