_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 them probably marginal habitat for the species. _Unverified species._--If the 10 species listed as of "unverified occurrence," it seems highly likely that as many as eight will be found to occur, or occurred within historic time, in Harding County. Among these eight are one steppe species (_Vulpes velox_), three with boreomontane affinities (_Sorex cinereus_, _Gulo gulo_, and _Lynx canadensis_), two (_Sciurus niger_ and _Urocyon cinereoargenteus_) that are associated with the eastern deciduous forests, and two (_Sorex merriami_ and _Lagurus curtatus_) that are Great Basin elements. It is noteworthy that the last-mentioned faunal unit is not known to be represented in northwestern South Dakota. ACKNOWLEDGMENTS