The first three columns represent the actual measurements of the various elements; the fourth column is the index established. Six different complexes (groups) of subspecies of _D. ordii_ have probably arisen as a result of geographical separation. The Great Plains complex consisting of _D. o. richardsoni_, _D. o. oklahomae_, _D. o. evexus_, _D. o. terrosus_, _D. o. luteolus_, _D. o. priscus_ and _D. o. medius_ are, with the exception of _D. o. priscus_, inhabitants of the high plains grassland habitat. _D. o. priscus_ inhabits the red Desert of Wyoming. The Gulf Coast complex, comprising _D. o. sennetti_ and _D. o. compactus_ are separable from all others by small auditory bullae and short tail. _D. o. compactus_ probably has differentiated from _D. o. sennetti_ since the cutting off, by wave action, from the mainland, of the islands on which _D. o. compactus_ lives. The Mexican complex consisting of _D. o. obscurus_, _D. o. fuscus_, _D. o. idoneus_ and _D. o. palmeri_ have probably differentiated by natural selection acting on fortuitous variations, but I lack first hand knowledge of the region concerned. The southwestern complex consists of _D. o. chapmani_, _D. o. extractus_, _D. o. attenuatus_ and _D. o. ordii_. _D. o. attenuatus_ and _D. o. chapmani_ are subspecifically distinct owing to geographic isolation, although both kinds show intergradation where their ranges approach that of _D. o. ordii_. The western desert complex, composed of _D. o. monoensis_, _utahensis_, _cineraceus_, _columbianus_, _cinderensis_, _fetosus_, _celeripes_, _marshalli_, _inaquosus_, _pallidus_, _panguitchensis_ and _fremonti_ was isolated from the other complexes of _D. ordii_ by the Quaternary uplift of the Wasatch Mountain mass, consisting of the Wasatch, Fish Lake and San Pitch mountains and the Wasatch, Aquarius, Paunsaugunt and Kaiparowits plateaus, and the concurrent reëstablishment of drainage systems. The drainages are those of the Colorado and Columbia rivers and that of the Snake River from Blackfoot, Idaho, to the junction with the Columbia. _D. o. fremonti_ has been isolated on the upper reaches of the Fremont River which arises from the eastern side of the Wasatch Divide. _D. o. panguitchensis_ has been isolated in Panguitch Valley as a result of the canyons formed by the Sevier River in Utah. _D. o. cineraceus_, although its subspecific and insular status are in doubt, appears to have been isolated on Dolphin Island, Great Salt Lake, Utah.