erence between the
two subspecies. The tympanic bullae of _M. m. pratincolus_ have
approximately a fourth more volume than those of _M. m. nanus_.

_Remarks._--Northwardly in the Bitterroot Valley, specimens of _M. m.
pratincolus_ morphologically approach _M. m. nanus_, especially in the
reduced degree of inflation of the tympanic portion of the bullae. On
geographic grounds we think that the geographic range of _M. m.
pratincolus_ extends southward to the southern end of the Bitterroot
Valley; we have not seen specimens from that area. Although we have not
examined the specimen reported upon by Davis (Murrelet, 18:26, September
4, 1937) from Canyon Creek, "a few miles west of Hamilton", Montana, we
think that it will be found to belong to _M. m. pratincolus_.

Our examination of specimens from localities in Montana east of the
range here ascribed to _M. m. pratincolus_ indicates that, among named
kinds of _Microtus_, those specimens are best referred to _M. m. nanus_.
These specimens are listed below under comparative materials. It should
be mentioned here that although Bailey (_loc. cit._) applied the name
_Microtus nanus canescens_ to Montanan specimens from Flathead Lake and
Hot Springs Creek, the subspecies _M. montanus canescens_ now is thought
to be restricted to Washington and the adjoining part of British
Columbia; _M. m. canescens_ does not occur so far east as Montana.

Grateful acknowledgment is made to those persons in charge of the
Biological Surveys collection for permission to study the specimens in
that collection, and to the Kansas Endowment Association for support of
the field work which yielded the specimens from six miles east of
Hamilton, Montana. The study here reported upon was aided also by a
contract between the Office of Naval Research, Department of the Navy,
and the University of Kansas (NR 161-791).

_Measurements._--The following measurements in millimeters are
those of the holotype and the average, maximum, and minimum,
respectively, of eleven adult males from various places in the
range of the subspecies. Except as noted below, we are unable to
detect significant morphological differences in the populations
sampled and believe that pooling of the measurements is justifiable
in this case. Measurements are: Total length, 129, 149 (156-141);
length of tail-vertebrae, 27, 37 (39-31); length of hind foot, 18,
19 (20-18) (all preceding