Comparisons.—Near topotypes of fossor can be distinguished from topotypes of Thomomys talpoides ocius as follows: Size larger throughout. Color: Darker throughout, being dark brown as opposed to grayish. Skull: Longer and narrower; nasals and rostrum longer; extension of supraoccipital posterior to lambdoidal suture markedly less; tympanic bullae markedly smaller; upper incisors longer and not as strongly recurved.
Among the races of Thomomys talpoides occurring in Utah, fossor most closely resembles Thomomys talpoides uinta in color and size, but differs from it as follows: Ears smaller, more pointed and with more darkly pigmented pinnae. Skull: Longer, narrower and weaker; rostrum longer; nasals longer, and rounded proximally as opposed to markedly emarginate; interparietal triangular instead of roughly pentagonal; tympanic bullae larger and more inflated ventrally; basioccipital narrower; palate narrower, palatal pits shallower; dentition lighter.
Remarks.—Bailey (1915:111) remarked that fossor was one form that held its distinctive characters over a wide range. At that time, its range was understood to include practically all of the mountainous parts of Colorado, Utah as far west as the central part of the state, and parts of New Mexico, Arizona and Wyoming. Subsequently three new forms have been named from central Utah, (Goldman 1938:334-337) thereby showing variation to be much more prevalent than formerly supposed. The range of fossor in Utah, as now understood, is limited to the mountainous parts of the state south and east of the Colorado and Green rivers in Grand and San Juan counties.
The Utah specimens are not typical. At first glance some differences are noted in the premaxillae and nasals. Four specimens in the collections of the Museum of Natural History, University of Kansas, three from 3 miles east of Creede, Mineral County, and one from 10 miles east of Lake City, Hinsdale County, Colorado, both of which lie north and east of the type locality of fossor show the same characters as the Utah specimens.
Eight specimens from Oak Spring are intergrades between fossor and ocius. In size and color they are like fossor, but the skulls are intermediate. Because the animals are more like fossor in the majority of characters, they are here referred to that race.
As a result of these studies and due to the paucity of specimens from Utah, it is advisable, for the present, to refer all these Utah animals to fossor. Additional specimens may reveal characters that will merit the separation of the Utah animals from typical fossor; a desertlike area unfavorable to Thomomys exists between the type locality and eastern Utah.
Specimens examined.—Total, 21, distributed as follows: Grand County: Oak Spring, Middle Fork Willow Creek, 15 mi. N Thompson, 8 (C. M.); La Sal Mountains, 1 (U. S. N. M.); Warner Ranger Station, La Sal Mountains, 3 (B. Y. U.). San Juan County: Geyser Pass, 18 mi. SE Moab, La Sal Mountains, 3 (1, B. Y. U.; 2, C. M.); 5 mi. W Monticello, 1 (C. M.); Cooley Pass, 8 mi. W Monticello, 2 (C. M.); Joshua Flat, Elk Ridge, 8,300 ft., 3 (M. V. Z.).
Thomomys talpoides parowanensis Goldman
Thomomys fossor parowanensis Goldman, Journ. Washington Acad. Sci., 28:334, July 15, 1938.
Thomomys talpoides parowanensis Goldman, Journ. Mamm., 20:234, May 14, 1939; Long, Journ. Mamm., 21:176, May 14, 1940.