RODENTIA-MURINAE—HESPEROMYS BOYLII. 
471 
HESPEROMYS BOYLII, Baird. 
Long-tailed Mouse. 
Hesperomys boylii, Baird, Pr. A. N. Sc. Phila. YII, April, 1855, 335. 
Sp. Ch.—B ody stout. Ears very large, -with very scant hairs, appearing almost naked. Tail stout, considerably longer 
than the body, with long hairs at the end, (and 32 vertebrae?) Above, mixed brown and yellowish brown ; paler on the 
sides. The outside of fore leg colored to near the wrist. 
This species is rather larger than H. leucopus, and is quite distinctly marked in other respects 
among the North American Hesperomys. The head is broad, and the snout quite acute and 
pointed ; the whiskers longer than the head. The ears are very large, and appear naked on 
both surfaces, especially the exterior, but a close examination shows them to be thinly coated 
with a very fine short pile ; the antitragus is almost obsolete. The tail is cylindrical, and con¬ 
siderably longer than the body ; the hairs are moderately close, and slightly elongated towards 
the tip. The feet are moderate, the hinder ones rather long. The under surface of the metatarsi 
is thinly coated with short hairs. The fur is everywhere full and soft, and the general bulk 
greater than in most other species. 
The upper parts are of a mixed glossy brown and pale yellowish brown, the former predomi¬ 
nating on the back and thighs, the latter on the sides, still brighter on the outside of the fore 
leg and the side of the head. The lower parts and feet are white, the fore feet, with a brownish 
tinge; the color of the sides reaches nearly to the carpus. The white of the sides is 
bordered above by a suffusion of reddish buff, which melts insensibly into the color of the upper 
parts. The outside of the fore leg is colored nearly to the wrist. The tail is white, with the 
upper side dusky, though the line of separation of the two is not as distinct as in some species. 
The specimen upon which this species was founded was collected by Dr. C. 0. Boyle, in Eldorado 
county, California, on the middle fork of the American river. It had 32 caudal vertebras, while 
a specimen from Monterey, of H. ccdifornicus, had but 24. It is the very appreciably greater 
length of the tail that distinguishes the two species at first sight. 
A specimen from Shoalwater Bay, collected by Dr. Cooper, has the tail still longer than the 
preceding. 
List of specimens. 
1 These measurements were taken before skinning. The dried specimen measures to root of tail 3.40; the tail to end of hairs 4.05. 
* Possibly H. calijornicus. 
