. r )(K) MONOGRAPHS OF NORTH AMERICAN RODBNTIA. 



todipus break apart across (he middle. This seems 1<> be chiefly due to (lie 

 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 resl of the skull. 



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 Cricetodlpus, 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 Pkuognathus. 



A. Notch of the car bounded in front by a slight, though distinct, lobe of the tragus. Whole fore leg 



white. 



a. Tail peuicillate, crested at the end, rather longer than the body and head. Sides with no fulvous 



stripe 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 saudy and blackish fasciatus. 



2. Tail not shorter thau head and body. Length about 3 inches. Pelage very hispid. Mixed 



cinnamon and blackish hispidus. 



B. Notch of ear formed in front directly by the edge of the conch. Fore leg mostly colored like the back. 



Tail simple; lateral stripe obscure monticola. 



PEROGNATHUS FASCIATUS, Maxim. 



Perognathus fasciatus, Maxim., N. Act. Akad. Leop.-Cres. Nat. Cur. xix, pt. i, 1839, 369, pi. 34 ; ReiseNord- 

 Am. i, 1839, 449; Arch. f. Naturg. 1861, p. — ; Verz. Reisc, 186'i, 175, pi 4, f. 6, 7.— Wagn., 

 Arch. f. Naturg. 1841, 45 ; Suppl. Schreb. iii, 1843, 612.— ScniNZ, Syn Mamm. ii, 1845, 259.— 

 LeC, Proc. Acad. Nat. Sci.rhila.vi, 1853, 224.— Add. & Bach., Q. N. A. iii, 1854, 341 (com- 

 piled.— Giehf.l, Suugeth. 1855, 572 (compiled).— Bd., M. N. A. 1857, 421 (Chihuahua).— Bo., 

 U. S. Mex. Bound. Surv. ii, pt. ii, 1859, Mamm. 42.— Suckl., P. R. R. Rep. xii, pt. ii, I860, 101 

 (compiled).— Gray, P. Z. S. 1868, 201.— Coues, Proc. Phila. Acad. 1875, 284 (monographic). 



Perognatus fasceatus, Lincecum, Am. Nat. vi, 1872, p. 369 (habits). 



Prognathic faceatus, LlNCF.CUM, Am. Sportsman, Feb. 28, 1874 (habits). 



Diagnosis. — Largest of the genus; considerably exceeding Hesperomys 

 leucopus, and approaching Tamias quadrivittatus, in size ; length four inches 



