SACCOMYIDyE — CHARACTERS OF THE FAMILY, ETC. 493 



GEOMYIME. SACCOMYID-ffi. 



(Geomys and Thomomys.) {Perognaihus, Cricetodipus, Dipodomys.) 



Squamosals roofing most of cerebral cavity, from Squamosals mostly or wholly restricted to the 



roof of which mastoids are excluded. orbits; mastoids roofing much of cerebral cavity. 



Tympauics coutracted, tubular. Tympanies more or less inflated, vestibular. 



Petrosals widely discrete. Petrosals approximating or in actual contact at 



their apices. 



Occipital broad, forming most of occipital plane. Occipital contiacted, scarcely or not entering 



but not mounting on top of skull. occipital plane, but mounting top of skull, to there 



embrace interparietal between its forks. 



Molars rootless. Molars rooted (except in Dipodomys). 



Large, erect, falcate coronoid overtopping condyle. Small,slopiug, prickle-like corouoid below condyle. 



Lower jaw large and strong. Lower jaw small and weak. 



General form heavy, squat, clumsy — Arvicoline. General form light, lithe, and graceful— Murine. 



Fore limbs highly fossorial ; the claws much eu- Hind limbs more or less saltatorial. Fore claws 



larged, fitted for digging. not enlarged or specially fossorial. 



Eyes and ears minute. Eyes and ears large. 



Tail much shorter than body. Tail nearly as long as or longer than body. 



Pelage usually soft, lustrous, mole-like. Pelage usually coarse and hispid, even spiny. 



Habits completely subterranean. Habits exposed. 



I may next illustrate the points of resemblance between SaccomyidcE and 

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

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

 the former family Saccomyidcn of Baird, group Soccomyina of Waterhouse. 



Saccomyid.e and Geomyidce zz 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 tar forward and low down. Zygomatic process of maxillary an 

 expanded perforate plate. Molars (pm. and m.) t. 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 pentadactyle ; 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 some 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 Cricetodipus, many of the expressions applicable to Dipodo- 



