_Zygomatic breadth._--The greatest distance across the zygomatic arches, at right angles to the long axis of the skull. _Squamosal breadth._--The greatest distance between the mastoidal processes of the squamosal. _Length of rostrum._--The shortest distance from the shallow notch that lies lateral to the hamulus of the lacrymal bone, to the tip of the nasal on the same side of the skull. _Breadth of rostrum._--The greatest width of the rostrum, anterior to the maxillae, transverse to the long axis of the skull. _Alveolar length of upper maxillary tooth-row._--Distance between the anterior margin of the alveolus of the first cheek-tooth and the posterior margin of the alveolus of the last upper cheek-tooth, on one side of the skull. _Least interorbital breadth._--The least distance across the frontal bones at the interorbital constriction as seen in dorsal view. PHYSIOGRAPHY _Thomomys bottae_ occurs in the Colorado Plateau Province (terminology of Fenneman, 1931), the Southern Rocky Mountain Province and a small part of the Great Plains Province. The Colorado Plateau Province, in the southwestern part of the state, is mostly above 5000 feet and is characterized by the great number of canyons cut by rivers and streams in the nearly horizontal strata. Prominent features of the landscape are cuestas, such as Mesa Verde, and laccoliths, such as Ute Peak. The Southern Rocky Mountain Province consists mainly of high granitic mountains running north and south, many of which extend to more than 14,000 feet above sea level. Included in this region are several large basins, such as North Park and South Park and the San Luis Valley. The San Juan Mountains, which separate the Colorado Plateau Province from the San Luis Valley, and the Sangre De Cristo and Wet mountains, which intervene between the San Luis Valley and the Great Plains, importantly influence the distribution of _Thomomys bottae_. The Great Plains Province is a broad highland that slopes gradually eastward from the Rocky Mountains. Of importance to the present study are two subdivisions of the Great Plains, the Colorado Piedmont and the Raton Section. The Colorado Piedmont is a much dissected fluviatile plain, roughly extending from the vicinity of the Arkansas River to the northern boundary of the state. In general the topography of the Colorado Piedmont is broadly rolling with greater relief than the high plains to the east; however, buttes and steep bluffs occur locally. The Raton Section imperceptibly blends into the southern boundary of the Colorado Piedmont and extends south into New Mexico and Texas. A trenched peniplane of greater relief and altitude than the Colorado Piedmont, it is characterized by high mesas, extensive dissected lava-capped plateaus, deep canyons, and mountains of volcanic origin. GEOGRAPHIC VARIATION