140 MICROTIN^ 



reside during the cold season, as compared with its freer and more 

 active mode of life in summer." 



Hind-feet remarkably broad, the width across the bases of 

 the digits equal to about one-half the length, all the toes being 

 rather short; hallux well developed, scarcely shorter than 

 digit V ; , digits II, III and IV longer and subequal, III being, 

 however, slightly longer than its neighbours ; all, including the 

 hallux, bear long, slender, curved, and sharply pointed claws, 

 which equal or surpass their digits in length. 



In both hands and feet the claws are covered and protected 

 by long hair growing from the dorsum and sides of each paw and 

 its digits; these hairs always project beyond the claw tips, the 

 shorter summer hairs being replaced by a longer growth in the 

 winter, which keeps pace with the increasing length of the claws. 

 Palms and especially the soles densely clad with long crisp hair, 

 which on each sole forms a stiff curling brush quite comparable 

 with that developed upon the foot of a hare. When the hairy 

 covering is clipped away the palms are seen to be quite smooth, 

 bearing, apart from the great pollical tubercle, no trace of the 

 pads present in normal Microtinse. Normally too, judging from 

 a specimen from Discovery Bay specially examined by me and 

 from two studied by TuUberg,^ no trace of the pads remains 

 upon the sole ; but Miller ^ states that he found " several minute, 

 faintly developed tubercles near the base of the toes." Mammae, 

 in adult females, 2 — 2 = 8. 



Colour in summer brownish or greyish above, whitish or 

 yellowish below; often with brilliant rufous suffusions upon the 

 fore-quarters and throat. In winter pure white above and below, 

 Dicrostonyx being the only genus among Simplicidentata that 

 shows the phenomenon of winter whitening completely developed.^ 

 Young like adults in summer pelage, but duller, and with a dark 

 stripe along the spine. This stripe, in some forms, persists also 

 in the adult summer pelage. 



Hair, with the exception of the special hairs (which are 

 whitish throughout) clothing the tail and feet, everywhere with 

 slaty bases, the colour, both in summer and in winter pelage, 

 being produced by the terminal parts of the hairs. In the 

 summer coat the hairs are shorter, 15-18 mm. long on the 

 back, and their terminal portions have from one to three diversely 

 coloured annulations, which vary both in number and hue with 

 the species or subspecies and with their precise situation upon 

 the body. In winter the hairs are longer, about 25 mm. on the 

 back, with pure white tips. The change is made by two moults, 

 one in autumn, the other in spring; it is brought about by 

 the rapid growth of new hairs and the more gradual shedding 



'^ TuLLBERG, Ueber das System der Nagetiere, p. 257, 1899. 

 " Miller, N. Amer. Fauna, No. 12, 1896, p. 37. 



^ In Lemmus obensis novosibiricus winter whitening takes place also i 

 (see p. 203). \ 



