SA(XX)MYir)iB— CIIARA0TER8 OP THE FAMILY, ETC. 



493 



OEOMYIUiE. 



(Geomya and Tkomomyt.) 



Sqnamosals rooflog most of cerebrnl cavity, from 

 roof of which mastoids are uxoluded. 

 TympauicB contracted, tabular. 

 Petrosals widely discrete. 



Occipital brood, forming most of occipital plane, 

 bat not mounting on top of sknll. 



Molars rootless. 



LaT|;e, erect, falcate uoronoid overtopping condyle. 

 Lower jaw large and strong. 

 General form heavy, squat, clumsy — Arvicoline. 

 Fore limbs highly fossorial ; the claws much en- 

 larged, fitted for digging. 

 Eyes and ears minnte. 

 Tail much shorter than body. 

 Pelage usually soft, lustrous, mole-like. 

 Habits completely subterranean. 



SACCOMYIDiE. 

 {Perognalhm, Cricetodipm, Dipodomyi.) 



Squamosals mostly or wholly restricted to the 

 orbits ; mastoids roofing much of cerebral cavity. 



Tympanies more or less inflated, vestibular. 



Petrosals approximating or in actual contact at 

 their apices. 



Occipital conti acted, scarcely or not entering 

 occipital plane, but mounting top of skull, to there 

 embrace interparietal between its forks. 



Molars rooted (except in Dipodomys), 



Small,slopiDg,prickle-like coronoid below condyle. 



Lower jaw small and weak. 



General form light, lithe, and graceful— Murine. 



Hind limbs more or less saltatorial. Fore claws 

 not enlarged or specially fossorial. 



Eyes and ears large. 



Tail nearly as long as or longer than body. 



Pelage usually coarse and hispid, even spiny. 



Habits cxposMl. 



I may next illustrate the points of resemblance between SaccomyidcB and 

 Geomyida as distinguished from other Rodents, thereby showing the charac- 

 ters of the "superfamily" Saccomyoidea as named, but not defined, by Gill — : 

 the former family Saccomyida of Baird, group Saccomyina of Waterhouse. 



Saccomyid.* and Geomyid.*; = Saccomyoidea. — Mastoid bone inordi- 

 nately developed, occupying much of the occipital or superior surface of the 

 skull, or both. Occipital correspondingly reduced. No postorbital processes ; 

 no anteorbital foramen as such, — in its stead a perforation in the side of the 

 maxillary far forward and low down. Zygomatic process of maxillary an 

 expanded perforate plate. Molars (pm. and m.) f Root of under incisor 

 more or less protuberant posteriorly. Descending process of mandible an 

 obliquely twisted plate, projecting outward and upward. Large external fur- 

 lined cheek-pouches with special muscle ; upper lip densely hairy, not visibly 

 cleft ; feet pentadactylc ; fore claws longer than hinder ones. Pelage without 

 under fur. 



It will be observed that the differences outweigh the resemblances, not- 

 withstanding the higher taxonomic value of somr of the latter.. The pecul- 

 iar state of the temporal bone, the position of the anteorbital foramen, the 

 shape of the jaw, and the presence of the pouches, are the main common 

 characters. In contrasting the two families, the very peculiar genus Dipodo- 

 mys offers the strongest points of difference ; but, when we come to consider 

 Perognathus and Cricetodipm, many of the expressions applicable to Dipodo- 



lii' 



