the smaller size-group (_E. m. minimus_ and _E. m.
consobrinus_), which occurs in the western part of the State, is
significantly smaller, in measurements of the skull and baculum, than
the larger size-group (_E. m. pallidas_, _E. m. confinis_, _E. m.
silvaticus_, and _E. m. operarius_) which occurs in the eastern part of
the State.

Although all the six subspecies of _E. minimus_ in Wyoming can be
differentiated from one another by color pattern, this species cannot
be divided, by means of color pattern, into two groups, comparable in
geographic range, to the two size-groups that were established above on
the basis of variations in the skull and baculum.

Thus, the subspecies of _E. minimus_ are morphologically differentiated
at two distinct levels; one level is based on differences in the skull
and baculum, while the other is based on differences in color.

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. L