LEMMUS 615 



without meatal valve ; skull very broad and low, the zygoma 

 conspicuously expanded at middle, the mesopterygoid space short 

 but continued forward over (under when skull is examined from 

 below) posterior margin of palate ; lower incisor with root lying 

 entirely on lingual side of cheek-teeth; molars rootless, the 

 enamel pattern characterized by great depth of outer re-entrant 

 angles in maxillary teeth, the points of these angles extending 

 practically to inner margin of crown ; m 3 with four transverse 

 loops and no closed triangles ; m 3 with three transverse loops and 

 one closed triangle. 



Remarks. — Except for the primitive condition of the lower 

 incisors characteristic of the lemmings as a group, the members 

 of the genus Lemmus are probably more specialized than those of 

 any of the related genera. In Dierostonyx* the ear is more 

 reduced and the claws reach their maximum of development ; 

 but the skull and molars are distinctly more generalized in form 

 and structure. Two species of Lemmus are now known to 

 inhabit the Old World, while several other forms have been 

 described from North America. Only one occurs in Europe, 

 west of Russia. 



LEMMUS LEMMUS Linnseus. 



1758. [Mus] lemmus Linnaeus, Syst. Nat., i, 10th ed., p. 59 (Sweden). 



1808. L[emmus] lemmus Tiedemann, Zoologie, I, p. 474. 



1820. Lemmus boreaMs Nilsson, Skand. Faun., i, p. 185 (Substitute for 



lemmus). 

 1822. Lemmus norwegicus Desmarest, Mammalogie, pt. II, p. 287 (Norway). 

 1895. Myodes lemmus Collett, Christiania Vetenskabs, Selskabs Forhand- 



linger, 1895, No. 3, p. 3. 



* While no living member of the genus Dierostonyx is now known to 

 occur in Europe west of the Gulf of Finland, sub-fossil remains are found 

 in many parts of central and western Europe associated with those of 

 Lemmus. Such specimens of the two genera may be distinguished as 

 follows : — 



Lemmus. — Skull very broad and low ; zygomata greatly expanded at 

 middle ; temporal ridges early uniting to form a knife-like crest in inter- 

 orbital region ; postorbital process shelf-like ; auditory bullse very large, 

 their substance conspicuously spongy ; a large foramen below and behind 

 alveolus of m 3 ; tooth-rows noticeably converging anteriorly ; upper molars 

 with outer re-entrant angles conspicuously deeper than inner ; to 2 , m 3 , and 

 m 3 with one or more transverse loops in addition to the terminal loop. 



Dierostonyx. — Skull moderately broad, not specially depressed; zygo- 

 mata not greatly expanded at middle ; temporal ridges never uniting, the 

 interorbital region with a noticeable longitudinal furrow; postorbital 

 process peg-like ; auditory bullse not unusually large, their substance scarcely 

 spongy ; no noticeable foramen below and behind alveolus of m 3 ; tooth- 

 rows nearly parallel ; upper molars with outer and inner re-entrant angles 

 approximately equal ; to 2 , to 3 and m 3 without transverse loops in addition 

 to the terminal loop. Externally Dierostonyx is distinguished from 

 Lemmus by the reduction of the ear to a low rim, the absence of any well 

 developed nail on thumb, and the bifurcate form of claws on third and 

 fourth fingers in winter. 



