392 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
the long, light-tipped hairs than is commonly seen in eastern specimens. 
The more tangible difference consists in the relatively larger size of the nasals, 
which not only are rather longer, as pointed out by Professor Baird, but are 
also broader, and consequently larger. While the general size of the skull 
is the same in the two forms, the nasals in var. epixanthus average 1.50 in 
length, against 1.21 in var. dorsatus, and 0.88 in width at the anterior end 
against 0.75 for the same measurement in var. dorsatus. In var. epixanthus, the 
average length of the nasals iu fourteen skulls exceeds the interorbital breadth 
of the skull, being a little over one-third (0.36) of the length of the skull; in 
var. dorsatus their average length in seventeen skulls is only equal to the 
interorbital breadth of the skull, or rather less than one-third (0.32) the length 
of the skull. Different individuals, however, vary greatly in respect to the 
length of the nasals, in var. dorsatus they ranging from 1.06 to 1.50, and in var. 
epixanthus from 1.20 tv 1.75. In var. dorsatus, they exceed 1.33 in one speci- 
men ouly, and 1.30 in only three, while in five examples they fall below 1.15, 
in var. epixanthus, three only fall below 1.30, while five attain or exceed 1.60. 
The series of skulls of var. epizanthus presents about the same degree of 
individual variation as already noted under var. dorsatus, the largest example 
measuring 4.40 in length by 3.08 in breadth, and the smallest 3.30 by 2.47. 
In ten very old skulls, the variation ranges from 3.85 to 4.40 in length, and 
from 2.80 to 8.08 in width; nasals, 1.38 te 1.75. In eight middle-aged skulls, 
the variation ranges from 3.30 to 3.70 in length, and from 2.47 to 2.87 in 
breadth ; nasals, 1.20 to 1.45. In both varieties, the animal increases greatly 
in size after the mature dentition is acquired. Specimens of corresponding 
ages of the two varieties in question present no material difference in size. 
In respect to geographical variation in specimens strictly corresponding 
in age, those from the northern parts of the United States are considerably 
larger than those from the Yukon and Peel’s Rivers. Thus three very old 
skulls from the Yukhou measure only 3.98 by 2.42, against 4.17 by 2.+9 for six 
skulls of corresponding age from the United States (Montana, Idaho, and 
Wyoming). Five middle-aged skulls from the Yukon and Peel’s Rivers average 
3.64 in length, while five skulls from Maine (var. dorsatus) average 3.73. 
GENERAL REMARKS ON ERETHIZON DORSATUS AND JTS VARIETIES. 
GEOGRAPHICAL DISTRIBUTION.—The Eastern or Canada Poreupine was 
formerly found throughout the greater part of New England and New York 
