residents made no claim to its presence except for a few vague
recollections of spotted skunks having been seen "years ago." Visher
(1914:91), however, reported that _Spilogale_ was much commoner than
_Mephitis_ in the early part of the century. However that may have been,
_Spilogale putorius_, as currently understood, would be judged to be a
widespread species except that recent evidence strongly suggests that
the plains race (_interrupta_) is not of the same species as spotted
skunks to the west (subspecies _gracilis_). Furthermore, the ranges of
the two are not in contact. Whatever its ultimate specific affinities
may be, _S. p. interrupta_ clearly is a plains mammal, and thus is here
considered in that zoogeographic unit.

_Sonoran species._--_Sylvilagus audubonii_, _Dipodomys ordii_,
_Reithrodontomys megalotis_, and _Onychomys leucogaster_ are invaders to
the Northern Great Plains from the Sonoran region to the southwest. The
latter two, however, are rather broadly distributed on the Great Plains
and their assignment as Sonoran species is somewhat arbitrary. It is of
interest that as many as nine mammals with southwestern affinities occur
as far north as southwestern South Dakota and adjacent Wyoming.

_Eastern species._--Only _Sylvilagus floridanus_ and _Peromyscus
leucopus_ can be identified as species primarily associated with the
eastern deciduous forest. The former is limited in northwestern South
Dakota to brushy habitats in riparian communities, whereas _P. leucopus_
is restricted to relatively good stands of deciduous timber and
presently is known to occur only in Deer Draw of the Slim Buttes.

Throughout its known range in the western part of the Northern Great
Plains, _P. leucopus_ is represented by small and disjunct populations
associated with riparian deciduous timber. The known population nearest
to Harding County is on the Black Hills to the south; next nearest are
several isolated or semi-isolated populations along the Yellowstone
River in eastern Montana. Presumably, this white-footed mouse spread
northwestward into the western part of the plains region along river
systems, in company with deciduous trees, in some post-glacial period
when the climate was warmer and wetter than now. Subsequent drying
altered substantially the distribution and perhaps composition of
riparian forests, and isolated populations of _P. leucopus_ evidently
survived only in restricted areas, such as Deer Draw, many of th