_Description._--_Color pattern_: Head Cinnamon, shaded on crown to Light Drab; ocular stripe Fuscous Black, with Cinnamon along margins; other facial stripes Fuscous mixed with Cinnamon; ears Fuscous Black, Ochraceous-Tawny on anterior margin, grayish white on posterior margin and on postauricular patch; dark dorsal stripes black with Ochraceous-Tawny along margins; outer pair of dark stripes often mainly Tawny; light dorsal stripes grayish white, outer pair usually creamy white; sides Ochraceous-Tawny, shaded in the region of the shoulder with Cinnamon; rump and thighs Cinnamon-Buff mixed with Smoke Gray; antipalmar surfaces of forefeet Cinnamon-Buff; antiplantar surfaces of hind feet Pinkish Buff; dorsal surface of tail Fuscous Black, overlaid with Pinkish Buff; ventral surface of tail Ochraceous-Tawny, Fuscous Black along margin, Pinkish Buff along outermost edge; underparts creamy white. _Skull_: Large; braincase well inflated; zygomatic arches strong and slightly appressed to skull. _Baculum_: Large; long and slender. _Comparisons._--From _E. q. hopiensis_, the only other subspecies in this species, _E. q. quadrivittatus_ differs in: Dorsal dark stripes blackish; crown grayer; rump and thighs grayer; general tone of upper parts darker. _Remarks._--Specimens from the Chuska Mountains, Zuni Mountains, and Blanco, New Mexico, are intergrades between _E. q. quadrivittatus_ and _E. q. hopiensis_, but are referable to _E. q. quadrivittatus_. In north-central Colorado _E. umbrinus_ occurs in the spruce and pine forests at higher altitudes, while to the south and east of this area _E. q. quadrivittatus_ occurs in growths of pinon in lower, semiarid areas. In the northern half of New Mexico and in south-central Colorado, _E. q. quadrivittatus_ occurs not only in semiarid habitats but also in the moist habitats of the forests of higher altitudes. Ecologically, _E. umbrinus_ thus replaces _E. q. quadrivittatus_ in north-central Colorado. This ecological replacement is comparable to the ecological replacement of _Thomomys bottae_ by _T. talpoides_ in Utah as shown by Durrant (1952:156). _Specimens examined._--Total number, 130. =Colorado=: _Larimer Co._: Arkins, 1 BS. _Jefferson Co._: W spur Lookout Mountain, near Golden, 1 WC. _Gunnison Co._: Sapinero, 3 BS. _Saguache Co._: 5 mi. N and 22 mi. W Saguache 10,000 ft., 1; 21 mi. W and 3 mi. N Saguache, 1. _Fremont Co._: 18 mi. S and 7 mi. W Colorado Springs, 1; Arkansas River, "about" 26 mi. below Canon City, 15 BS. _San Juan Co._: Silverton, 1 BS. _Mineral Co._: 3 mi. E Creede, 1. _Alamosa Co._: Sangre de Cristo Range, 24 mi. E Hooper, 2 CM. _La Plata Co._: 2 mi. NE Bondad 6,100 ft., 1; Bondad, 15 mi. S Durango 6,050 ft., 1. _Archuleta Co._: Chromo, 1 CM. _Las Animas Co._: Trinidad, 6 BS. _Baca Co._: unspecified, 1.