The structure of the baculum most closely resembles that of _E. speciosus_ and the geographic ranges of these two species are in juxtaposition.
_Specimen examined_: One from Coal Kilns, Panamint Mountains, Inyo Co., California (CN).
General tone of upper parts dark; size medium to large; skull of medium size; braincase narrowed.
Baculum: Shaft thick; keel low, 1/4 of length of tip; tip 36 to 50 per cent of length of shaft; angle formed by tip and shaft 100°; distal 1/2 of shaft markedly compressed laterally; base markedly widened; shaft short to medium, 2.51 to 3.03 mm.
Differs from _E. bulleri_, in shaft shorter, keel smaller, ridges on either side of tip distinct; distal 1/2 of shaft strongly compressed laterally. Does not differ from _E. palmeri_.
For comparisons with all other species of chipmunks of western North America, see the accounts of those species.
Specimens of _E. umbrinus montanus_ from north-central Colorado have, in the past (Howell 1929:82), been referred to _E. quadrivittatus quadrivittatus_. In many features these two kinds of chipmunks resemble each other closely; their bacula, nevertheless, differ markedly (compare figs. 8-9 with 17-18).
_E. umbrinus umbrinus_: Wyoming: _Uinta Co._: 10 mi. S and 1 mi. W Robertson, 8,700 ft, 1. Utah: _Uintah Co._: Paradise Park, 21 mi. W and 15 mi. N Vernal, 10,050 ft., 12.
_E. u. adsitus_: Utah: _Beaver Co._: Britts Meadow, Beaver Range Mountains, 8,500 ft, 1 (BS). _Wayne Co._: Donkey Lake, Boulder Mountain, 10,000 ft., 1 (UU). _Garfield Co._: Wildcat R. S., Boulder Mountain, 8,700 ft., 1 (UU).
_E. u. sedulus_: Utah: _Garfield Co._: Mount Ellen, Henry Mountains, 1 (BS).
_E. u. inyoensis_: California: _Tulare Co._: Mount Whitney, head of Big Cottonwood Creek, 2 (BS). Nevada: _Elko Co._: W side Ruby Lake, 3 mi. N Elko County Line, 1.
_E. u. nevadensis_: Nevada: _Clark Co._: Sheep Mountains, 1 (MM).
_E. u. fremonti_: Wyoming: _Sublette Co._: 31 mi. N Pinedale, 8,025 ft., 1; 19 mi. W and 2 mi. S Big Piney, 1.
_E. u. montanus_: Colorado: _Boulder Co._: 3 mi. S Ward, 9,000 ft., 1; 1/2 mi. E and 3 mi. S Ward, 9,400 ft., 1.
General tone of upper parts grayish tawny; ocular stripe pale; skull, rostrum, nasals, and upper incisors shorter than in _E. umbrinus_.