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

Six subspecies of Thomomys bottae occur in Colorado. T. b. aureus and T. b. howelli occupy the Colorado Plateau Province (see fig. 1) and are characterized by a yellowish color; nasals posteriorly truncate or rounded; posterior extensions of premaxillae long; basioccipital wide; and interpterygoid space U-shaped with a median spicule.

T. b. internatus, T. b. cultellus, and a new subspecies from the vicinity of Cañon City described on page 376, inhabit the Sangre De Cristo and Wet mountains in the Southern Rocky Mountain Province and adjacent parts of the Colorado Piedmont and Raton Section of the Great Plains Province (see fig. 1). This group of closely related subspecies is characterized by reddish color; posterior margins of nasals forming a V; posterior extensions of premaxillae short; basioccipital narrow; and interpterygoid space V-shaped, lacking a median spicule.

T. b. pervagus occupies part of the San Luis Valley to the west of the Rio Grande (see fig. 1). In Colorado T. b. pervagus is iso lated from T. b. internatus and T. b. cultellus by the Sangre De Cristo and Culebra ranges and is separated from T. b. aureus by the San Juan Mountains. T. b. pervagus occupies an area geographically intermediate between T. b. aureus to the west and T. b. internatus and T. b. cultellus to the east and has some characters in common with these subspecies. T. b. pervagus resembles T. b. aureus in having long posterior extensions of the premaxillae and in sometimes having rounded posterior margins of the nasals. T. b. pervagus resembles T. b. internatus and T. b. cultellus in color, the presence of a V-shaped interpterygoid space, and a narrow basioccipital. Kelson (1951:69) has pointed out that in New Mexico the separation of the ranges of T. b. pervagus and T. b. cultellus is probably complete, but probably incomplete between T. b. pervagus and T. b. aureus. Nevertheless, the similarities between T. b. pervagus and T. b. cultellus and T. b. internatus suggest that T. b. pervagus was originally derived from the more eastern stock.

T. b. aureus is a variable subspecies which, according to Durrant (1952:211), intergrades with T. b. howelli in Utah. Specimens of T. b. aureus showing the greatest amount of geographic variation cranially are from the ecotone between the Piñon-juniper and Douglas Fir zones at the edge of the range of the subspecies.

T. b. howelli is a markedly distinct subspecies that shows certain similarities to T. b. aureus, but the degree of cranial difference from T. b. aureus suggests an isolation of long duration, or a rapid evolution from the parent stock.

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