A New Subspecies of Microtus montanus from Montana and Comments on Microtus canicaudus Miller

BY E. RAYMOND HALL and KEITH R. KELSON University of Kansas Publications Museum of Natural History Volume 5, No. 7, pp. 73–79 October 1, 1951 University of Kansas LAWRENCE 1951
University of Kansas Publications, Museum of Natural History Editors: E. Raymond Hall, Chairman, A. Byron Leonard, Edward H. Taylor, Robert W. Wilson Volume 5, No. 7, pp. 73–79 October 1, 1951 University of Kansas Lawrence, Kansas PRINTED BY FERD VOILAND, JR., STATE PRINTER TOPEKA, KANSAS 1951 23–7438

BY E. RAYMOND HALL AND KEITH R. KELSON
Microtus montanus pratincolus new subspecies
Type. —Female, adult, skull and skin, No. 34004, Univ. Kansas, Mus. Nat. Hist.; from 6 mi. E Hamilton, 3700 ft., Ravalli County, Montana; obtained on August 14, 1949, by John A. White; original number 477.
Geographic distribution. —Flathead and Bitterroot valleys of western Montana.
Diagnosis. —Size small for the species (see measurements). Color: Essentially as in Microtus montanus nanus . Skull: Small, slender, and comparatively smooth; rostrum moderately depressed distally; nasals moderately inflated distally and extending posteriorly not quite to tips of premaxillary tongues; nasals usually truncate posteriorly, but rounded in some individuals; premaxillary tongues terminating posteriorly in a short medial spine; zygomatic arches lightly constructed and usually more widely spreading posteriorly than anteriorly; interparietal comparatively long and terminating in a small, but distinct, medial spine, otherwise approximately rectangular in shape; exposed parts of upper incisors short and, for the species, only slightly procumbent; molar dentition weak and, in most specimens, especially so posteriorly; tympanic bullae large and well inflated, especially ventrolaterally; basioccipital narrow owing to the encroachment of the tympanic bullae.
Comparison. —Among named forms, Microtus montanus pratincolus most closely resembles Microtus montanus nanus . The geographic range of M. m. nanus adjoins that of M. m. pratincolus on three sides; there is no conspecific subspecies adjoining the range of M. m. pratincolus on the north. From M. m. nanus , M. m. pratincolus differs as follows (measurements are all of males, those of M. m. nanus being of nine topotypes and near topotypes from central Idaho): size smaller (149 mm. as opposed to 165), tail shorter (37 as opposed to 39), hind foot shorter (19 as opposed to 20), upper molar series shorter (expressed as a percentage of basilar length, 25.5 as opposed to 26.3), mastoidal region broader (expressed as a percentage of basilar length, 48.6 as opposed to 46.7), braincase slightly more vaulted (depth of braincase expressed as a percentage of basilar length, 31.3 as opposed to 30.4) and more inflated laterally; tympanic bullae more inflated, this inflation being the most conspicuous difference between the two subspecies. The tympanic bullae of M. m. pratincolus have approximately a fourth more volume than those of M. m. nanus .