474 
U. S. P. R. R. EXP. AND SURVEYS-ZOOLOGY GENERAL REPORT. 
HESPEROMYS SONORIENSIS, Leconte. 
Hesperomys sonoriensis, Leconte, Pr. A. N. Sci. Phila. VI, October, 1853, 413. (Sonora.) 
Arvicola (Hesperomys) sonoriensis, Aud. & Bach. N. Am. Quad. Ill, 1854, 296, (from Leconte.) 
? Mus leucopus, Rich. Zool Jour. Ill, 1828 .—Ib. F. B. Am. I, 1829, 142. 
Sp. Ch.—Y oung, light ashy gray ; adult, with a tinge of yellowish brown ; in neither is there any dorsal stripe. Tail 
scarcely longer than the body, exclusive of the head. Posterior tubercle of sole small, rounded, far forward. Soles hairy 
for half their length. Tail white, except a narrow line above of dusky. Ears large, with long hairs ; one-half of the ear 
hoary, in strong contrast with the dusky of the remaining portion. 
This species has the general characters of the white-footed mouse of the eastern States; and 
it is only after the comparison of extensive series that I have been able to detect differences 
which, though slight, are so constant and of such a character as to appear something more than 
a mere local variation. I shall, however, he obliged to indicate the differences rather by com¬ 
parison than as absolute characters. 
The hands, feet, and tail are considerably stouter than in eastern white-footed mice; the 
second finger is longer. The tubercles of the soles are smaller, especially the hinder one, which 
is a simple, small rounded cone, instead of an elongated eminence. This, too, is situated con¬ 
siderably more forward, its base anteriorly being about half way from the heel to the end of the 
balls of the toes, instead of less. The soles are more hairy, too, the hairs covering fully half 
the sole from heel to balls of toes, or as far as the posterior tubercle. 
The tail is very thick and quite short in its proportions, scarcely longer than the body, 
exclusive of the head. It is unusually thick and densely hairy, so as to conceal the annuli 
and junction of the vertebras, even in the dried specimen. 
The distribution of color in this animal is much as in the Hesperomys leucopus. The tints, 
however, are considerably different. Most specimens are of an ash gray, finely lined with black, 
nearly uniform all over the whole back, without forming a dorsal band. The under parts, with 
the feet, are very pure white. The hairs on the concavity of the ears are unusually long aod 
of a hoary gray ; the margin of the back of the ear is also of this color, forming a strong 
contrast to the dark brown of the remaining portion. In Hesperomys leucopus the hairs are 
shorter, with little or none of the hoary. The colors of the tail are in a very strong contrast, 
the dorsal dusky being quite intense and much more restricted than in the other species, 
occupying scarcely one-third of the entire circumference, instead of nearly one-half. 
In some specimens there is a slight tinge of yellowish in the ash gray of the upper parts. 
This is, however, not so vivid as in the other species. I have seen no specimens of a dark slate 
color. The specimens of Hesperomys, upon which the preceding description has been based, are 
from the upper Missouri. 
