41)2 MONOGRAPHS OF NOltTLI AMERICAN RODENTIA. 



or mil attaining top of condyle ; mandible small and weak in comparison wil h 

 rest of skull. Cervical vertebrae* 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.) { !, 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 (mure or less). Fore (daws moderate, not obviously fossorial. 

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

 furnished with a special muscle (these pouches as in Geomyidee, 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. The 

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

 bring the comparative characters of Saccomyidce and GeomyidcR into stronger 

 relief, the following may be adduced: — 



GEOMYID.~£. SACCOMYIDCE. 



(Geomyi and Thomomys.) (Perognathus, Cricetodipus, Dipodomys.) 



Skull massive, angular, in general of an Arvicoline Skull delicate, with rounded-oil* angles and slight 



superficies. ridges, if any.- 



Interorbital constriction narrower than rostrum. Interorbital space much broader than rostrum. 



Interzygomatic width the greatest diameter of Intermastoid width the greatest diameter of tho 



the skull. skull. 



Palate strongly sloping downward far below level Palate nearly horizontal, little, if any,below tho 



of zygomata. level of zygomata. 



Nasals not produced beyond incisors; rostrum Nasals produced beyond incisors; rostrum corn- 

 broad, blunt, parallel-sided. pressed, tapering, acute. 



Zygomata strong, flaring, with stout, short malar, Zygomata slender, parallel, with long thread-like 



having ordinary connections. malar, almost or actually abutting against tympanic. 



Frontal compressed. Frontal very broad. 



Parietals compressed, irregularly linear, remote Parietals broad, triangular or pentangular, com- 



from orbits. iug to edge of orbits. 



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

 to saltatorial progression, and a special condition of the cervical vertebra; and of certain elements of the 

 temporal bone, snch development of the bind legs being often associated with anchylosis of cervical 

 vertebra 1 , and great inflation of the bones of the ear. I)q»w and Dipodomys illustrate these conditions 

 very strougly. 



