f tympanic bulla and length of maxillary
tooth-row which are approximately the same. From topotypes of
_Perognathus fasciatus infraluteus_, geographically adjacent to the
south, _P. f. olivaceogriseus_ differs in: Upper parts paler and less
drab; lateral line brighter and more ochraceous; underparts never buffy;
skull averaging larger in all measurements taken except interparietal
breadth and length of maxillary tooth-row which are smaller. From
topotypes of _Perognathus fasciatus litus_, geographically adjacent to
the southwest, _P. f. olivaceogriseus_ differs in: Upper parts darker;
skull, when specimens of equal age are compared, averaging larger in
mastoidal breadth, interparietal breadth and basal length. From
topotypes of _Perognathus fasciatus callistus_, _P. f. olivaceogriseus_
differs in: Upper parts slightly darker, pelage not silky, coarser in
appearance; hind foot shorter; skull averaging smaller in all
measurements taken (especially mastoidal breadth), except interparietal
breadth which is more.

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 a