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MONOGEAPUS OF NOKTU x\.MElUOAN KODENTIA. 



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or not attaining top of condyle ; mandible small and weak in comparison with 

 rest of skull. Cervical vertebrjE* sometimes anchylosed. Incisors variable 

 (the upper compressed and sulcate in North American genera, said to be broad 

 and smooth in Heteromys). Molars (i. e. pm. and m.) i"^, rooted or rootless. 

 General form Murine ; body slender and graceful ; ears and eyes well devel- 

 oped. Hind limbs somewhat or very decidedly saltatorial. Tail as long as 

 the body (more or less). Fore claws moderate, not obviously fossorial. 

 External cheek-pouches ample, not connected with the mouth, furry inside, 

 furnished with a special muscle (these pouches as in Geomyida, but no other 

 Rodents). Pelage generally coarse and hispid, said to be sometimes mixed 

 with flattened spines (in Heteromys) ; always without under fur. The species 

 very active and quick in their movements, like Mice ; some of them leaping 

 like Jerboas. 



The foregoing characters amply suffice for the determination of this 

 family, though others might be adduced. The skull is very peculiar ; its 

 main features result from the exaggerated state of the mastoids, which pro- 

 duce great width behind, roof much of the cerebral cavity, pinch the occip- 

 ital, and push the squamosal into the orbit. The next most prominent features 

 are the slight thread-like malar, attenuated rostrum, and flat palate. Tiic 

 curious position of the anteorbital foramen is shared by the Geomyidfr. To 

 bring the comparative characters of Saccomyidee find Geomyidoi into stronger 

 relief, the following may be adduced : — 



OEOMYID.fi. . 



(Geomyxand Thomornyt.) 



Skull miuaive, angular, in general of an Axvicolino 

 finpnrficies. 



Interorbital conHtriction narrower than roHtrnm. 



Interzygomatic width the greatest diameter of 

 the skull. 



Palate strungly sloping downward far below levol 

 of zygomata. 



Nasals not produced beyond inolsors; rostrum 

 broad, blunt, parallel-sided. 



Zygomata xtrong, flaring, with stout, short mala'r, 

 having orilinary conntMjtions. 



Frontal compressed. 



Pariotals compressed, irvogularly linear, remote 

 from orbits. 



SACCOMYID.*!. 

 (rtrognaihut, CHoetodiput, Dipodomyf.) 



Skull delicate, with ronnded-off angles and slight 

 ridges, if any. 



Interorbital space ranch broader than rostrum. 



Intcrmastoid width the greatest diameter of the 

 skull. 



Palate nearly horizontal, little, if any, below the 

 level of zygomata. 



Nasals produced l^yond tnoisors ; rostrum com* 

 pressed, tapering, acute. 



Zygomata slender, parallel, with long thread-like 

 malar, almost or actually abutting against tympanic. 



Frontal very broad. 



Parictals broad, triangular or pentangular. Dom- 

 ing to edge of orbits. 



* There is a cnrions coincidence or correlation between the elongation of the hind legs in adaptation 

 to saltatorial pmgrossion, and anpocial condition of the cervical vertcbrm and of certain elements of th ' 

 temporal hone, Huch development of the hind legs being often associated with anchylimis of cervical 

 Tortebrn, and great iuHatiou of thu lH)ues of the ear. Diput and Dipodomyt iihistrato these couditions 

 very strongly. 



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