516 bulletin: museum of comparative zoology. 



the lack of the small postero-internal lobe in the tAvo anterior upper 

 cheek-teeth (Plate 1, fig. 1). This character, Avhich seems first to 

 have been pointed out by INIajor (1872), sharply separates hudsonius 

 from other American species. The posterior lower molar (Plate 1, 

 fig. 6) normally lacks the small antero-internal enamel fold that is 

 present in the other continental species, so that there are but two 

 closed spaces on each side. 



Measurements. — Six adults from Labrador average as follows : — 

 total length 148 mm., tail 20.3, hind foot 22.1. The largest speci- 

 men, a female from Xain, Labrador, measures 160, 22, 24 for these 

 three dimensions. 



The skull of the latter (Carnegie Mus. 1,124) measures: greatest 

 length 32 mm., basal length 30.5, palatal length 18.7, nasals (median 

 length) 10, zygomatic width 21.3, mastoid width 16, interorbital 

 width 4.8, upper tooth-row (alveoli) 8.6, lower tooth-row (alveoli) 7.8. 



Molt. — A certain amount of individual variation in the time of 

 molt is to be expected, and there is also variation in progress of the 

 change in different parts of the bod}'. In the series studied, an April 

 5th specimen from the Belcher Islands, Hudson Bay, is beginning to 

 show a yellowish wash over the head, lower back, and rump, where 

 the new hairs with their buffy subterminal rings appear among the 

 old white hairs of winter. A thin sprinkling of new black hair-tips 

 darkens these parts slightly, while at the nape they form a narrow 

 stripe. The enlarged fore claws have disappeared. An adult from 

 Great Whale River, Hudson Bay, April 20th, still retains the enlarged 

 fore claws, but shows a dark forehead and a blackish line from the 

 nape almost to the rump, with a buiTy appearance of new hairs in a 

 narrow area over the shoulders on either side. Elsewhere, the white 

 coat of winter is retained. In a specimen from Xastapoka River, 

 May 8th, tiie long white winter pelage is retained in small tufts behind 

 each ear, on the sides of the abdomen, the hips, and rump, and below 

 in a narrow band across the chest. Scattered white hairs elsewhere 

 give a gray tint to the head, neck, and fore shoulders, but in the 

 center of the back is a small oval patch of the fully developed brown- 

 ish summer coat. The dark line down the back is not present except 

 in the center of this patch. A second skin from the same locality, 

 June 1st, retains a little patch of long white hair just behind each 

 shoulder, on the left hip, and on the rump and lower leg. Elsewhere 

 the summer pelage is nearly matured, and the enlarged fore claws 

 are nearly ready to fall away. The retention of the winter pelage 



