1 29 specimens were used in taking measurements of the skull.


Remarks.—Bailey (1900:20) had only 7 specimens from northeastern Wyoming and western South Dakota, of M. pennsylvanicus and thought that Arvicola insperatus Allen (1894:347) was not subspecifically distinct from modestus. Subsequently Bailey (1920:72) had adequate numbers of specimens and described Microtus pennsylvanicus wahema from eastern Montana and western North Dakota. Anderson (1943:92) concluded that wahema was not distinct from insperatus and therefore the name M. p. insperatus (Allen) is applicable to this subspecies. On the basis of specimens that I have examined from Montana, South Dakota, and Wyoming I concur with Anderson. Bailey's characterization of wahema is applicable to insperatus as I interpret it. In the Black Hills, Microtus longicaudus longicaudus (Merriam) occurs together with insperatus.

Specimens examined.—Total 123. Wyoming: Sheridan Co.: 3 mi. WNW Monarch (=Kleeburn), 3800 ft., 4; 4 mi. NNE Banner, 4100 ft., 26; 5 mi. NE Clearmont, 3900 ft., 3. Johnson Co.: 5½ mi. W, 1 mi. S Buffalo, 5520 ft., 1; 5½ mi. W, 1½ mi. S Buffalo, 1; 1 mi. W, 4/5 mi. S Buffalo, 4800 ft., 36; ¼ mi. E Klondike, 5160 ft., 1. Campbell Co.: Belle Fourche River, 45 mi. S, 13 mi. W Gillette, 5350 ft., 2. Crook Co.: 3 mi. S, 2 mi. E Rocky Point, 3800 ft., 6; Bear Lodge Mts., 6½ mi. SSE Alva, 1 (Mich); 15 mi. N Sundance, 5500 ft., 3; 15 mi. ENE Sundance, 3825 ft., 6; 3 mi. NW Sundance, 5900 ft., 1; 11/3 mi. NW Sundance, 5000 ft., 4; Sundance, 1 (USBS). Weston Co.: 1½ mi. E Buckhorn, 6150 ft., 26; Newcastle, 1 (USBS).

GENERAL REMARKS

The region considered in this paper differs in several regards from the state of Pennsylvania, where variation in the skulls of this species has been studied in detail by Snyder (1954) who referred all populations there to a single subspecies. In some characteristics of the skulls, populations within Pennsylvania differed as much or more than the subspecies from Wyoming and Colorado. In other characteristics of the skulls and of the skins differences are greater between populations in Wyoming and Colorado. The region discussed here is approximately five times as large as the state of Pennsylvania. Populations of M. pennsylvanicus are less continuously distributed than in Pennsylvania owing to major physiographic and climatic barriers and also owing to competition with one or more of the five other species of Microtus occurring in this region. The distribution of three of these species has been discussed by Findley (1945:419). Large areas of relatively greater aridity, such as the region occupied by the subspecies insperatus, occur in Wyoming and Colorado. I have pointed out that the populations which I have designated as subspecies are not absolutely uniform. Also the different subspecies are not of exactly equal degrees of difference. However, there is considerable uniformity of populations occupying conveniently mapped geographic areas. In my opinion, the use of subspecific nomenclature is justified in this case, although not completely unambiguous.

LITERATURE CITED

Allen, J. A.

1894. Descriptions of five new North American mammals. Bull. Amer. Mus. Nat. Hist., 6:347-350, December 7.

Anderson, R. M.

1943. A prior name for the bean mouse revived. Canadian Field-Nat., 57:92, October 17.

Anderson, S.

1954. Subspeciation in the montane meadow mouse, Microtus montanus, in Wyoming and Colorado. Univ. Kansas Publ., Mus. Nat. Hist., 7(7):489-506, 2 figs. in text, July 23.

Bailey, V.

1900. Revision of American voles of the genus Microtus. N. Amer. Fauna, 17:1-88, 5 pls., 17 figs., June 6.
1920. Identity of the bean mouse of Lewis and Clark. Jour. Mamm. 1:70-72, March 1.
1927. A biological survey of North Dakota. N. Amer. Fauna, 49:vi+ 226, 21 pls., 8 figs. in text, January 8.
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Baird, S. F.

1858. Explorations and surveys for a railroad route from the Mississippi River to the Pacific Ocean. War Department. Mammals, Part I, xxxii + 757, pls. 17-60, 35 figs. in text, July 14.

Dalquest, W. W.

1948. Mammals of Washington. Univ. Kansas Publ., Mus. Nat. Hist., 2:1-444, 140 figs. in text, April 9.

Davis, W. B.

1939. The Recent mammals of Idaho. The Caxton Printers, Caldwell, Idaho, 400 pp., 2 full page half tones, 33 figs. in text, April 5.

Durrant, S. D.

1952. Mammals of Utah, taxonomy and distribution. Univ. Kansas Publ., Mus. Nat. Hist., 6:1-549, 91 figs. in text, 30 tables, August 10.

Ellerman, J. R.

1941. The families and genera of living rodents. Volume II. Family Muridae. The British Museum. xii + 690 pp., 50 figs. in text, March 21.

Findley, J. S.

1954. Competition as a possible limiting factor in the distribution of Microtus. Ecology, 35:418-420, July.

Hall, E. R., and E. L. Cockrum

1952. Comments on the taxonomy and geographic distribution of North American microtines. Univ. Kansas Publ., Mus. Nat. Hist., 5:293-312, November 17.
1953. A synopsis of the North American microtine rodents. Univ. Kansas Publ., Mus. Nat. Hist., 5:373-498, 149 figs. in text, January 15.

Hibbard, C. W.

1940. A new Pleistocene fauna from Meade County, Kansas. Trans. Kansas Acad. Sci., 43:417-425, December 23.

Howell, A. B.

1926. Anatomy of the wood rat. Monogr. Amer. Soc. Mamm. No. 1, x + 225 pp., 35 figs., Williams and Wilkins Co., Baltimore.

Snyder, D. P.

1954. Skull variation in the meadow vole (Microtus p. pennsylvanicus) in Pennsylvania. Annals of the Carnegie Museum, 33:201-234, September 21.

Warren, E. R.

1942. The mammals of Colorado. Univ. Oklahoma Press, xviii + 330 pp., 50 pls.

Transmitted June 30, 1955.