Specimens of T. b. rubidus differ from topotypes of T. b. pervagus in darker color; rostrum wider posteriorly; posterior extensions of premaxillae shorter; bullae smaller, proportionately more inflated posteriorly; zygomatic arches more rounded; wider across squamosals; alveolar length of upper maxillary tooth-row greater.
From topotypes of T. b. cultellus, T. b. rubidus differs as follows: paler; larger in all measurements taken; rostrum proportionately wider; zygomatic arches more rounded, less angular; angle formed by zygomatic arch and rostrum greater; bullae proportionately smaller, not so pointed anteriorly; alveolar length of upper maxillary tooth-row shorter.
Remarks.—The range of T. b. rubidus is surrounded by the range of T. b. internatus; nevertheless, intergradation has not been found. For a discussion of the geographic relation of T. b. rubidus to T. b. internatus see page 374.
Specimens examined.—Total 7. Fremont Co.: Garden Park, Cañon City, 5344 ft., 1; 2-9/10 mi. E Cañon City, 5344 ft., 6.
SUMMARY
A study of 249 specimens of Thomomys bottae from Colorado reveals six subspecies in the state. T. b. aureus and T. b. howelli occupy the Colorado Plateau Region in the western and southwestern parts of the state. T. b. internatus, T. b. cultellus, T. b. pervagus, and the newly named T. b. rubidus occupy part of the Southern Rocky Mountain Region and a narrow strip of the Great Plains.
The greatest amount of geographic variation, in Thomomys bottae in Colorado, occurs in the ecotone between the grassland and coniferous forest at the edge of the Great Plains, and in the ecotone between the Piñon, juniper, and sage of the Colorado Plateau and the Coniferous forest of the southern Rocky mountains.
Table 1. Measurements, in Millimeters, of Thomomys bottae