larify the pattern of
distribution. My study was based, in addition, on 762 specimens
that are listed under "specimens examined" in the accounts of
subspecies, and on comparative material from other states. Most of
these specimens are skins with skulls but some are skins only and
others are skulls only. Some localities are represented by too few
adult individuals to permit significant comparisons. Owing to
damaged skulls, certain measurements of some specimens were omitted
from the calculations. If it seemed that the damaged skull was
exceptionally large or small or a deviant in any other regard it
was not used, in order not to bias the computed averages, which
might be used in comparing proportions of the skulls. In the lists
of specimens examined, localities that are omitted from Figure 2
because overlapping or undue crowding of the symbols would have
resulted are _italicized_.

[Illustration: FIGURE 2. Distribution of the subspecies of _Microtus
pennsylvanicus_ in Wyoming and Colorado. Solid dots represent
localities from which specimens have been examined, and triangles
represent localities reported in the literature from which I have not
examined specimens. The question mark in southern Colorado denotes a
questionable record discussed in the text.

A. _M. p. pullatus_
B. _M. p. insperatus_
C. _M. p. uligocola_
D. _M. p. finitus_
E. _M. p. modestus_
F. _M. p. aztecus_]

I am grateful to Professor E. Raymond Hall for critical reading of
the manuscript and helpful suggestions, to Dr. Rollin H. Baker and
various of my fellow students at the Museum of Natural History for
stimulating comments pertinent to the problems involved in this
study, to my wife, Justine Anderson, for assistance in the
preparation of the manuscript, to numerous members of field parties
from the Museum of Natural History, who collected much of the
material studied, and to the curators and other persons, at the
museums listed below, who courteously made specimens available for
study. The field work of the Museum of Natural History was assisted
by the Kansas University Endowment Association. A National Science
Foundation Fellowship made it possible for me to visit the museums
listed below. An honorarium awarded by the American Society of
Mammalogists enabled me to present this paper at the 34th Annual
Meet