_T. b. internatus_ and _T. b. cultellus_ probably intergrade east of the Sangre De Cristo Range in the vicinity of the Colorado-New Mexico boundary. The amount of intergradation is obscured by the great amount of geographic variation occurring in _T. bottae_ at the edge of the plains and by the lack of specimens from this area. _T. b. internatus_ is a widespread subspecies showing its greatest variation at the edge of the plains. This area is an ecotone between the coniferous forest and the grassland and is by nature an area of change owing to the alternation of wet and dry periods such as the pluvial, interpluvial, and postpluvial periods. This seems to support Durrant's observation (1952:496) that "the greatest range of morphological variation is in animals from the least stable environments." Specimens from a small area north of the Arkansas River in the vicinity of Cañon City (see fig. 1) differ sufficiently from _T. b. internatus_ to be given nominal recognition. High mountains and the Arkansas River isolate the new subspecies found at Cañon City from populations of _T. b. internatus_ to the west and south; however there are no apparent geographic barriers between the newly named subspecies and populations of _T. b. internatus_ twelve miles to the north or from the vicinity of Pueblo to the east. This new subspecies is the most extreme of the variants occurring in the unstable environment at the edge of the plains. =Thomomys bottae aureus= Allen _Thomomys aureus_ Allen, Bull. Amer. Mus. Nat. Hist., 5:49, April, 1893; Warren, Colorado College Publ., 19:252, January, 1906; Warren, Colorado College Publ., 33:77, January, 1908; Warren, Mammals of Colorado, p. 79, 1910; Cary, N. Amer. Fauna, 33:136, August 17, 1911. _Thomomys bottae aureus_, Goldman, Proc. Biol. Soc. Washington, 48:156, October 31, 1935; Warren, Mammals of Colorado, p. 158, 1942. _Thomomys apache_ Bailey, Proc. Biol. Soc. Washington, 23:79, May 4, 1910. Holotype from Lake La Jara, 7500 feet, Rio Arriba County, New Mexico. _Thomomys perpallidus aureus_, Bailey, N. Amer. Fauna, 39:74, November 15, 1915. _Thomomys perpallidus apache_, Bailey, N. Amer. Fauna, 39:75, November 15, 1915. _Thomomys bottae apache_, Goldman, Proc. Biol. Soc. Washington, 48:157, October 31, 1935; Warren, Mammals of Colorado, p. 160, 1942. _Thomomys bottae optabilis_ Goldman, Jour. Washington Acad. Sci., 26:116, March 15, 1936. Holotype from Coventry, 6500 feet, Montrose County, Colorado; Warren, Mammals of Colorado, p. 159, 1942, part. _Holotype._--Adult female, skin and skull number 5243/4123, American Museum of Natural History, obtained at Bluff City, San Juan County, Utah, May 12, 1892, by Charles P. Rowley. _Distribution._--Colorado Plateau Province of southwestern Colorado (see fig. 1), northwestern New Mexico, southeastern Utah, and northeastern Arizona.