512 bulletin: museum of comparative zoology. 



ent in color from the adults. At the other extreme of development is 

 the Alaskan Lemming, which has an additional enamel lobe on certain 

 of its molars, and in summer has in the adult a brilliantly variegated 

 pelage. It merges by insensible degrees into the subspecies richard- 

 soni, the type of which is from Fort Churchill, west coast of Hudson 

 Bay. This is a nearly uniform ruddy-gray race, quite different in 

 color from the Alaskan form. On the west, the animal of Unalaska 

 Island has been named as a distinct species and all too briefly diagnosed 

 from skulls, but it will probably be found to represent at most a sub- 

 species of the Alaskan Lemming. The lemming of St. LawTence 

 Island, Bering Sea, is here described as new. It is a gray, pall-id 

 member of the Alaskan Lemming group. The Greenland Lemming, 

 so far as present evidence goes, seems to be a species distinct from 

 any of its neighbors, though it may yet be found to intergrade with 

 the Alaskan species, of which it is perhaps a depauperate form. The 

 Labrador Lemming, on account of its less degree of specialization in 

 color and in tooth-structure, is possibly to be regarded as the most 

 ancestral of the living forms, and has probably been isolated for 

 a long period. 



For the loan of specimens, including the type of D. unalascensis, 

 I am particularly indebted to the U. S. Biological Survey and the 

 U. S. National Museum; thanks are also due to Dr. J. A. Allen of 

 the American Museum of Natural History; the Carnegie Museum of 

 Pittsburgh through Mr. 0. J. Murie; Dr. Joseph Grinnell of the 

 Museum of Vertebrate Zoology of the University of California; and 

 the Field Museum of Natural History. 



Generic and Subgeneric Characters. 



The genus Dicrostonyx has been sufficiently characterized by 

 Miller (1896, p. 38). It is an arvicoline, externally modified for 

 subterranean life by its short tail, much reduced ear conch, and power- 

 ful third and fourth claws of the fore feet. These claws become 

 singularly enlarged in winter, so as to appear double, by the great 

 enlargement of the ventral portion of the claw in a vertical direction. 

 The thumb is very greatly reduced. It is the onl}' known true rodent 

 (the hares and rabbits are now considered a separate order, Lago- 

 morpha) that becomes white in winter, an adaptation to its Arctic 

 habitat. 



The broad flat skull is characterized by a peculiar pointed process 



