See figure 1.

_Comparisons._--Comparisons with subspecies named as new in this
paper will be found in the accounts of those subspecies beyond.
From _Microtus montanus fusus_ Hall, the subspecies to the south,
_M. m. nanus_ from Idaho differs as follows: averages smaller;
slightly darker and less reddish and less yellowish in color;
slightly wider braincase (see measurement of prelambdoidal
breadth); larger bullae.

_Measurements._--Average (= arithmetical mean) measurements of 34
specimens, both male and female, from several localities in eastern
Idaho are: total length, 151; length of tail, 39; hind foot, 19.2;
condylobasilar length of the skull, 25.0; zygomatic breadth, 15.0;
alveolar length of upper molar tooth-row, 6.4; prelambdoidal
breadth, 8.9; and lambdoidal breadth, 11.7.

Average and extreme measurements of six adult males from near
Pocatello, Bannock County, Idaho, and nine adult males from near
Afton, Lincoln County, Wyoming, are, respectively, as follows:
total length, 143(135-150), 163(143-179); length of tail,
35.1(33-38), 42.8(36-49); caudal index, 32.0(28.0-33.1),
35.7(30.6-41.9); hind foot, 18.9(18-20), 18.8(17-20);
condylobasilar length of skull, 24.4(24.0-26.0), 25.6(24.5-26.2);
alveolobasilar length, 14.1(13.7-14.5), 14.6(13.8-15.0); palatilar
length, 13.2(12.9-13.6), 13.8(13.2-14.5); alveolar length of upper
molar tooth-row, 6.3(6.1-6.5), 6.3(6.0-6.6); depth of braincase,
7.7(7.5-7.9), 8.0(7.7-8.3); lambdoidal breadth, 11.4(11.0-11.7),
12.0(11.3-12.7); prelambdoidal breadth, 9.1(8.6-9.4), 8.7(8.0-9.4);
zygomatic breadth, 14.3(13.8-14.7), 15.3(14.4-16.3); interorbital
breadth, 3.6(3.5-3.7), 3.5(3.3-3.7). The average length of the
nasal bones in the series from Pocatello is 7.1 mm. The averages,
which have not been included in Table 1, for three measurements of
the series from Carbon County, Wyoming, are as follows (Encampment,
males; Encampment, females; Savery, males; and Savery, females,
respectively): alveolobasilar length, 14.4, 14.3, 14.5, 14.3;
interorbital breadth, 3.5, 3.4, 3.5, 3.4; depth of braincase, 7.8,
7.6, 7.9, 7.6. Additional measurements are included in Table 1 for
other series.

_Discussion._--The name _Microtus montanus caryi_ Bailey is here placed
in synonymy under _M. m. nanus_ (Merriam). Vernon Bailey