Measurements.—The average and the extremes for some measurements of 34 males and females, 27 from the type locality and 7 from other localities in the range assigned to this subspecies, are as follows: total length, 165 (146-186); length of tail, 44.2 (35-55); hind foot, 19.6 (17-21); condylobasilar length of the skull, 25.5 (24.0-27.5); zygomatic breadth, 15.6 (14.7-16.6); alveolar length of upper molar tooth-row, 6.6 (6.2-7.0); prelambdoidal breadth, 8.8 (8.1-9.5); lambdoidal breadth, 12.0 (11.2-12.8). As an indication of variability and for comparison with other series the coefficient of variability and two times the standard error of the mean for each measurement in this series are included in [Table 1]. The averages for some measurements of 27 topotypes are as follows: total length, 162; length of tail, 45.5; hind foot, 19.9; condylobasilar length, 25.6; palatilar length, 14.0; molar series, 6.6; alveolobasilar length, 14.9; zygomatic breadth, 15.6; interorbital breadth, 3.5; lambdoidal breadth, 12.1; prelambdoidal breadth, 8.9; depth of braincase, 7.8.
Discussion.—Three species of Microtus were collected by James W. Bee at the type locality. Microtus montanus codiensis, Microtus longicaudus mordax, and Microtus pennsylvanicus modestus were taken in the same runways in the same meadow, at the same time. Microtus ochrogaster haydeni, although not taken at this locality, occurs in the Big Horn Basin. These four species differ in their geographic ranges, being largely allopatric, except M. montanus and M. longicaudus which are sympatric. Although the different species have ecological preferences and habits which differ, several species of Microtus may occur together in local areas such as the above. Certain of the characteristics of M. m. codiensis are intermediate between those of the species M. montanus on one hand and those of the other three species on the other hand. Could interspecific hybridization between "good species" 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); 3⅕ mi. E, ⅗ 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).
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.