362 PROCEEDINGS OF THE ACADEMY OF [1896. 



posed outer faces. Upper incisors rooted ou the inferior bases 

 of the premaxillaries. Diameter of upper incisor wider than 

 deep, its face strongly and broadly grooved. 



la. Nasals Compressed and narrowed anteriorly ; bony palate 

 longer than width of postpalatal fossa ; narrow incisive 

 foramina terminating opposite anterior alveolus of pm. 1 ; 

 narrow premaxillary process falling short of base of 

 nasal ; breadth of rostrum ojjposite bases of pm. T shorter 

 than distance from alveolus of W^_\ to alveolus of poste- 

 rior incisor; total length of adult skull never exceeding 

 100 mm. (95 to 99 mm.), molars narrow, rounded — 



arcticns. 

 lb. Similar to la — hangd. 



Ic. Nasals broad, equilateral, flattened ; bony palate shorter 

 than width of postpalatal fossa ; the wide incisive fora- 

 mina reaching nearly opposite base of P5L^2; broad pre- 

 maxillary process reaching to or beyond base of nasal ; 

 breadth of rostrum equal to or greater than distance be- 

 tween alveolus of pm. 1 and the base of corresponding 

 secondary incisor ; total length of adult skull always ex- 

 ceeding 100 mm. (101 to 115 mm.) ; molars very broad 

 and angular — tsehuktschorum. 



II. Jaws prognathous ; upper and lower incisors meeting at angles 

 of 35 to 50 degrees. Upper incisors rooted on the anterior 

 floor of the maxiliaries. Diameter of upper incisor deeper than 

 wide, its slender sulcus filled with a functional, indurated, stri- 

 ate cementum approaching the consistency of enamelled dentine 

 at the cutting edge. 



2a. Nasals compressed and narrowed anteriorly ; bony palate 

 shorter than width of postpalatal fossa ; incisive foramina 

 reaching opposite anterior alveolus of pm J ; narrow pre- 

 maxillary process falling short of base of nasal ; breadth 

 of rostrum opposite bases of pm. 1 equal to or shorter than 

 distance between the base of pm. 1 and the apex of the 

 incisive foramina ; total length of adult skull exceeding 

 100 mm. ; molars broad, angular, very massive as com- 

 pared with slender incisors — grcenlandicus. 



External characters. 



I. Size medium, length of hind foot \h times that of ear from 

 crown. Tail always white. Upjier body fur in summer, dark 

 tawny gray to nearly pure white. 



