(4), six (5), six
(10), five (4). Three adult males taken in the same period had testes
that measured 7, 7, and 8, whereas those of two May-taken males measured
12 and 6.

Molt from winter to summer pelage was in progress, from anterior to
posterior, on both the dorsum and venter of many May- and June-taken
animals. Some individuals had completed molt, or had but a small patch
of winter pelage remaining on the rump, as early as the last week in
June.

~Reithrodontomys montanus albescens~ Cary, 1903

Plains Harvest Mouse

_Specimens examined_ (3).--2 mi. N, 5 mi. W Ludlow, 2; 1/2 mi. W Reva,
1.

This harvest mouse is uncommon in northwestern South Dakota, although
the species probably occurs sparingly in upland grassy habitats
throughout Harding County. Our specimens, along with one in the
collections of the University of Michigan Museum of Zoology, not
previously reported, from 11 mi. S Mandan, Morton Co., North Dakota,
represent the northernmost known records of this mouse.

A young adult female, obtained on June 21, carried three embryos that
measured 17 and was in summer pelage; an adult male taken on June 27
still was in a worn winter pelage.

At the locality 1/2 mi. W Reva, where traps were set in sparse to
relatively lush grassy areas along South Dakota Highway 20, the
following small mammals were taken in the same trapline (or adjacent
lines) in which one plains harvest mouse was captured: _Spermophilus
tridecemlineatus pallidus_, _Reithrodontomys megalotis dychei_,
_Peromyscus maniculatus nebrascensis_, _Microtus ochrogaster haydenii_,
and _Microtus pennsylvanicus insperatus_.

~Peromyscus leucopus aridulus~ Osgood, 1909

White-footed Mouse

Seven adults of this woodland inhabitant were trapped along
shrub-covered banks of the spring-fed stream and small impoundment in
Deer Draw of the Slim Buttes (10 mi. S and 5 mi. W Reva). Deciduous
trees grew in the bottom of the draw, but the slopes above supported
ponderosa pine and juniper. No white-footed mice were found along the
generally treeless tributaries of the Moreau and Grand rivers to the
east of Slim Buttes nor were these mice found along the Little Missouri
River or in likely-looking habitat in the North Cave Hills. The _P.
leucopus_ of Deer Draw likely represent, therefore, an isolated segment
of a formerly much more broadly distributed population of white-footed
mice on the Northern Great Plains in post-Wisconsin times. Other such
populat