102 Proceedings of the Biological Society of Washington. 



Lemmus yukonensis. Biological Survey collection, 5 skins with skulls. 



In the lack of a revisionary study of the genus, the original descriptions of 

 the various species remain our main source of published information, and 

 the several small series before me show certain disagreements with those 

 descriptions. The principal discrepancy lies in the characterization of 

 alascensis as of small size, whereas it appears to be the largest of the Alaskan 

 forms. This seems evident, both from externals and from the skulls, but 

 unfortunately collectors' measurements can not be tabulated to show the 

 differences. Measurements by different individuals, by different methods, 

 and with a proportion of evident mistakes, are factors that debar compari- 

 sons. The four Alaskan forms of Lemmus before me present the following 

 characteristics: 



Lemmus alascensis. Size large; skull large and massive; coloration bright 

 ruddy. 



Lemmus yukonensis. Size small; skull small but audita! bullse large in 

 proportion, as compared with alascensis; coloration bright ruddy. 



Lemmus nigripes. Size small; skull small with small audital bullae; 

 coloration dark brown. 



Lemmus harroldi. Size larger, nearly as in alascensis, and skuU most 

 nearly as in that form. Coloration dark brown. 



The above color comparisons are between specimens taken at the same 

 season and presumably in comparable condition. In the Nunivak Island 

 series there is a mummified specimen, a "pick-up," that is more ruddy than 

 the rest and may represent winter conditions. In the Pribilof Island series 

 two skins collected in February are more ruddy than the others, taken 

 from June to August. I have not seen Lemmus minusculus Osgood. In 

 the description thereof comparisons are all made with alascensis, and it is 

 not apparent how minusculus and yukonensis are to be distinguished. 



Meabitkembnts of Skulls. 



