276 



PROCEEDINGS OF THE ACADEMY OF 



GEOMYID^E. 



(Geomys and Thomomys.) 



Palate strongly sloping downward 

 far below level of zygomata. 



Nasals not produced beyond inci- 

 sors ; rostrum broad, blunt, parallel- 

 sided. 



Zygomata strong, flaring, with 

 stout, short malar, having ordinary 

 connections. 



Frontal compressed. 



Parietals compressed, irregularly 

 linear, remote from orbits. 



Squamosals roofing most of cere- 

 bral cavity, from roof of which mas- 

 toids are excluded. 



Tympanies contracted, tubular. 



Petrosals widely discrete. 



Occipital broad, forming most of 

 occipital plane, but not mounting on 

 top of skull. 



Molars rootless. 



Large erect falcate coronoid over- 

 topping condyle. 



Lower jaw large and strong. 



General form heavy, squat, clumsy 

 arvicoline. 



Fore limbs highly fossorial. 



Eyes and cars minute. 



Tail much shorter than body. 



Pelage usually soft, lustrous, mole- 

 like. 



Habits completely subterranean. 



SACCOMYIDiE. 



(Perognqthus, Cricetodipus, 

 Dipodomys.) 



Palate nearly horizontal, little if 

 any below the level of zygomata. 



Nasals produced beyond incisors ; 

 rostrum compressed, tapering, acute. 



Zygomata slender, parallel, with 

 long thread-like malar, almost or 

 actually abutting against tympanic. 



Frontal very broad. 



Parietals broad, triangular or pent- 

 angular, coming to edge of orbits. 



Squamosals mostly or wholly re- 

 stricted to the orbits ; mastoids roof- 

 ing much or most of cerebral cavity. 



Tympanies more or less inflated, 

 vestibular. 



Petrosals approximating or in act- 

 ual contact at their apices. 



Occipital contracted, scarcely or 

 not entering occipital plane, but 

 mounting top of skull, to there em- 

 brace interparietal between its forks. 



Molars rooted (except in Dipodo- 

 mys). 



Small, sloping, prickle-like cor- 

 onoid below condyle. 



Lower jaw small and weak. 



General form light, lithe, and grace- 

 ful murine. 



Hind limbs more or less saltatorial. 



Eyes and ears large. 



Tail nearly as long as or longer 

 than body. 



Pelage usually coarse and hispid. 



Habits exposed. 



We may next illustrate the points of resemblance between Sac- 

 comyidx and Geomyidx as distinguished from other rodents, 

 thereby showing the characters of the " superfamily" Saccomyoi- 

 dea as named, but not defined, by Gill the former family Sac- 

 comyidee of Baird. 



