_Distinctive characters._--Size large (see measurements); usually pale in western part of range, dark in eastern part; posterior extensions of premaxillae long, wide, and deeply serrated; posterior margins of nasals truncate or slightly rounded (see fig. 2); interpterygoid space U-shaped, with median spicule; basioccipital wide; bullae well inflated, rounded ventrally. _Comparisons._--For comparisons with _T. b. howelli_ and _T. b. pervagus_, see accounts of those subspecies. _Remarks._--_T. b. aureus_ is a variable subspecies, which differs considerably from _T. b. internatus_, _T. b. cultellus_, and _T. b. rubidus_ and includes several microgeographic races distinguishable to a taxonomist specializing in the group. These slightly varying populations are here not considered sufficiently distinct for nominal recognition. Characters such as color of the pelage and conformation of the bullae and zygomatic arches vary with the locality, and to some extent vary among specimens from a single locality. The name _Thomomys bottae optabilis_, given to specimens from Coventry by Goldman (1936:116), is here placed in synonymy under _T. b. aureus_ Allen. The characters originally used to describe _T. b. optabilis_ are of the type that vary between populations only a few miles apart, or often vary within a population. The skulls of specimens from Coventry are not lighter in structure than those of _T. b. aureus_. The premaxillae are not narrower, nor is the frontal region narrower or more constricted than in _T. b. aureus_. The name _Thomomys bottae apache_, given to specimens from Lake La Jara, New Mexico, by Bailey (1910:79), and later applied to specimens from Colorado by Bailey (1915:75), is here also placed in synonymy under _T. b. aureus_. Specimens from Lake La Jara, New Mexico, and nearby localities in Colorado may be separated from topotypes of _T. b. aureus_ on the basis of color only. The topotypes of _T. b. aureus_ are mostly pale; some, however, are dark. The number of pale specimens in any given series decreases gradually in a clinal pattern from west to east. Since there is no noticeable step in the cline and since all specimens show close cranial similarity, it is felt that nominal recognition of the darker specimens does not present a realistic picture of the relationships of the relatively unisolated populations in the Colorado Plateau Province. Since _Thomomys bottae_ in the Colorado Plateau Province is especially plastic, varying from locality to locality, emphasis is here placed on similarities that unite specimens from different localities. The individual and microgeographic variations are outlined below.