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).

Discussion.—This subspecies is separated from M. m. codiensis to the west by the Big Horn Basin. A series from along Buffalo Creek, 27 mi. N, 1 mi. E Powder River, 6075 ft., in Natrona County, Wyoming, is intermediate between the topotypes of zygomaticus and nanus in the characters cited above as distinguishing the two, but shows greater resemblance to zygomaticus in the shape of the zygomatic arch, in color which is paler than in topotypes of zygomaticus, and in the short hind foot. On these and on geographic grounds this population is referred to zygomaticus. Unfortunately we cannot be certain in many cases that an intermediate condition in a certain character indicates a genetically intermediate population and therefore true intergradation between the two subspecies to which the population is geographically intermediate. The topotypes of this subspecies are the most distinct of all the series which I have studied from the eastern Rocky Mountains, in terms of the degree of morphological departure from the norm for the species. After zygomaticus the following populations are arranged according to their degree of deviation from this norm (codiensis deviates most): topotypes of codiensis, fusus and a population from southern Sweetwater County, Wyoming, and lastly the nanus-caryi complex. Within the latter group, as I have mentioned, there are a number of local variants most of which do not differ significantly and do not conform to any geographic pattern.

Specimens examined.—Total, 55, 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): Wyoming: Big Horn Co.: Medicine Wheel Ranch, 9000 ft., 28 mi. E Lovell, 30; W slope, head of Trappers Creek, 9500 ft., 2 (USBS). Washakie Co.: 9 mi. E, 5 mi. N Tensleep, 7400 ft., 1. Johnson Co.: 7½ mi. W, 1 mi. S Buffalo, 6500 ft., 3; Big Horn Mountains, 3 (USBS). Natrona Co.: Buffalo Creek, 27 mi. N, 1 mi. E Powder River, 6075 ft., 16.

Microtus montanus fusus Hall

Microtus nanus, Bailey, N. Amer. Fauna, 17:30, June 6, 1900 (part); Cary, N. Amer. Fauna 33:123, August 17, 1911.

Microtus montanus fusus Hall, Proc. Biol. Soc. Washington, 51:131-134, August 23, 1938; Warren, The Mammals of Colorado, Univ. of Okla. Press, p. 229, 1942.

Type.—Male, adult, skin and skull; No. 61281, Museum of Vertebrate Zoology; 2½ miles east of summit of Cochetopa Pass, Saguache County, Colorado; Sept. 21, 1933; collected by Annie M. Alexander; original number 2568. Type not seen by me.

Range.—Southern Colorado and northern New Mexico.

Comparisons.—For comparison with M. m. nanus, the subspecies to the northward, see the preceding account of that subspecies. For comparison with M. m. amosus the subspecies to the west see Hall (1938) and Durrant (1952). I have not examined specimens of amosus.

Measurements.—Average and extreme measurements for 17 adults including both males and females from several localities in southern Colorado are as follows: total length, 160 (136-179); length of tail, 42 (35-55); hind foot, 19.2 (17-23); condylobasilar length of the skull, 25.2 (24.0-26.0); zygomatic breadth, 15.0 (14.1-15.5); alveolar length of upper molar tooth-row, 6.4 (6.0-6.7); prelambdoidal breadth, 8.7 (8.3-9.2); lambdoidal breadth, 11.7 (11.1-12.6).

Average and extreme measurements of 4 adults (2 males and 2 females) from the type locality and 11 adults (4 males and 7 females) from other localities in southern Colorado are as follows: total length, 162 (157-168), 157 (137-169); length of tail (means only), 44.5, 40.5; hind foot, 18.8 (18-19), 18.6 (18-23); condylobasilar length of skull, 24.5 (24.0-24.7), 25.2 (24.3-26.1); alveolobasilar length, 14.2 (13.9-14.5), 14.6 (14.1-15.1); palatilar length, 13.2 (13.0-13.4), 13.5 (13.1-14.2); alveolar length of upper molar tooth-row, 6.3 (6.0-6.6), 6.4 (6.3-6.7); zygomatic breadth, 15.0 (14.3-15.5), 14.9 (14.1-15.5); interorbital breadth, 3.5 (3.3-3.6), 3.5 (3.3-3.7); lambdoidal breadth, 11.8 (11.1-12.6), 11.7 (11.2-12.3); prelambdoidal breadth, 8.6 (8.3-9.2), 8.8 (8.3-9.0); depth of braincase, 7.5 (7.2-7.8), 7.6 (7.1-7.9).

Discussion.—There is no sharp boundary between M. m. fusus of southern Colorado and the subspecies to the north, M. m. nanus. Although the line separating these two subspecies is drawn somewhat arbitrarily, on the whole the samples from north of this line more closely resemble nanus. All of the means for total length given above are larger than the maximum given in Hall's description of fusus. The caudal index (38 and 35% in two series) is slightly larger than that cited by Hall (33.3%) and is not significantly different from that in nanus (35.2%). The color in both young and old mice is variable, but in general is more yellowish, and less grayish, than in any other series studied.

There is a large area in western Colorado and eastern Utah, between the known ranges of M. m. fusus and M. m. amosus from which there are no specimens. Probably the species occurs only at certain places in this arid region which seems to be a partial