496 
MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
ica. These may be readily distinguished by much stronger characters’ than 
any hitherto adduced by other writers :— 
Genus Perognathus. 
Occiput nearly plane, i. e., the mastoids not pro- 
jecting noticeably back of the occipital bone. 
Apices of petrosals separated by the whole width 
of the basisphenoid. 
Genus Cricetodipus. 
Occiput with a broad emargination, %. e., the 
mastoids bulging decidedly back of the occipital 
bone. 
Apices of petrosals almost meeting beneath the 
basisphenoid. 
Parietals perfectly pentagonal, with nearly equal 
sides. 
Inuterparietal elliptical, much broader than long, 
embraced between narrow plates of occipital. 
Ear with a distinct upright lobe of the antitragus, 
and generally also a lobe of the tragus. 
Sole naked to the heel, at least along a central 
stripe. 
Size of Mus musculus, or much larger. 
Parietals imperfectly pentagonal, inzquilateral. 
Interparietal pentagonal, shield-shaped,embraced 
between mere spurs of the occipital. ; 
Ear with no vestige of a lobe either of antitragus 
or tragus. ‘ 
Sole entirely hairy on the posterior half. 
Very diminutive; less in size than Mus musculus. 
The cranial characters above adduced, it may be observed, are all coér- 
dinated with the single main feature of much greater development of the 
mastoid in Cricetodipus than in Perognathus, the state of the parts in the 
former being an evident approach to the peculiarities of Dipodomys itself. 
The difference in the shape of the occiput is very striking when skulls of the 
two genera are laid beside each other; the part in Perognathus being quite 
flat, as in most Rodents, while Cricetodipus shows an emargination, much shal- 
lower and comparatively much broader than in Dipodomys indeed, but still 
well-marked. These cranial peculiarities, substantiating a genus Cricetodipus 
distinct from Perognathus, do not appear to have been noted before the 
appearance of my “Review”. They are correlated with the excellent and 
readily appreciable external characters of the feet and ears presented by 
Professor Baird. 
Genus PEROGNATHUS, Maxim. 
= Perognathus, Maxim., Nov. Act. Acad. Cws.-Leop. Carol. xix, 1839, 369. (Type P. fasciatus.)—CouEs, 
Proc. Phila. Acad. 1875, 279. (Excludes Cricetodipus.) 
< Perognathus, LECONTE, Proc. Acad. Nat. Sci. Phila. 1853, 224. (Includes Cricetodipus.)—BatrD, Mamm. 
N. A. 1857, 416. (Includes Cricetodipus.)—ALSTON, Proce. Zodl. Soc. Lond. 1876, 88. (Includes 
Cricetodipus. ) 
= ?Abromys,* GRAY, Proc. Zod]. Soc. Lond. 1868, 202. (Type “A. lordi”, sp. n. =P. monticola?.) 
Having already indicated the generic characters of Perognathus, I need 
only here give some further details respecting the skull and teeth, following with 
* duct. E.R. Alston, epist. ined. Londini, 25 Nov. 1876.—In penning my original account of this group 
for Proc. Phila, Acad., I was at a loss to know what to do with Abromys ; sol simply copied Gray’s notice 
into my text, stating that I could not make it out at all, though I failed to see any difference between 
“Abromys ” and Perognaihus, and suspected “A. lordi” to be P. monticola. In this impression, it seems, I 
