3 ( J2 MONOGRAPHS OP NORTH AMERICAN RODENTIA. 



the long, light-tipped hairs than is commonly seen in eastern specimens. 

 The mure tangible difference consists in the relatively larger size of the nasals, 

 which nut only are rather lunger, 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 in 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. dorsal us 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 LOG to 1.50, and invar. 

 epixanthus from 1.20 tu 1.75. In var. dorsatus, they exceed 1.33 in une speci- 

 men only, and 1.30 in only three, while in five examples they fall below 1.15; 

 in var. epixanthus, three only fall beluvv 1.30, while five attain or exceed 1.60. 



The series of skulls of var. epixanthus 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 uld skulls, the variation ranges from 3.85 to 4.40 in length, and 

 from 2.80 to 3.08 in width ; nasals, 1.38 to 1.75. In eight middle-aged skulls, 

 the variation ranges from 3.30 to 3.7 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 uf 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 Yukon measure only 3.98 by 2>2, against 4.17 by 2>9 for six 

 skulls of corresponding age from the United States (Muntana, Idaho, and 

 Wyoming). Five middle-aged skulls from the Yukon and Peel's Rivers average 

 3.64 in length, while 'Cisc skulls from Maine (var. dorsatus) average 3.73. 



GENERAL REMARKS ON EEETHIZ0N DORSATUS AND ITS VARIETIES. 



Geographical distribution. — The Eastern or Canada Porcupine was 

 formerly found throughout the greater part of New England and New York 



