SACCOMYIDAI—CHARACTERS OF THE FAMILY, ETC. 
GEOMYID&. 
(Geomys and Thomomys.) 
Squamosals roofing most of cerebral cavity, from 
roof of which mastoids are excluded. 
Tympanics contracted, tubular. 
Petrosals widely discrete. 
Occipital broad, forming most of occipital plane, 
but not mounting on top of skull. 
Molars rootless. 
Large, erect, faleate coronoid 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 minute. 
Tail much shorter than body. 
Pelage usually soft, lustrous, mole-like. 
Habits completely subterranean. 
I may next illustrate the points of 
Geomyide as distinguished from other 
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SACCOMYID. 
(Perognathus, Cricetodipus, Dipodomys.) 
Squamosals mostly or wholly restricted to the 
orbits; mastoids roofing much of cerebral cavity. 
Tympanics more or less inflated, vestibular. 
Petrosals approximating or in aetual contact at 
their apices. 3 
Occipital contracted, scarcely or not entering 
occipital plane, but mounting top of skull, to there 
embrace interparietal between its forks. 
Molars rooted (except in Dipodomys). 
Small,sloping, 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 exposed. 
resemblance between Saccomyide and 
Rodents, thereby showing the charac- 
ters of the “superfamily” Saccomyoidea as named, but not defined, by Gill— 
the former family Saccomyide of Baird, group Saccomyina of Waterhouse. 
Saccomyip# and Gromyip# — SaccomyomeEs.—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. 
expanded perforate plate. Molars (pm. and m.) 4. 
more or less protuberant posteriorly. 
obliquely twisted plate, projecting outward and upward. 
Zygomatic process of maxillary an 
Root of under incisor 
Descending process of mandible an 
Large external fur- 
lined cheek-pouches with special muscle ; upper lip densely hairy, not visibly 
cleft; feet pentadactyle; fore claws longer than hinder ones. 
under fur. 
Pelage without 
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- 
