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A New Subspecies of Microtus montanus from Montana

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

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

BY

E. RAYMOND HALL AND KEITH R. KELSON

In 1949, for the Museum of Natural History of the University of Kansas, Mr. John A. White collected two specimens of the species _Microtus montanus_ in the Bitterroot Valley of Montana, that did not fit the description of any named subspecies. These were laid aside until we could examine the additional specimens from Montana in the Biological Surveys collection in the United States National Museum, some of which previously had been reported by Bailey (N. Amer. Fauna, 17:31, June 6, 1900) under the name _Microtus nanus canescens_ Bailey [=_Microtus montanus canescens_]. Our examination reveals that the animals from the Bitterroot and Flathead valleys belong to an heretofore unrecognized subspecies which is named and described below.

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_.


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