_Remarks._--Geographic variation is evident in the material examined. In the initial study, one of us, Montague, separated the material from the Medicine Bow Range in Wyoming as a subspecies different from that at Laramie and the adjoining mountains to the eastward because of the darker color of the western animals and the smaller size of males. Acquisition of more material from still farther west (Sierra Madre) in Wyoming and the examination of material in the United States Biological Surveys Collection from Colorado discloses that there is a cline of increasing intensity of color from the geographic range of _T. t. cheyennensis_ at Pine Bluffs, Wyoming, westward to the eastern side of the Sierra Madre at a locality three miles east and five miles north of Savery, Wyoming. A further deterrent to setting apart the animals of the Medicine Bow Mountains as a separate subspecies is the large size of males from the North Platte River Valley southeast of Saratoga. The males from the valley of the North Platte are intermediate in size between those from the Medicine Bow Mountains and those from the Laramie River Valley. Females from the same places are available in longer series and show less variation. If there is a difference in size in the females, those from the mountains are larger than those from lower elevations on either side. The examination that one of us, Hall, has made of the related materials from Colorado reveals, as we supposed would be the case, that a large area formerly assigned to the geographic range of _Thomomys talpoides fossor_ is to be assigned to the geographic range of the newly named _Thomomys talpoides rostralis._ It should be added that, at this writing, the lack of ideally complete material from southwestern Colorado leaves some doubt as to the range of variation properly to be included within the geographic range of _T. t. fossor._ Consequently, study of a larger number of specimens from more localities in Colorado may show that the boundary between the geographic ranges of _T. t. fossor_ and _T. t. rostralis_ should be shifted from where we have tentatively placed it. _Specimens examined._--Total number, 168. Unless otherwise indicated, those from Colorado are in the United States National Museum, Biological Surveys Collection, and those from Wyoming are in the Museum of Natural History of the University of Kansas.