^Mi\k 



392 



MONOOKAPnS OF NOUTII AiMBllIUAN ItODENTIA. 



tlic long, liglit-li|»|)C(l luiirs than is commonly seen in eustcrn specinK-ns. 

 The more tangihle (lillcrenec consists in the relative!)' larger size of the nasals, 

 which uot only are rather longer, as pointed out by Professor Baird, but arc 

 also broader, and conseiiitently larger. While the general size of the skull 

 is the same in the two fernis, the nasals in var. cpixanthux averagii 1.50 in 

 length, against l."Jl in var. r/or«(/tf,v, and U.8S in width at the anterior end 

 against 0.75 for the .xame measurement in var. ilorsutus. In var. ejn.atnthus, the 

 average length of the nasals in tburtecn skulls exceeds the interorbital breadth 

 of the skull, being a little over one-third (0.3G) of the length of the skull; in 

 var. duitiulus their average length in seventetui skulls is only eciiud to the 

 interorbital breadth of tliQ skull, or rather less than one-third (032) the length 

 of the skull. Ditlerent individuals, however, vary greatly in respect to the 

 length ofilie nasals, in var. </or«tf/MA' they ranging from l.OtJ to 1.50, and invar. 

 ejiixanthu.s from 1.20 to 1.75. In \iiv. tlursatiin, they exceed l.'d'd in one speci- 

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

 in var. cj/ixanthus, three oidy fall below 1.30, while five attain or exceed l.GO. 



The series of skulls of siw.epixantkus presents about the same degree of 

 individual variation as already noted under var. dorsatu>t, 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. -10 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 to3.'i0in length, and from 2.47 to 2.87 in 

 brea<lth ; 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 (juestion present no material dilference 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.S2, against 4.17 by 2.^i) for six 

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

 Wyoming). Five niiddle-age<l skulls from the Yukon and Peel's Rivers average 

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



GENEUAL REMARKS ON ERETHIZON DORSATUS AND ITS V.vRIETIES. 



Geographical distribution. — The Eastern or Canada Porcupine was 

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



