NATURAL SCIENCES OF PHILADELPHIA. 275 



betwixt forks of the occipital. Squamosal almost entirely re- 

 stricted to the orbit. Tympanic more or less inflated. Mastoid 

 enormously enlarged and bullous, mounting to the top of the 

 skull, and also forming much or most of the occipital surface, 

 the occipital being correspondingly reduced, and scarcely or not 

 forming part of the general occipital surface. Petrosals mode- 

 rate^ inflated, their apices closely approximated or even in mutual 

 contact. Coronoid process of mandible slender, sloping, prickle- 

 like, scarcely or not attaining top of condyle ; mandible small and 

 weak in comparison with rest of skull. Incisors variable (the 

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

 broad and smooth in Saccomys and Heteromys). Molars |:|, rooted 

 or rootless. General form murine ; body slender and graceful ; 

 ears and eyes well developed. Hind limbs somewhat or 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 spe- 

 cial muscle (these pouches as in Geomyidee, but no other rodents). 

 Pelege generally coarse and harsh, said to be sometimes " spiny," 

 alwa3's without under fur. The species very active and quick in 

 their movements, like mice. 



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 produce great width behind, roof much of 

 the cerebral cavity, pinch the occipital, 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 Geomyidse. 

 To bring the comparative characters of Saccomyidse and Geomyi- 

 dse into strong relief, the following may be adduced : 



GEOMYIDSE. SACCOMYIDSE. 



(Geomys and Tlwmomys.) (Perognathtis, Cricetodipus, 



Dipodomys. ) 



Skull massive, angular, in general Skull delicate, with rounded-off 



of an arvicoline type. angles and slight ridges, if any. 



Interorhital constriction narrower Interorbital space much broader 



than rostrum. than rostrum. 



Interzygomatic width the greatest Intermastoid width the greatest 



diameter of the skull. diameter of the skull. 



