MURIDA—SIGMODONTES—H. LEUCOPUS SONORIENSIS. 19 
HESPEROMYS LEUCOPUS SONORIENSIS (LeC.). 
Mus leucopus, RicHarpson, Zool. Journ. iii, 1318; Fn. Bor.-Am. i, 1829, 142. 
Hesperomys sonoriensis, LECOXTE, Proc. Acad. Nat. Sci. Phila. vi, 1853, 413 (Sonora; type, No. 146, Mus. 
Smiths.).—Aup. & Bacu., Q. N. A. iii, 1854, 296 (after LeConte).—Batrp, M. N. A. 1857, 474; 
U.S. & Mex. Bound. Surv. ii, pt. ii, 1859, 43. 
Hesperomys ( Vesperimus) leucopus sonoriensis, COUES, Proc. Acad. Nat. Sci. Phila. 1874, 179. 
Hesperomys sonoriensis var. nebrascensis, Barrp, M. N. A. 1857, 462, in text (based on the below-enu- 
merated Nebraskan examples). See choy. 2% ante 
Diacnosis—Hesp. leucopo via differt nisi caudé breviore, truncum sine 
capite subequante, necnon coloribus dilutioribus in speciminibus meridionalibus, 
in arcticis obscurioribus minime fulvescentibus. 
A variety of Hesperomys leucopus, differing in having the tail shorter 
(only about equal to the body without the head), and the colors either paler 
as in prairie specimens, or darker as in Arctic examples, but neither of these 
exhibiting the particular coloration of typical dewcopus. 
Hasirar.—tInterior of North America, west of the Mississippi, from the 
Arctic regions to Mexico; usually occupying this range to the exclusion of 
true leucopus, but in some places mixed therewith. 
It only remains to discuss the intricate question touching the White-footed 
Mice from the interior of North America. We approach this subject with a full 
knowledge of the normal variability of /eucopus, after settling the eastern and 
Pacific varieties, and in possession of hundreds of specimens from all locali- 
ties, among them the type of sonortensis. Only a part of our material appears 
in the following tables, the rest being ineligible on account of immaturity, bad 
taxidermy, and other causes. 
The type of sonoriensis (No. 146, Mus. Smiths.) is a young animal about 
three fourths grown, in a gray pelage identical with that of the same age of 
leucopus. While nothing, therefore, can be predicated upon its absolute size 
or its color, it may be known at a glance from ordinary /ewcopus by the short- 
ness of the tail; this member falling short of two inches, which is never the 
case with even three-fourths grown deucopus, so far as we know. The trunk 
measures 3.25, the tail 1.93; and, after allowing for stretching, we may safely 
say that the tail is more than an inch shorter than the trunk. 
The followimg table shows that this same short-tailedness marks a large 
proportion of the mice from the regions indicated. While the average length 
of the series from the United States is about 3.25, the tail averages only 
about 2.25, and several tails, as of Nos. 1782, 1932, 3149, 3352, &c., fall 
