MURIMS— SIGMODONTES— H. LEUCOPUS SONOUIENSIS. 79 



HESPEROMYS LEUCOPUS SONORIENSIS (Led.). 



Mus leucopus, Richardson, Zool. Journ. iii, 1818; Fu. Bor.-Am. i, 1829, 142. 



Hesperomys sonoriensis, LuCoxte, Proc. Acad. Nat. Sci. Phila. vi, 1853, 413 (Sonora ; type, No. 146, Mus. 

 Smiths.).— Aun. & Bach., Q. N. A. ii.i, 1854, 29G (after LeCoute).— Baird, M.N. A. 1857, 474; 

 U. S. & Mex. Bound. Surv. ii, pt. ii, 1859, 43. 



Hesperomys (Vesperimw) leucopus sonoriensis, Coues, Proc. Acad. Nat. Sci. Phila. 1874, 179. 



Hesperomys sonoriensis var. nebrascensis, Baiud, M. N. A. 1857, 4G2, iu text (based on the below-enu- 

 merated Nebraskau examples). 



Diagnosis. — Hesp. leucopo vix differt nisi caudd breviore, truncum sine 

 capite subcequante, necnon coloribus dilutioribus in speciminibus meridionalibus, 

 in arcticis obscurioribus minime fulvescentihus. 



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 leucopus. 



Habitat. — Interior 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 leucopus, after settling the eastern and 

 Pacific varieties, and in possession of hundreds of specimens from all locali- 

 ties, among them the type of sonoriensis. 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 leucopus by the short- 

 ness of the tail ; this member falling short of two inches, which is never the 

 case with even three-fourths grown leucopus, 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 following 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 



