HYSTRICID.E— ERETHIZON— E. DORS AT DS VAR. EPIXANTEUS. 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 821 and 815 

 (M. C. Z. Coll.), measuring respectively 3.40 and 3. 'JO 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: 



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 inform 

 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. EPJXANTHUS. 



Western Porcupine. 



Similar in external appearance to var. dorsalus, 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 



