Remarks.--Merriam (N. Amer. Fauna, 5:119, July 30, 1891) named this subspecies on the basis of two specimens collected in the Black Hills of South Dakota in July, 1888, and assigned it to the species Evotomys [= Clethrionomys] gapperi. He reported the diagnostic characteristics as: "Similar to E. gapperi, but with larger ears and shorter tail. The hazel of the dorsal area is not so bright as in gapperi; the sides are the same golden brown." Of the cranial and dental characteristics he wrote: "Much as in E. gapperi."
Bailey (Proc. Biol. Soc. Washington, 11:129, May 13, 1897), in his "Revision of the American voles of the genus Evotomys," with one additional specimen available, raised the Black Hills population to specific status, re-emphasizing the shortness of the tail, and pointing out a few slight cranial differences ("zygomatic arches low and flaring out, so that the inner instead of the outer side shows in top view; auditory bullae as large as in gapperi, but less rounded").
Bailey (loc. cit.) remarked that: "though based on so scanty material, the characters distinguishing the species are fairly pronounced. Its range is isolated and widely separated from that of any other members of the genus by open prairie country and a wide belt of the Transition zone. There seems to be no valid reason for considering it a subspecies."
Additional specimens have been taken in recent years from the Black Hills of South Dakota and northeastern Wyoming. This material has shed light on the relationships and morphological characteristics of the red-backed mice of this region. Bole and Moulthrop (Sci. Publ. Cleveland Mus. Nat. Hist., 5:153, September 11, 1942) listed, as comparative material, eight specimens from Bull Springs, Custer County, South Dakota, under the name Clethrionomys gapperi brevicaudus (Merriam). They gave no reason for arranging brevicaudus as a subspecies of C. gapperi.
Twenty adults (11 skins and skulls, 9 skulls only) from Pennington County, South Dakota (specimens in the University of Michigan Museum of Zoology), have the following measurements (averages of external measurements based on 11 specimens only): Total length, 142 (123-155); tail, 35 (30-39); hind foot, 19.5 (18.6-21.0); basal length, 23.3 (21.7-24.5); condylobasilar length, 23.3 (21.9-24.5); zygomatic breadth, 13.7 (12.9-14.7); lambdoidal breadth, 11.7 (11.3-12.9); alveolar length upper cheek-teeth, 5.5 (5.2-5.8); interorbital breadth, 3.9 (3.6-4.1); length of nasals, 7.7 (7.1-8.5); breadth of rostrum, 3.2 (2.9-3.6); and length of incisive foramina, 5.0 (4.6-5.3).
Measurements of the type and one "more fully adult topotype" (as given by Bailey, op. cit.) are: Total length, 125, 130; tail length, 31, 32; hind foot, 19, 19; basal length, 21.2, 21.8; length of nasals, 6.6, 7.0; zygomatic breadth, 12.5, 12.8; mastoid breadth, 11.3, 11.0; alveolar length of upper molar series, 5.4, 5.3. In every measurement the figures for Bailey's specimens are smaller than the average of the same measurement in the 20 adults from Pennington County, and, in most measurements, are even lower than the minimum of the latter series. Therefore, we conclude that the material available to Merriam (op. cit.) and Bailey (op. cit.) consisted of only subadults.
In comparison with a series of 23 adult Clethrionomys gapperi galei from 28 mi. E Lovell, Big Horn County, Wyoming, C. g. brevicaudus has a slightly shorter tail, longer hind foot, greater basal and condylobasilar lengths, greater zygomatic and lambdoidal breadths and conspicuously longer nasals.
In comparison with three adult C. g. loringi from Elk River, Sherburne County, Minnesota, C. g. brevicaudus has a greater total length, longer hind foot, greater basal length, conspicuously greater zygomatic and lambdoidal breadths, much longer nasals, and a narrower rostrum.
Clethrionomys gapperi brevicaudus, although isolated geographically and although morphologically more distinct than many of the currently recognized subspecies of C. gapperi, is probably best arranged as a subspecies of C. gapperi rather than as a full species. In certain characters, such as interorbital breadth and breadth of rostrum, it is intermediate between C. g. galei and C. g. loringi, but it resembles C. g. galei more than it does any other named kind.
Specimens examined.--Total, 66. Unless otherwise indicated, specimens from Wyoming are in the University of Kansas Museum of Natural History and specimens from South Dakota are in the University of Michigan Museum of Zoology. Specimens are distributed as follows: