129 specimens were used in taking measurements of the skull.
Remarks.—Bailey (1900:20) had only 7 specimens from northeastern Wyoming and western South Dakota, ofM. pennsylvanicusand thought thatArvicola insperatusAllen (1894:347) was not subspecifically distinct frommodestus. Subsequently Bailey (1920:72) had adequate numbers of specimens and describedMicrotus pennsylvanicus wahemafrom eastern Montana and western North Dakota. Anderson (1943:92) concluded thatwahemawas not distinct frominsperatusand therefore the nameM. 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 ofwahemais applicable toinsperatusas I interpret it. In the Black Hills,Microtus longicaudus longicaudus(Merriam) occurs together withinsperatus.
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,4/5mi. 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;11/3mi. NW Sundance, 5000 ft., 4; Sundance, 1 (USBS).Weston Co.: 1½ mi. E Buckhorn, 6150 ft., 26; Newcastle, 1 (USBS).
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,4/5mi. 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;11/3mi. 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 ofM. pennsylvanicusare 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 ofMicrotusoccurring 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 subspeciesinsperatus, 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.
Anderson, R. M.
Anderson, S.
Bailey, V.
Baird, S. F.
Dalquest, W. W.
Davis, W. B.
Durrant, S. D.
Ellerman, J. R.
Findley, J. S.
Hall, E. R., and E. L. Cockrum
Hibbard, C. W.
Howell, A. B.
Snyder, D. P.
Warren, E. R.
Transmitted June 30, 1955.