748 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
and throat pale fulvous. The tail has also the hairs of the lower surface pale 
yellowish at the base, then black, broadly tipped with white. These speci- 
mens, judging from De Saussure’s description, are the more common or normal 
phase of the species. No. 8495, from Nicaragua, is white, except a broad 
dorsal band, extending from the occiput to the tail, which is intense brownish- 
black. The hairs of the lower surface of the tail are white at base, then 
black, broadly tipped with white, giving a wholly pure white surface to the 
tail throughout. No. 8628, from Costa Rica (about half-grown), has the 
whole body pale yellow or yellowish-white, with an indistinct brownish dor- 
sal band. The hairs of the tail are wholly black at base, broadly tipped 
with whitish. Two other specimens, from Southern Mexico, are almost 
wholly black, being only slightly varied with gray above and on the tail. 
This species differs from most other American species in its slenderness, 
the great length and narrowness of the ears, and the excessive length of the 
tail, which, with the hairs, is one-fifth to one-fourth longer than the head and 
body. Dr. Gray's S. dorsalis agrees perfectly with the white, black-backed 
specimen (No. 8495) above described. The Macrorus melania of the same 
author corresponds with the black phase of this species, and his JZ maurus 
with the black phase which has the under parts more rufous. De Saussure, in 
his paper on the Mexican Squirrels (as above cited), describes this species 
as being usually ferrugineous beneath,* but gives the following phases of col- 
oration under the head of three unnamed varieties :—a, wholly black, with 
the hairs more or less fulvous at the base; }, blackish, with the ventral sur- 
face gray, fulvescent, or dusky; c, body wholly black. 
The original description of Wagler refers toa phase with the lower 
parts strongly rufous. he length is given as 12 inches from the nose to the 
end of the tail; the tail (vertebrae only?) as 11.75. Wagner gives the length 
from nose to base of tail as 12 inches; of the tail-vertebrae 12.09; tail to end 
of hairs 14.75. 
Tam quite confident that the Sciwrus nigrescens of Bennett, described 
in 1833 as ‘from that part of California that adjoins Mexico”, is referable to 
the dark phase of the present species. The great length of the tail as com- 
pared with the body+ renders it almost certain that it can refer to no other 
*“Subtus ruto-ferrugineus, frequenter pilis nigris intermixtis, mento nigrescente.”—(Zev. et 
Mag. de Zool. 1861, p. 5.) 
t According to Bennett, head and body 10.50; tail-vertebrax 10.50; tail to end of hairs 14.00; or, 
according to Bachman’s measurements of the same specimen, head and bedy 12.37; tail to end of 
hairs 15,37, 
