Although there is considerable controversy concerning the glacial chronology in the mountains of western North America (Flint 1947:302-303), it is generally agreed that in Wyoming, in Wisconsinan time (the latest glacial age), glaciers covered a large part of the Yellowstone-Teton-Wind River highlands, the Big Horn Mountains, the southern part of the Laramie Range, the Medicine Bow Range, Sierra Madre Range, and the northern foothills of the Uinta Mountains. With this in mind, a possible explanation of the geographic variation in _E.

minimus_ of Wyoming, is here attempted.

In Sangamonian time, _E. minimus_-like chipmunks occurred over most of the region which is now Wyoming, and were divided into two size-groups, much as _E. minimus_ is today.

When permanent snow fields were formed in Wisconsinan time, these chipmunks were restricted in their ranges, not, of course, occurring on the glaciers.

When the glaciers melted at the end of Wisconsinan time, new habitats were thus "uncovered." The chipmunks which moved into these ice-free areas, then, became adapted to the new habitats. This then accounts for the subspeciation of _E. m. consobrinus_, _E. m. confinis_, and _E. m.

operarius_.

The Black Hills were not covered by glaciers. In late Pleistocene time these hills were probably of low relief. Subsequent differential erosion produced relief sufficient to provide a different habitat. The chipmunks that continued to occupy this area adapted themselves in color to the new habitat and became _E. m. silvaticus_.

LITERATURE CITED

CARY, M.

1917. Life zone investigations in Wyoming. N. Amer. Fauna, 42:1-96, 15 pls., 17 figs.

FLINT, R. F.

1947. Glacial geology and the Pleistocene Epoch. John Wiley and Sons, New York, pp. xviii + 589, 88 figs., 27 tables, 6 pls.

HALL, E. R.

1926. Changes during growth in the skull of the rodent Otospermophilus grammurus beecheyi. Univ. California Publ. Zool., 21:355-404, 43 figs., March 9.

1946. Mammals of Nevada. Univ. California Press, Berkeley, California, pp. xi + 710, 11 pls., 485 figs., July 1.

HOWELL, A. H.

1929. Revision of the American chipmunks (genera _Tamias_ and _Eutamias_). N. Amer. Fauna, 52:1-157, 10 pls., 9 maps, November 30.

JOHNSON, D. H.

1943. Systematic review of the chipmunks (genus Eutamias) of California. Univ. California Publ. Zool., 48:63-148, 1 pl., 12 figs., December 24.

LARRISON, E. J.

1949. Variation in the chipmunks of west-central Washington.

Murrelet, 29:34-43, 1 map, March 1.

SHAW, W. T.

1944. Brood nests and young of two western chipmunks in the Olympic Mountains of Washington. Jour. Mamm., 25:274-284, 1 pl., 4 figs., September 8.

WHITE, J. A.

1953. Taxonomy of the chipmunks, Eutamias quadrivittatus and Eutamias umbrinus. Univ. Kansas Publ., Mus. Nat. Hist., 5:563-582, 6 figs. in text, December 1.