_Specimens examined._--Total number, 9, as follows: WYOMING: _Carbon County_: 8 mi. SE Lost Soldier, 6700 ft., 1 (USBS). _Fremont County_: Granite Mts., 1 (MZ). _Natrona County_: 5 mi. W Independence Rock, 6000 ft., 4 (KU); Sun, 1 (USBS); 16 mi. S, 11 mi. W Waltman, 6950 ft., 1 (KU). _Sweetwater County_: 27 mi. N Table Rock, 1 (MZ). =Perognathus fasciatus callistus= Osgood _Perognathus callistus_ Osgood, N. Amer. Fauna, 18:28, September 20, 1900, type from Kinney Ranch, near Bitter Creek, Sweetwater County, Wyoming. _Geographic distribution._--East of the Green River in central and southern parts of Sweetwater County, Wyoming, and adjacent parts of Moffat County, Colorado (see Figure 1). _Remarks._--For comparisons with _Perognathus fasciatus litus_, geographically adjacent to the north, see account of that subspecies. When Osgood (_loc. cit._) described _P. f. callistus_ as a full species, he characterized it as having " ... the attractive coloration of _fasciatus_, but softer and more delicate. Its position is evidently between _fasciatus_ and _apache_, and its nearest relations are clearly with the latter." He further remarked that, "Its large size immediately separates it from _fasciatus_ which it resembles externally, especially before maturity." Comparison of the average and extreme external measurements of ten adult _P. f. callistus_ from Sweetwater County, Wyoming, with adult individuals of other subspecies of _P. fasciatus_ shows that _callistus_ does not average larger than _P. f. olivaceogriseus_ except in length of hind foot and that it averages slightly smaller in all external measurements than topotypes of _P. f. fasciatus_. Moreover, judging from the accounts of Goldman (1918:24) and Durrant (1952:235), _Perognathus apache caryi_ (the subspecies of _P. apache_ nearest to the range of _P. f. callistus_) is significantly larger externally and has no trace of olivaceous in the dorsal pelage. Comparison of the skulls of _callistus_ with a skull of _Perognathus apache apache_ from Wingate, New Mexico (USBS 137388), reveals the following differences: Interparietal bone wider in _callistus_, averaging 4.5 (as opposed to 4.0) and more pentagonal; lacrimal bone shorter and stouter in _callistus_; tympanic bullae more inflated in _callistus_; interorbital foramina larger in _callistus_; lower premolar approximately the same size as the last lower molar in _callistus_, approximately half the size of the last lower molar in _apache_. Conversely, comparison of skulls of _callistus_ with those of _P. fasciatus_ as concerns the above mentioned features reveals that they closely resemble each other. In view of this resemblance it seems best to arrange _callistus_ as a subspecies of _P. fasciatus_.