390 MONOGRAPHS OF NORTH AMERICAN RODENTIA, 
the base of the tail; toward the end and on the sides of the tail, they pass 
again into long, thick bristles and stiff hairs. The lower surface of the body 
is clothed merely with brownish-black hair, scantily mixed with fur. The 
long, white-tipped hairs are generally most abundant on the head and the 
sides of the neck and shoulders and hips, disappearing entirely over the mid- 
dle of the belly. 
Different individuals vary greatly in respect to the abundance of the long 
light-tipped hairs, they being most abundant in the younger animals, in which 
they often give a decidedly whitish cast to the general color. In one specimen 
(No. 1309, M. C. Z. Coll.), the long bristly hairs are black, tipped occasionally 
with whitish. The pelage is very long and full, through which very few of the 
quills are visible. In another specimen (No. 5038, M. C. Z. Coll.), the exposed 
portion of the long bristly hairs is generally yellowish-white. » In other speci- 
mens, there is a mixture of the wholly black, bristly hairs with those that are 
whitish-tipped. In young and full-pelaged specimens, the quills are generally 
entirely concealed; in the majority of the examples before me, they are visible 
on the head, sides of the neck, hinder part of the back, and the basal portion 
of the tail. In old specimens, in worn pelage, the spines are visible over 
most of the dorsal surface. The under-fur varies in different specimens from 
brownish-black to grayish. The young are born without quills, and of a 
uniform black color. 
Full-grown specimens average about 35 to 40 inches in total length; the 
head about 6; tail (to end of vertebrae) about the same. Seventeen skulls, 
all of which have attained mature dentition, average 3.82 in length and 2.69 
in breadth; nasals, 1.21; muzzle (from anterior border of intermaxillaries to 
the first molar), 1.29. As usual, there isa considerable range of variation 
in size, in color, and in the proportion of parts in specimens of corresponding 
ages. The few specimens in which the sex is known seem to indicate little, 
if any, strictly sexual variation in either size or color. Seven very old skulls 
vary in size as follows: length, 3.90 to 4.25; breadth, 2.60 to 3.00; nasals, 
1.20 to 1.50; muzzle, 1.25 to 1.60. Three middle-aged skulls vary in length 
from 3.55 to 3.90; in breadth from 2.50 to 2.80; nasals from 1.12 to 1.33 ; 
muzzle from 1.12 to 1.30. Five younger skulls range in length from 3.40 to 
3.65; in breadth from 2.35 to 2.60; nasals from 1.06 to 1.15; muzzle from 
1.00 to 1.22. The largest skull measures 4.25 by 3.00; the smallest, 3.40 by 
2.35. The nasals vary from 1.06 to 1.50 (in the very old skulls from 1.20 to 
1.50); the muzzle from 1.00 to 1.62 (in the very old skulls from 1.25 to 1.62). 
