_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 as follows: total length, 159(150-175); length of tail, 37.6(31-46); hind foot, 18.6(17-20); condylobasilar length of the skull, 25.3(24.2-26.7); zygomatic breadth, 15.3(14.1-16.7); alveolar length of upper molar tooth-row, 6.2 (5.7-6.8); prelambdoidal breadth, 8.7(8.3-9.4); lambdoidal breadth, 11.9(11.0-12.5). Average and extreme measurements of a series of 12 adult male topotypes are as follows: total length, 159(144-174); length of tail, 36.4 (30-41); hind foot, 18.2(16-20); condylobasilar length of skull, 25.8(24.7-26.7); alveolobasilar length, 14.8(13.8-15.3); palatilar length, 13.8 (12.7-14.2); alveolar length of upper molar tooth-row, 6.4(5.9-6.6); zygomatic breadth, 15.9 (15.0-16.7); interorbital breadth, 3.6(3.4-3.7); lambdoidal breadth, 12.1 (11.5-12.5); prelambdoidal breadth, 8.6(8.3-8.9); depth of braincase, 8.0 (7.6-8.3).