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_.

_Specimens examine