72 Osgood — A New Vole from Montague Island, Alaska. 



about as in M. innuitus ; braincase narrower; rostrum shorter; width 

 across lacrymal processes of frontal greater; audital bullae relatively 

 smaller; upper incisors less projecting anteriorly. 



Meosurcmenls. — The type and two topotypes, respectively : Total length, 

 201 ; 191; 180; tail vertebrae, 40 ; 40; 35; hind foot (dry), 23.5; 23; 22. 

 Skull oftype: Basal length, 31.1 ; basilar length, 29.2 ; postpalatilar length, 

 12.1; zygomatic width, 18.6; mastoid width, 13.8 ; length of nasals, 9.2 ; 

 interorbital constriction, 3.9 ; maxillary toothrow, 7.4. 



Rnnarks. — This insular form differs from its mainland relatives chiefly 

 in decidedly larger size. It belongs to the so-called '' operarius group" 

 which properly includes, besides a number of Alaskan forms, several of 

 wide distribution in Eurasia. Representatives of this group doubtle.ss en- 

 tered Alaska from Asia at a time not very remote, for although a number 

 of Alaskan forms are now diflferentiated, all are very closely allied and 

 none show any marked departure from the Asiatic forms. The one here 

 described seems as worthy of specific rank as any of the others but the 

 amount of cranial variation in all the forms and the general uniformity 

 of coloration leads one to believe that they might well be ranked as sub- 

 species. If this were done, however, M. oeconomus, M. kamschaticus, and 

 probably M. ratticeps ought to be included as they difTer from M. operarius 

 and other Alaskan forms only very slightly. 



Skulls of Microlus operarius Group. 



99,373 — M. innuitus. Type. 98,991 — M. operarius. Topotype. 



98,005 — M. yakutatensis. Type. 107,472 — M. unalascensis. Topotype. 



137,323— J»/. elyrnocetes. Type. 



