): Each locality is followed by the month
(or months) of capture, the number of specimens, and the average value
for color.

_Montana_: Glacier County, August, 6, 1.8; Hill and Chouteau
counties combined (Mich), July, 24, 1.5; Malta, Philips County,
August, 14, 1.5; Sheridan County, August, 6, 1.5; Fergus County
(USBS), August, 5, 2.4; Ravalli County (KU and USBS), August, 12,
2.8; Silver Bow County, August, 7, 3.0; Sweet Grass County (Mich),
June and July, 7, 2.7; Park County, August, 10, 2.6. _Idaho_:
Pocatello and vicinity, November and December, 5, 3.4. _Wyoming_:
Park County, August, 6, 2.8; Sheridan County, September, 9, 1.2;
Johnson County, August, 12, 1.5; Campbell and Crook counties, July,
11, 1.4; Weston County, July, 7, 1.6; Teton County, September, 8,
3.4; Teton County (Mich), June, 17, 3.1; Afton and vicinity,
Lincoln County, July, 10, 4.2; Sage, Lincoln County, July, 5, 5.0.
_South Dakota_: Pennington County (Chi), June, 14, 2.1; Pennington
County (Mich), December and January, 8, 1.1; Walworth County, July,
4, 3.7; Buffalo County, July, 6, 3.2. _Colorado_: Loveland and
vicinity, Larimer County (KU and USBS), July, 13, 2.8; Boulder
County (Chi), September, 34, 2.6; Park County (Denv), March, 8,
1.9; Colorado Springs (ERW), March, April, and May, 5, 2.8;
Saguache County (USBS), August, 46, 3.0; Conejos County, June, 4,
3.0; Wray, Yuma County (USBS), 3, 4.7. _Nebraska_: Dundy County,
August and November, 14, 4.6. _New Mexico_: Colfax County, June, 8,
3.2. Variation in color is discussed in the accounts of the
subspecies concerned.

[Illustration: FIGURE 1. Geographic variation in color in _Microtus
pennsylvanicus_ in the Rocky Mountains. Paler colors are represented
by smaller numbers. Numbers are derived from the series of specimens
listed in the text by the method described there. The subspecies that
occur in the region studied are as follows:

_a. M. p. pullatus_
_b. M. p. insperatus_
_c. M. p. uligocola_
_d. M. p. finitus_
_e. M. p. modestus_
_f. M. p. aztecus_
_g. M. p. drummondi_
_h. M. p. pennsylvanicus_]

For each of the series listed in Table 1 all adult mice having
skulls that measured more than 24.0 mm. in condylobasilar length
were studied. Total length, length of tail, and length of hind foot
were taken from the collector's field labels. The measurem