Distinctive characters.—Dark (topotypes); size medium (see measurements); posterior tongues of premaxillae short; posterior margins of nasals forming a V (see fig. 5).
Comparisons.—From T. b. pervagus, topotypes of T. b. cultellus differ as follows: darker, not so reddish; smaller; skull smaller; zygomatic arches relatively longer; bullae proportionately smaller and less inflated; basioccipital proportionately wider; posterior tongues of premaxillae shorter.
Topotypes of T. b. cultellus most closely resemble those of T. b. internatus but differ as follows: darker; zygomatic arches more widely spreading, not so nearly parallel; nasals not so wide; bullae slightly more inflated.
For a comparison with T. b. rubidus see the account of that subspecies.
Remarks.—Kelson (1951:64) named T. b. cultellus on the basis of six dark specimens (Dark Reddish Brown 5YR 3/4 and 2/2). Nowhere else within the range of this subspecies, as defined by Kelson, do any specimens resemble the topotypes in color.
After comparing topotypes of T. b. cultellus with topotypes of T. b. internatus of approximately equal age, I disagree with Kelson (loc. cit.) on some of the characters which he used to separate cultellus from internatus. My findings indicate that T. b. cultellus is not smaller, that its skull is not smaller and not less angular, and that the tympanic bullae are not less pointed ventrally. Further collecting is needed better to limit and diagnose this subspecies.
Specimens examined.—Total 13. Colorado: Las Animas Co.: Fishers Peak, about 8000 ft., 1 (USNM). New Mexico: Union Co.: Near Folsom, 4 (CMNH); Colfax Co.: Philmont Ranch, Cimarroncito, 8100 ft., 2. Mora Co.: Halls Peak, 6 (USNM).
Figs. 2-7. Dorsal views of skulls of Thomomys bottae. × 1.
Fig. 2. Thomomys b. aureus, 3 mi. W Durango, La Plata Co., Colorado. No. 72967, ♀.