egion. Four other
species, _Erethizon dorsatum_, thought of primarily as a mammal of
coniferous forests, and _Ursus arctos_, _Taxidea taxus_, and
_Odocoileus hemionus_, all more or less western taxa, are not so broadly
distributed as are other members of this grouping. Of the five bats,
three are year-round residents, but _Lasiurus cinereus_ and evidently
_Lasionycteris noctivagans_ are migrants.

The remaining 25 kinds of mammals are representative of four regional
faunal groupings as follows: boreomontane species (10), steppe species
(nine), species with Sonoran affinities (four), and species of the
eastern deciduous forest (two).

_Boreomontane species._--Of the 10 mammals in this faunal group, three
(_Eutamias minimus_, _Tamiasciurus hudsonicus_, and _Microtus
pennsylvanicus_) are distributed both in the boreal forests to the north
of the plains and in montane areas to the west. Six species (_Myotis
evotis_, _Myotis volans_, _Plecotus townsendii_, _Thomomys talpoides_,
_Neotoma cinerea_, and _Ovis canadensis_) are primarily montane in
distribution and evidently reached northwestern South Dakota from the
west in late Wisconsin or post-glacial times; all but the pocket gopher
occur there now only in the vicinity of coniferous timber or rocky
buttes. The remaining species, _Zapus hudsonius_, is a glacial "relic."
The nearest populations now are far to the north, and this jumping mouse
occupies only restricted habitats in northwestern South Dakota and
adjacent regions. In Harding County, _Z. hudsonius_ presently is known
only from Deer Draw in the Slim Buttes.

_Steppe species._--Taxa intimately associated with the Great Plains are:
_Lepus townsendii_, _Cynomys ludovicianus_, _Spermophilus
tridecemlineatus_, _Perognathus fasciatus_, _Perognathus hispidus_,
_Reithrodontomys montanus_, _Microtus ochrogaster_, _Mustela nigripes_,
and _Spilogale putorius_ (subspecies _interrupta_). A few of these are
endemic to the plains, but most occur in grassland habitats beyond the
borders of the region. All clearly are well adapted to, and therefore
presumably evolved in response to, the environment of the interior
grasslands; this zoogeographic unit, then, is characterized by truly
steppe species that have relatively narrow habitat requirements and
largely concordant patterns of distribution.

The case of the spotted skunk deserves brief commentary. This species
was not taken or observed by members of our field parties and local