Swenk (_loc. cit._) originally described _P. f. olivaceogriseus_ as a subspecies of _Perognathus flavescens_. I have examined the holotype, kindly made available to me by Dr. Seth B. Benson of the Museum of Vertebrate Zoology, University of California, and the paratype, in the collection of the University of Nebraska State Museum. Neither is _P. flavescens_, but both are clearly specimens of _P. Fasciatus_ on the basis of olive-gray dorsal coloration, larger over-all dimensions of the skull, and in that the tympanic bullae do not meet anteriorly. In addition, a third specimen from Glen, Nebraska, purported by Swenk to be intermediate between _flavescens_ and _olivaceogriseus_, has been examined and found to be an immature _fasciatus_. [Illustration: FIGS. 2-7. Skulls of two species of pocket mice. All x 1-1/2. In comparison with _P. flavescens_ note that _P. fasciatus_ has the tympanic bullae separated anteriorly and that the skull is slightly larger in over-all dimensions. FIGS. 2-4. _Perognathus fasciatus fasciatus_. Buford, Williams County, North Dakota, May 6, 1910, [** Male], No. 168599 USBS. FIGS. 5-7. _Perognathus flavescens flavescens_, Kennedy, Cherry County, Nebraska, July 9, 1935, [** Female], No. 66883 MVZ.] In the original description Swenk indicated the type locality as Chadron, Dawes County, Nebraska. The locality given on the specimen label of the holotype, however, is 5 mi. E Chadron. In addition, Swenk (_loc. cit._), in the paragraph preceding the description of _olivaceogriseus_, states that the holotype was actually taken on Little Bordeaux Creek, sec. 14, T. 33 N, R. 48 W, 3 mi. E Chadron, on the farm of L. M. Gates, who obtained the holotype. It seems best to restrict the type locality to the place last mentioned. Four specimens from southeastern Alberta recently reported by Moore (1953:143) have been examined and seem best referred, among known subspecies of _fasciatus_, to _P. f. olivaceogriseus_. They differ from the latter in several minor cranial features and more drab back, sides and lateral line. When adequate material is available they may prove to be subspecifically different from _olivaceogriseus_. Moore referred these specimens to _P. f. fasciatus_. Specimens from extreme northwestern Nebraska, including the holotype, are slightly darker than specimens from eastern Wyoming. A specimen from Ft. Steele, Wyoming, shows approach to _P. f. litus_ in pale dorsal coloration.