Numerous members of summer field parties from the Museum of Natural History at the University of Kansas collected most of the specimens studied and wrote field notes that have been helpful. I am grateful to these persons and to Professor E. R. Hall and Dr. R. H. Baker for their assistance and helpful suggestions. Specimens in the following museums were made available by their respective curators: Chicago Natural History Museum by Mr. Colin C. Sandborn, The Museum of Zoology at the University of Michigan by Dr. E. T. Hooper, The American Museum of Natural History by Mr. G. G. Goodwin, The United States National Museum by Dr. David H. Johnson and the Biological Surveys Collection by Miss Viola S. Schantz. A fellowship from the National Science Foundation made possible the studies at the museums other than at the University of Kansas.

Microtus montanus nanus (Merriam)

Arvicola (Mynomes) nanus Merriam, N. Amer. Fauna, 5:62, July 30, 1891.

Microtus montanus nanus, Hall, Proc. Biol. Soc. Washington, 51:131, August 23, 1938.

Microtus nanus, Bailey, N. Amer. Fauna, 17:30, June 6, 1900 (part).

Microtus montanus caryi Bailey, Proc. Biol. Soc. Washington, 30:29, February 21, 1917.

Microtus nanus nanus, Cary, N. Amer. Fauna, 42:43, October 3, 1917 (part).

Type.—Adult male No. 23853/31253, U. S. National Museum, Department of Agriculture collection, from Pahsimeroi Mountains, Custer County, Idaho; obtained by C. Hart Merriam and Vernon Bailey, September 16, 1890.

Range.—Idaho; southwestern Montana; most of the southwestern half of Wyoming; southward to central Colorado. See [figure 1].

Comparisons.—Comparisons with subspecies named as new in this paper will be found in the accounts of those subspecies beyond. From Microtus montanus fusus Hall, the subspecies to the south, M. m. nanus from Idaho differs as follows: averages smaller; slightly darker and less reddish and less yellowish in color; slightly wider braincase (see measurement of prelambdoidal breadth); larger bullae.

Measurements.—Average (= arithmetical mean) measurements of 34 specimens, both male and female, from several localities in eastern Idaho are: total length, 151; length of tail, 39; hind foot, 19.2; condylobasilar length of the skull, 25.0; zygomatic breadth, 15.0; alveolar length of upper molar tooth-row, 6.4; prelambdoidal breadth, 8.9; and lambdoidal breadth, 11.7.

Average and extreme measurements of six adult males from near Pocatello, Bannock County, Idaho, and nine adult males from near Afton, Lincoln County, Wyoming, are, respectively, as follows: total length, 143(135-150), 163(143-179); length of tail, 35.1(33-38), 42.8(36-49); caudal index, 32.0(28.0-33.1), 35.7(30.6-41.9); hind foot, 18.9(18-20), 18.8(17-20); condylobasilar length of skull, 24.4(24.0-26.0), 25.6(24.5-26.2); alveolobasilar length, 14.1(13.7-14.5), 14.6(13.8-15.0); palatilar length, 13.2(12.9-13.6), 13.8(13.2-14.5); alveolar length of upper molar tooth-row, 6.3(6.1-6.5), 6.3(6.0-6.6); depth of braincase, 7.7(7.5-7.9), 8.0(7.7-8.3); lambdoidal breadth, 11.4(11.0-11.7), 12.0(11.3-12.7); prelambdoidal breadth, 9.1(8.6-9.4), 8.7(8.0-9.4); zygomatic breadth, 14.3(13.8-14.7), 15.3(14.4-16.3); interorbital breadth, 3.6(3.5-3.7), 3.5(3.3-3.7). The average length of the nasal bones in the series from Pocatello is 7.1 mm. The averages, which have not been included in [Table 1], for three measurements of the series from Carbon County, Wyoming, are as follows (Encampment, males; Encampment, females; Savery, males; and Savery, females, respectively): alveolobasilar length, 14.4, 14.3, 14.5, 14.3; interorbital breadth, 3.5, 3.4, 3.5, 3.4; depth of braincase, 7.8, 7.6, 7.9, 7.6. Additional measurements are included in[ Table 1] for other series.

Discussion.—The name Microtus montanus caryi Bailey is here placed in synonymy under M. m. nanus (Merriam). Vernon Bailey (1917) in his description of caryi made four assumptions that have been found to be entirely or partly invalid. First, he assumed that this is an "extreme variant which gradually changes in characters across Nevada and Utah, and reaches its maximum variation in Wyoming." The differences pointed out in subsequent descriptions of subspecies found in the above area do not show a gradual change in any character, or in the number of characters, nor is caryi an extreme when compared with the other subspecies. Second, Microtus nanus was not, as Bailey assumed, a different species than Microtus montanus. Third, he assumed that the characteristics of adults of nanus were adequately ascertainable from the thirteen topotypes available to him. Subsequent sampling from Idaho shows that the series of specimens available to Bailey was made up mostly of young and subadult animals. Finally, caryi does not occupy as Bailey stated "the meadows along streams in the arid sagebrush country of the Bear River, Green River, and Wind River valleys" exclusively, or characteristically. When the localities from which the species actually is known are plotted, it seems that the arid basin serves as a barrier and that the species is more commonly and abundantly found in montane meadows in the Transition and Canadian life-zones.

Certain samples, here assigned to M. m. nanus, that vary from the average of the subspecies deserve comment. For example, mice from the area in Wyoming southwest of the Green River (in the Uinta Mountains) have relatively smaller feet, but are larger in both total length and size of skull. Specimens from near Afton, Lincoln County, Wyoming, are relatively large in both total length and size of skull. This series and specimens from Teton County, Wyoming, are intermediate between nanus from Idaho and the newly named subspecies from near Cody, Park County, Wyoming, described below, in terms of both darkness and the amount of reddish color. Mice from Laramie County are more richly reddish-brown. The specimens from near Savery, in Carbon County, Wyoming, are darker. The alveolobasilar length relative to the condylobasilar length is smaller in the series from along Deer Creek, 16 mi. S, 11 mi. W Waltman, Natrona County, Wyoming. The series from the southern tier of counties in Wyoming and some of the specimens from Colorado have relatively wider zygomatic arches. The specimens from southern Sweetwater County, Wyoming, are relatively paler, have a relatively longer tail and longer hindfeet, lesser condylobasilar length, and wider braincase. Most of these variations are of questionable significance; they may be chance variations owing to errors in sampling.

Much of the south-central part of the state is relatively low and relatively arid. This area includes the arid basin of the Green River and its major tributaries and the arid Red Desert along the continental divide in Sweetwater County. This area might have acted as a barrier to the mice; gene flow might have been prevented between the populations of the western part of the state and those farther east in the Medicine Bow Mountains and Laramie Mountains. Nevertheless geographic variations of subspecific worth have not taken place. The barrier has either not been of as long duration, or has not been so complete and effective, as the other barriers in the state, namely the Absaroka Range, the Big Horn Basin, the Shoshone Basin, and the valley of the North Platte River. These four barriers presumably have led to the differentiation of the two subspecies that are newly named beyond. Each of the two areas which is set apart by these barriers and in which one of the newly named subspecies has evolved is small; therefore there is a lesser amount of suitable habitat available for each of the newly named mice than there is for M. m. nanus. It is conceivable, therefore, that in periods of adverse conditions in each of the small areas the size of the effective breeding population may have been so small that random genetic drift could have operated effectively, or that selection was more critical than in a larger, more stable population. It is difficult to test these possibilities because the selective value of the taxonomic characters is unknown. The observed pattern of variation and facts of distribution are, however, not contradictory to the above possibilities.

Specimens examined.—Total, 993, distributed as follows: All specimens unless otherwise indicated are in the University of Kansas Museum of Natural History. Specimens in other museums are labeled as follows: Chicago Natural History Museum (Chi); University of Michigan, Museum of Zoology (Mich); American Museum of Natural History (AMNH); United States National Museum (USNM); Biological Surveys Collection (USBS). Localities that are not represented in [Fig. 1] because overlapping or crowding of the symbols would result are Italicized. Localities are arranged from north to south by states, within a state from northwest to southeast by counties, and within a county from north to south.

Wyoming: Yellowstone Park: Canyon Camp, 1 (USBS); Lower Geyser Basin, 1 (USBS); Upper Yellowstone River, 2 (AMNH); North end of Lake, Yellowstone National Park, 2 (AMNH). Teton Co.: Pacific Creek, 1 (USBS); Big Game Ridge, 3 (USBS 1, Mich 2); Whetstone Creek, 7 (Mich); Moran and environs (6 localities within a 5 mile radius), 28 (USBS 2, Mich 5); S fork Buffalo River, 7 (AMNH); 2 mi. W pass, Black Rock Creek, 1 (USBS); Jenny Lake, 5 (Mich); Bar BC Ranch, 2½ mi. NE Moose, 6500 ft., 2; Teton Pass above Fish Creek, 1 (USBS); Jackson and environs, 142 (Mich 141); Sheep Creek, 2 (Mich). Lincoln Co.: 13 mi. N, 2 mi. W Afton, 2; 10 mi. N, 2 mi. W Afton, 4; 9½ mi. N, 2 mi. W Afton, 3; 9 mi. N, 2 mi. W Afton, 9; 7 mi. N, 1 mi. W Afton, 12; Afton, 1 (USBS); Labarge Creek, 1 (USBS); Border, 6 (USBS); Cokeville, 2 (USBS); 6 mi. N, 2 mi. E Sage, 1; Cumberland, 5 (USBS). Sublette Co.: 34 mi. N, 4 mi. W Pinedale, 1; 33 mi. N, 2 mi. W Pinedale, 6; 32 mi. N, 1 mi. W Pinedale, 1; 31 mi. N Pinedale, 4; Dell Creek, on Ferris Ranch, 7 (Mich); Horse Creek, 7800 ft., Merna, 4 (USBS); Big Piney, 1 (USBS). Fremont Co.: 17½ mi. W, 2½ mi. N Lander, 9500 ft., 3; 17 mi. W, 2 mi. N Lander, 9300 ft., 4; Milford and environs (5 localities within a 1 mile radius), 23 (USBS 4); 15½ mi. S, 7½ mi. W Lander, 9200 ft., 1; South Pass City, 8000 ft, 8 (USBS); 23½ mi. S, 5 mi. W Lander, 8600 ft., 7. Natrona Co.: Deer Creek, 16 mi. S, 11 mi. W Waltman, 6950 ft., 44; 6 mi. S, 2 mi. W Casper, 5900 ft., 4; 6-4/5 mi. S, 2 mi. W Casper, 6100 ft., 1; 7 mi. S, 2 mi. W Casper, 6370 ft., 3; 10 mi. S Casper, 7750 ft., 33; Sun, 2 (USBS); 5 mi. W Independence Rock, 6000 ft., 4; 5 mi. W, 1 mi. S Independence Rock, 2. Converse Co.: Beaver, 1 (USBS). Uinta Co.: 1½ mi. W, ½ mi. S Cumberland, 6; 16 mi. S, 2 mi. W Kemmerer, 6700 ft., 3; 10 mi. SW Granger, 3 (Mich); Fort Bridger, 6650 ft., 25 (USNM 6); 9 mi. S Robertson, 8000 ft., 9; 9½ mi. S, ½ mi. W Robertson, 8600 ft., 1; 10 mi. S, 1 mi. W Robertson, 8700 ft., 25; 14 mi. S, 2 mi. E Robertson, 9000 ft., 5; 4 mi. S Lonetree, 1 (USBS). Sweetwater Co.: Farson, 3; Bitter Creek, 3 (AMNH); Kinney Ranch, 21 mi. S Bitter Creek, 6800 ft., 9 (USNM 1, AMNH 2); 32 mi. S, 22 mi. E Rock Springs, 7025 ft., on Vermillion Creek, 15. Carbon Co.: 18 mi. NNE Sinclair, 6500 ft., 10; Bridgers Pass, 18 mi. SW Rawlins, 7500 ft., 7; Saratoga, 1 (USBS); 6 mi. S, 13 mi. E Saratoga, 8500 ft., 5; 6 mi. S, 14 mi. E Saratoga, 8800 ft., 1; Lake Marie, 10,440 ft., 2; 1 mi. S Lake Marie, 2; ½ mi. S, 2 mi. E Medicine Bow Peak, 10,800 ft., 1; Encampment (12 localities from 10 mi. N, 14 mi. E to 9 mi. N, 3 mi. E Encampment and from 6500 to 8400 ft.), 63; 1/4 mi. N Riverside, 7380 ft., 2; S base Bridger Peak, 8800 ft., Sierra Madre Mountains, 1; 2 mi. S Bridger Peak, 9300 ft., 2; Savery (10 localities from 8 mi. N, 21 mi. E to 4 mi. N, 8 mi. E Savery and from 7300 to 8800 ft.), 80. Albany Co.: 30 mi. N, 10 mi. E Laramie, 6760 ft., 6; 29¾ mi. N, 9½ mi. E Laramie, 6350 ft., 1; 26 mi. N, 4½ mi. E Laramie, 6960 ft., 8; 26¾ mi. N, 6½ mi. E Laramie, 6700 ft., 3; 3 mi. N, 13 mi. E Laramie, 7500 ft., 1; 7 mi. N, 2 mi. E Laramie, 1 (Chi); 5 mi. N Laramie, 7400 ft., 15; Laramie, 4 (AMNH); 1 mi. E Laramie, 7160 ft., 4; 7-7/10 mi. SSW Laramie, 7200 ft., 4; 6½ mi. S, 8¾ mi. E Laramie, 8200 ft., 1; Headquarters Park, 10,200 ft., Medicine Bow Mountains, 3 (USBS); Centennial, 8120 ft., 1; 2-1/4 mi. ESE Brown's Peak, 10,300 ft., 3; 3 mi. ESE Brown's Peak, 10,000 ft., 12; 2 mi. S Brown's Peak, 10,600 ft., 1; Pole Mountain, 15 mi. SE Laramie, 4 (USBS 3); 1 mi. SSE Pole Mountain, 8350 ft., 4; 2 mi. SW Pole Mountain, 8300 ft., 13; 3 mi. S Pole Mountain, 8100 ft., 1; Sherman, 2 (AMNH). Laramie Co.: 5 mi. N, 1 mi. W Horse Creek P. O., 7200 ft., 1; Meadow, 2 (USBS); 11 mi. N, 5½ mi. E Cheyenne, 5450 ft., 7; 7 mi. W Cheyenne, 6500 ft., 10; Cheyenne, 3 (USNM).

Colorado: Moffat Co.: Lay, 6160 ft., 1 (AMNH). Routt Co.: Wright's Ranch, Yampa, 7700 ft., 2; Gore Range, 8 mi. E Toponas, 8000 ft., 2 (USBS). Larimer Co.: 12½ mi. W, 1½ mi. S Rustic, 1; 11 mi. W, 1 mi. S Rustic, 1; Cache La Poudre River, 1 (Chi); Estes Park, 3 (USBS 1, AMNH 2); 19½ mi. W, 2½ mi. S Loveland, 7280 ft., 6; 16 mi. W Loveland, 6840 ft., 1; 6 mi. W, ½ mi. S Loveland, 5200 ft., 1. Rio Blanco Co.: Meeker, 1 (USBS); 9½ mi. SW Pagoda Peak, 7700 ft., 3; 5 mi. S Pagoda Peak, 9100 ft., 2. Eagle Co.: Eagle, 1 (USBS); Pando, 2 (USBS). Grand Co.: Mt. Whiteley, 2 (USBS); Arapahoe Pass, Rabbit Ear Mountains, 2 (USBS); Coulter (near Granby), 5 (USBS); Arrowhead (near Dale), 1 (USBS). Boulder Co.: ¾ mi. N, 2 mi. W Allenspark, 8400 ft., 4; 3 mi. S Ward, 9000 ft., 3; Nederland, 16 (Chi). Clear Creek Co.: Mt. McLellan, 2 (USBS); Berthoud Pass, 4. Park Co.: Trout Creek Ranch, 2 mi. N Garo, 1 (USBS).

Specimens examined of M. m. nanus from eastern Idaho and Montana are as follows: Idaho: Custer Co.: Challis, 7 (USBS); Mill Creek, Challis Nat. Forest, 1 (USBS); Pahsimeroi Mts., 12 (USBS); Lost River Mts., 1 (USBS). Fremont Co.: N fork Snake River, 10 mi. SW Island Park, 6200 ft., 2 (AMNH); Black Springs Creek, 4 mi. W Ashton, 5200 ft., 1 (AMNH); 5 mi. W St. Anthony, 5000 ft., 1 (AMNH). Camas Co.: Camas Prairie, Corral, 5100 ft., 2 (USBS). Blaine Co.: Alturas Lake, 3 (USBS); Sawtooth Lake, 2 (USBS); Craters of the Moon, Laidlow Park, 2 (Mich); Ticura, 10 mi. S Picabo, 1 (USBS); 19 mi. NE Carey (Lava Lake), 8 (Mich). Butte Co.: 26 mi. SW Arco, 12 (Mich). Bingham Co.: Shelley, 6 (USBS). Bonneville Co.: 10 mi. SE Irwin, 4 (USBS). Owyhee Co.: Three Creeks, 3 (USBS). Twin Falls Co.: Castleford Fenced Plot, 11 mi. W, 9 mi. S Twin Falls, 1. Minidoka Co.: Heyburn, 2 (USBS). Cassia Co.: 2 mi. S, 2 mi. W Burley, 5. Bannock Co.: Pocatello, 23 (USBS 4); Swan Lake, 1 (USBS). Bear Lake Co.: Montpelier Creek, 6700 ft., 3 (USBS). Montana: Gallatin Co.: W. Fork of W. Fork, Gallatin River, 1 (USBS). Park Co.: Lamar River, 7000 ft., 1 (USBS); Gardiner, 1 (USBS). Sweet Grass Co.: 14 mi. S Big Timber, 1 (USBS); McLeod, 1 (USBS); West Boulder Creek, 18 mi. SE Livingston, 2 (USBS).

Microtus montanus codiensis, new subspecies