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.

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.

Specimens from Bedrock have zygomatic arches that are heavy anteriorly. Specimens from Coventry are dorsally almost uniformly Strong Brown (7.5YR 5/6) and lack a strong dorsal stripe. The venters are Reddish Yellow (7.5YR 8/6). Specimens from 15 miles west of Cortez are the palest specimens of T. b. aureus from Colorado, and closely resemble topotypes. The basic color varies from Reddish Yellow (7.5YR 7/6 and 6/6) to Strong Brown (7.5YR 5/6). Specimens are marked with a narrow dark dorsal stripe. The venters are white. Specimens from Ute Peak and Cortez have Reddish Yellow (7.5YR 6/6) flanks and are slightly darker dorsally. Many specimens from Mesa Verde are indistinguishable from specimens from Coventry and from Cortez. Others have dark diffuse dorsal stripes. The venters are Pink (7.5YR 7/4) or Pinkish White (7.5YR 8/2). Some specimens from the Mancos River have wide dorsal stripes. Specimens from three miles west of Durango have especially wide-spreading zygomatic arches posteriorly and have wide black dorsal stripes. The venters are Pink (7.5YR 7/4). One specimen from Florida is dark and grizzled and has a dark dorsal stripe. Another specimen is pale and has only a small dorsal stripe. Specimens from 12 miles west of Pagosa Springs have thin rostra and diffuse dorsal stripes. Specimens from Bondad have a V-shaped interpterygoid space and in it a small median spicule. One specimen is uniformly grizzled and lacks a dorsal stripe, giving an overall effect of Dark Yellowish Brown (10YR 3/3). Another specimen has Strong Brown (7.5YR 5/6) flanks and is only slightly darker dorsally.

Thomomys bottae howelli Goldman

Remarks.—T. b. howelli most closely resembles T. b. aureus; however, since only one adult specimen of T. b. howelli is known, it is impossible to appraise adequately its characters. Durrant (1952:211) records intergradation between T. b. howelli and T. b. osgoodi, and between T. b. howelli and T. b. aureus in Utah.

An attempt to collect specimens of T. b. howelli, in March, 1957, by Richard S. Miller and the writer was unsuccessful.