p00 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
todipus break apart across the middle. This seems to be chiefly due to the 
delicate state of the zygomata, which afford no stable connection between 
the fore and aft parts. The break occurs at the basispheno-occipital, squamo- 
mastoid, and fronto-parietal sutures; the parietals, temporals excepting squam- 
osals, with the occiptal, coming away from the rest of the skall. 
There appears to be something peculiar, in the habits perhaps, rather 
than in the scarcity, of the species, which prevents the acquisition of large 
series of specimens in this family. While hundreds of examples of animals 
no larger or more conspicuous than these are readily amassed, collections are 
all deficient in Perognathus and Cricetodipus, and not very full in Dipodomys. 
I have not been able to examine more than a hundred specimens altogether, 
and of these more than half were Dipodomys. Of Perognathus, I have, how- 
ever, specimens of all the described North American species, including all 
of Baird’s types and considerable additional material. After protracted exam- 
ination, I endorse the validity of all the species admitted by that author in 
1857, and find indications of the probable existence of one or two more. ‘This 
point, is fully discussed beyond. The species that appear to be established 
may be readily determined by the following analysis :— 
Analysis of the Species of PEROGNATHUS. 
A. Notch of the ear bounded in front by a slight, though distinct, lobe of the tragus. Whole fore leg 
white. 
a. Tail penicillate, crested at the end, rather longer than the body and head. Sides with no fulvous 
GUY npORe a COO ROreRSmomcos op mos see.ncos cacbod ecawisn cdetos Sdcornoasenasnes PENICILLATUS. 
b. Tail simple; sides with a strong fulvous stripe. 
1. Tail decidedly shorter than the body and head. Length about 4 inches. Pelage moderately 
stiff.) Mixed ‘sandy-and blackish’: 2-4: caseeeee aoe etoaeereon sce se atasiece eet FASCIATUS, 
2. Tail not shorter than head and body. Length about 3 inches. Pelage very hispid. Mixed 
cinnamon:and blackish. 2 - .<).252.)) -ose5 coceuss bape cecine= eee eae one nese eeaee HISPIDUS. 
B. Notch of ear formed in front directly by the as of the conch. Fore leg seats colored like the back. 
Taiksimple;, lateral stripe obscure: .=- .-..22- 12 oe eee tee oet eee eee eee eee MONTICOLA. 
PEROGNATHUS FASCIATUS, Maxim. 
Perognathus fasciatus, Maxt., N. Act. Akad. Leop.-Czes. Nat. Cur. xix, pt. i, 1839, 369, pl. 34; Reise Nord- 
Am. i, 1839, 449; Arch. f. Naturg. 1861, p. —}; Verz. Reise, 1862, 175, pl. 4, f. 6, 7.—Waaun., 
Arch. f. Naturg. 1841, 45; Suppl. Schreb. iii, 1843, 612—Scu1nz, Syn Mamm. ii, 1845, 259.— 
LEC., Proc. Acad. Nat. Sci. Phila. vi, 1853, 224.—Aup. & Bacu., Q.N. A. iii, 1854, 341 (com- 
piled.—GieBEL, Siiugeth. 1855, 572 (compiled).—Bp., M. N. A. 1857, 421 (Chibuahua).—Bp., 
U.S. Mex. Bound. Surv. ii, pt. ii, 1859, Mamm. 42.—Suckt., P. R. R. Rep. xii, pt. ii, 1860, 101 
(compiled).—Gray, P. Z. S. 1868, 201.—Cours, Proc. Phila. Acad. 1875, 284 (monographic). 
Perognatus fasceatus, LINCECUM, Am, Nat. vi, 1872, p. 369 (habits). 
Prognathus faceatus, LINCECUM, Am. Sportsman, Feb. 28, 1874 (habits). 
Dracnosis.—Largest of the genus; considerably exceeding Hesperomys 
leucopus, and approaching Tumias quadrivittatus, in size; length four inches 
