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522 



MONOGllAPDS OF NOKTO AMERICAN KODENTIA. 



(iceipitiil seimrnted l)y a continuous fissure from petrosals. Mnndible small, 

 stout, witli u very slight coroiu)i(l. (Z> Dental.) — Superior incisors sulcate, con- 

 riivent, ])()iuting strongly backward ; deeper than wide. Molars (pm. and m., 

 f4) simple, rootless, (c. External.) — General form Jerboa-like; hind legs very 

 long, sallatorial. Tail rather longer tluin head and body, penicillate. Soles 

 densely furry. Feet with 1st digit rudimentary, l)ut bearing a claw. Eyes 

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

 the small nasal [md. Wiiiskers half as long as the wliole body. Ujjper lip not 

 cleft. Cheek-pouches ample. Pelage long and very soft. Pictura of body 

 and tail bicolor. Size of a half-grown rut {Mus decumanus). 



The skull of Dipodomijs, 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 developrhent 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, arc 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 i\\z petrosals with each other. In Geomyidce, 

 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 liearing must be exquisitely acute, if co-ordi- 

 nated with the osseous state of the parts. 



Notwithstanding the singular condition of the skull of Dijtodomys, result- 

 ing from the hypertrophy of certain parts and the reduction of others, the 

 relations with that of Geomyidce are both close and clear; while Perognathus 

 constitutes, in many respects, an excellent connecting link. Numerous coin- 

 cidences could be pointed out showing hov/ 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. 



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