﻿38 



NORTH AMERICAN FAUNA. 



[No. 12. 



Genus DICROSTONYX Gloger. 



1830. Cuniculus Wagler, Nat. Syst. cl. Ampbibien, p. 31, 1830 (part). 

 1877. Cu)nculus Coues, Mouogr, N. Am. Rodentia, p. 243, 1877. 



1841. Dicrostonyx Gloger, Gemeiun. Hand- u. Hilfsbnch d. Naturgescb., pp. XXXI, 97^ 

 1841. Type, an American species, probably Mas hudsonlns Pall. 



1854. " Myolemmus Pomel, Ann. Sci. Soc. Auvergne, 1854" (fide Trouessart). 



1855. Misothermus Hensel, Zeitscbr. der Deutscb. geolog. Gesellscb., VII, p. 492, 1855. 



Type Myodes torquatus Pall. 

 1881. Borioilon Polyakoff, Mem. Acad. Imp. Sci. St. Petersbourg, XXXIX, suppl. p. 34^ 

 1881. Type Myodes torquatus Pall. 



Geographic distribution of type species. — Arctic America. 

 Geographic distribution of genus. — Arctic region in both hemispheres. 

 Essential characters: 



Upper incisors witbout grooves. 



Lower incisors witb roots on inner (lingual) side of molars. 

 Molars rootless. 



Enamel pattern cbaracterized by approximate equality of reentrant angles. 



m 1 witb 7 closed triangles and 2 transverse loops. 



m 3 witb 3 or 4 closed triangles and 2 transverse loops. 



Feet bigbly modified. 



Palms smootb; soles witb rudimentary tubercles. 

 Thumb witb a rudimentary nail. 

 Tail sborter tban bind foot, terete. 

 External ear rudimentary. 



Skull. — The sknll of Dicrostonyx (PI. I, iig. 14) in a general way 

 resembles that of Lemmus, but is smaller and more lightly built. The 

 zygomata are less broadly flaring and the expansion near the middle is 

 comparatively slight. The outer face of the expanded portion, as in 

 Lemmus, is strongly oblique. The rostrum is also lighter and more 

 slender. While the pterygoids are iDroportionally longer than in Lem- 

 7nuSj the posterior edge of the bony palate is formed exactly as in the 

 latter (PI. II, figs. 12 and 14). The anterior edge of the squamosal 



gives off a conspicuous peg-shaped postorbital 

 process very different from the postorbital proc- 

 ess in Lemmus or any of the other Microtinw, 



These pegs are especially conspicuous when the 

 sknll is viewed from the ventral aspect. 



YiG. 13. -Enamel pattern of Tccth. — luclsors cssentially as iu Lemmus. 



molar teetli, Dicrostonyx from MolarS rOOtlCSS. Pattern of CUamcl folding (fig, 



Ungava, Labrador, (x 5.) vcry different from that of either of the other 



genera of Lemmi and in some respects resembling that of the Microti. 

 The reentrant angles on the opposite sides of the teeth are approxi- 

 mately equal in depth, 'thus producing closed triangles of nearly the 

 same size on the two sides. The first lower molar contains seven closed 

 triangles in addition to a transverse loop at each end. The second 

 lower molar contains a posterior loop followed by four alternating 

 closed triangles and an anterior transverse loop, which is much flattened 



