﻿July, 1896.] 



GENUS DICROSTONYX. 



39 



^^^^^^ 



Fig. li. — Ear, (a) Dicrostonyx, (b) Lemmus 

 (double natural size' 



and so small that the tips of the salient angles do not reach to the level 

 of the tips of the other salient angles of the tooth. Occasionally the 

 anterior outer triangle opens into the transverse loop. The posterior 

 lower molar has a posterior transverse loop followed by three large 

 closed or nearly closed triangles (two on the inner side), and a fourth 

 smaller triangle on the outer side.^ The maxillary teeth have each a 

 large anterior loop. This is followed in the first hj five alternating 

 closed triangles and a small postero- 

 external loop, in the second by four 

 closed triangles and a small postero- 

 external loop, and in the third by four 

 closed triangles and a small rounded 

 terminal loop. 



External form. — In external form the 

 species of Bicrostonyx are even more 

 specialized than the members of the 

 genus Lemmus. As in the latter, the 

 head is very large, the tail is reduced to 

 a stub, shorter than the hind foot, and 

 the feet are highly modified for digging. 

 The external ears are, however, mere naked folds of integument lying 

 just behind the meatus (fig. 14 a). The fur is long and dense, much fis in 

 Lemmus. The palms and soles are densely furred, and the tail is pro- 

 vided with a stiff pencil of bristle-like hairs, longer than the tail vertebrae. 

 The hind feet are very broad, the breadth at base of toes being about 

 one-half length of foot.^ On the hind foot there are several minute, 

 faintly developed tubercles near the base of the toes. The palms are^ 



however, perfectly smooth. The claws on 

 the hind feet are large and well formed, 

 though in no way different from those of 

 Lem m us. Those on the front feet are very 

 highly modified, and present seasonal 

 changes in size and form unknown else- 

 where among the 2[icrotln(v. The thumb 

 (fig. 15) is greatly reduced in size. The 

 thumb nail is so small ^s readily to escape 

 notice, but the ball of the thumb projects 

 as a distinct tubercle, the surface of which 

 is covered with a thick layer of corneous tissue. The claws on the 

 second and fifth fingers are large, though not peculiar in form. The 

 two middle claws, on the contrary, while in summer not different 

 from those of Lemmus, are in winter very greatly enlarged (fig. 15), and 



Fig. 15.— Left front foot of tliree speci 

 mens of Dicrostonyx from Alaska, show 

 ing successive stages in tlie develop 

 ment of the claws (hair removed). 



^ In Dicrostonyx torqiiatus there is a niiuute supplemental anterior internal loop 

 wbicli is absent in the species that occurs in Labrador. 

 ■2 In Lemmus this breadth is only about one-third length of foot. 



