1895.] NATURAL SCTENCKS OF PHILADELPHIA. '- >S;> ' 



of difference between them. Certain of his categories are here omitted, 

 as not hearing upon the points to he discussed and some verbal changes 

 have been made to render the nomenclature consistent with that used 

 in the body of this paper. 



GEOMYIDiE. 



1. Skull massive, angular, in 1. 



general of an arvicoline su- 

 perficies. 



2. [nterorbital space narrower 



than rostrum. 



:;. Skull widest across zygomatic :'•. 

 arches. 



4 Palate sloping strongly down- 4. 

 ward, far below level of zygo- 

 mata. 



5. Nasals not produced beyond 5. 



incisors, rostrum broad, 

 blunt, parallel sided. 



6. Frontals compressed. 6. 



7. Parietals compressed, irregu- 7. 



larly linear, remote from 

 orbits. 



8. Squamosal roofing most of 



cerebral cavity, from roof of 

 which mastoids are ex- 

 cluded. 

 'K Auditory meatus contracted, 9. 

 tubular. 



10. Tympanic bullae widely dis- 



crete. 



11. Supraoccipital broad, forming 11 



most of occipital plane, but 

 not mounting on top of 

 skull. 



12. Molars rootless. 12. 



13. Large, erect, falcate corouoid, 13. 



ovei topping condyle. 



14. Lower jaw large and strong. 14. 



15. Fore-limbs highly fossorial; 15. 



claws much enlarged, fitted 

 for digging. 



16. Eyes and ears minute. 16. 



17. Habits completely subter- 17. 



raneau. 



HETEROMYIDjE. 



( excl. Heteromys. ) 



Skull delicate, with rounded 

 oil angles and slight ridges, 

 if any. 



Luterorbital space much 

 wider than rostrum. 



Skull widest across mastoids. 



Palate nearly horizontal, lit- 

 tle, if any, below level of 

 zygomata. 



Nasals produced beyond in- 

 cisors, rostrum compressed, 

 tapering, acute. 



Frontals very broad. 



Parietals broad, triangular or 

 pentagonal, coming to edge 

 of orbits. 



Squamosals mostly or wholly 

 restricted to orbits; mastoids 

 roofing much of cerebral 

 cavity. 



Meatus inflated, vestibular. 



Tympanies approximated or 



in contact at apices. 



Supraoccipital contracted, 

 scarcely or not entering occi- 

 pital plane, but mounting 

 to top of skull to there em- 

 brace interparietal between 

 its forks. 



Molars rooted (except in 

 Dipodomyince). 



Small, sloping, prickle-shaped 



corouoid, below condyle. 



Lower jaw small and weak. 



Hind-limbs saltatorial. Fore- 

 claws not enlarged or spe- 

 cially fossorial. 



Eyes and ears large. 



Habits exposed. 



The two families agree in the following respects: (1) The mastoids 

 are much inflated (except in Heteromys) and occupy much of the 

 superior or occipital surfaces of the skull, or both. (2) The infra- 



