HYSTRICIDH—ERETHIZON—E. DORSATUS VAR. EPIXANTHUS. 391 
As usual, skulls of the same general size vary greatly in the relative size 
of different parts, and consequently in general form. Thus Nos 821and 815 
(M. C. Z. Coll.), measuring respectively 3.40 and 3.90 in length, have each a 
breadth of 2.60, while another specimen (No. 823, M. C. Z. Coll.), with the 
same length as No. 821, has a breadth of only 2.35. Specimens Nos. 815 
and 816 (M. C. Z. Coll.), of essentially the same general size as respects 
length and breadth, vary remarkably in the form of the body of the skull, 
as shown by the following measurements : 
p} Fi a 
3a . = 
= a 2 
oe =] aw 
Samet oy ce IS 
ro e - a i) oe 
2° a Ss a 
rs) S p= oO we) ast 
i= ms; S a) ep = A 
Beil eeeia| Geer se Pr Alaae 2 
A eh My efes eGeaal| ees E 
8i5| 3.90| 260] 0.85] 1. 0.70 
a6] 3.95) 270] 1.08 t 0. 82 
No. 816 is a rather short, broad, stout skull, while No. 815 is remark- 
ably narrow and attenuated, with an unusually great expansion of the zygo- 
matic arches. While the total breadth differs but little, the body of the 
skull in 815 is fully one-eighth narrower than in 816, with a correspondingly 
less capacity. This is indicated by the lesser interorbital breadth and narrower 
nasals in 815, as is shown by the above given comparative measurements. 
The measurements, however, inadequately express the great contrast in form 
between these two examples—a difference that might ordinarily be regarded 
as sufficient to indicate well-marked specific diversity. Both are from the 
same locality, Oxford County, Maine. 
Var. EPIXANTHUS. 
Western . Porcupine. 
Similar in external appearance to var. dorsatus, but with the light tips of 
the long hairs pale greenish-yellow rather than yellowish-white. Nasals 
broader and longer. 
The Porcupines from the region west of the Missouri River differ ex- 
ternally from those of the eastern half of the continent almost wholly through 
a slight difference in the color of the lighter portions of the long hairs of the 
dorsal surface, which have a greenish-yellow tint instead of yellowish-white. 
The few examples before me of this variety have also a greater profusion of 
