class="smcap">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

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.

P. f. callistus is distinct cranially from all other subspecies of P. fasciatus in the narrowness of the interparietal, the greater length of the tympanic bulla and the greater mastoidal breadth. The fine, silky nature of the pelage is shared, to some extent, with P. f. litus.

A specimen from 27 mi. N, 37 mi. E Rock Springs, Wyoming, referred to callistus resembles litus in pale dorsal coloration and slightly wider interparietal. Four immature specimens from 25 mi. N, 38 mi. E Rock Springs, also referred to callistus, have extremely pale juvenal pelage and also are judged to be intergrades with litus.