pecies" of _Microtus_ take place in nature and possibly alter the
characteristics of a local population?

_Specimens examined._--Total, 50, distributed as follows
(abbreviations for collections are given in the account of _M. m.
nanus_; localities that are not represented in Fig. 1 because
overlapping or crowding of the symbols would result are
Italicized):

MONTANA: _Carbon Co._: Beartooth Mountains, 2 (USBS); _Beartooth
Lake_, 1 (USBS).

WYOMING: _Park Co._: Black Mountain, head of Pat O'Hara Creek, 3
(USBS); 13 mi. N, 1 mi. E Cody, 5200 ft., 1; SW slope Whirlwind
Peak, 9000 ft., 1; _5 mi. N Cody, 6300 ft._, 1 (USBS); 3-1/5 mi. E,
3/5 mi. S Cody, 31; Ishawooa Creek, 6300 ft., 2 (USBS); _Valley_, 1
(USBS); Needle Mountain, 10,500 ft., 4 (USBS). _Hot Springs Co._: 3
mi. N, 10 mi. W Thermopolis, 4950 ft., 3.

=Microtus montanus zygomaticus=, new subspecies

_Type._--Male, adult, skin and skull, No. 32761, Museum of Natural
History, University of Kansas, from Medicine Wheel Ranch, 9000 ft.,
28 mi. E Lovell, Big Horn County, Wyoming; obtained by R. Freiburg,
original number 105.

_Range._--The Big Horn Mountains of north-central Wyoming.

_Diagnosis._--A large _Microtus montanus_ with a relatively short
tail; short molar series; broad zygomatic arches well rounded in
lateral outline when viewed from above; small and flattened bullae;
raised basioccipito-basisphenoid suture.

_Comparisons._--For comparison with _M. m. codiensis_ from the
west, on the other side of the Big Horn Basin, see the account of
that subspecies. In comparison with _nanus_ this subspecies is
slightly paler, in this respect showing more resemblance to
_codiensis_ although not so pale, and more grizzled or unevenly
colored. This difference in color between _zygomaticus_ and
_codiensis_ may not be of taxonomic significance. From both the
topotypes of _nanus_, and the series of it from Wyoming,
_zygomaticus_ differs on the average in having a relatively shorter
tail, a relatively shorter upper molar tooth-row, relatively more
rounded and relatively more wide-spread zygomatic arches, and
smaller more flattened bullae.

_Measurements._--Average and extreme measurements of 24 adult males
and females from several localities here referred to _M. m.
zygomaticus_ are