_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 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.