﻿July, 1896.] 



KEYS TO GENERA. 



29 



may be called a key — is a rough groiipiug of the subgenera of 3[icrotus 

 according to the essential characters used in the classification here 

 adopted. The keys are in all cases based on the characters of adults 

 only. 



1. KEY TO THE GENEEA OF MICKOTIN^. 



[Based primarily on tlie skull.] 



Skull of adult more than 50 mm. long FWer 



Skull of adult less than 45 mm. long. 



Molars rooted; skull always less than 30 mm, long. 



Posterior border of palate a thin-edged shelf, continuous between 



alveoli of posterior molars JEvotomijs 



Posterior border of palate not forming a shelf Pkenacomys 



Molars rootless; skull often more than 30 mm, long. 



Middle part of zygoma expanded so as to form an oblique 



plate about 4 mm. broad Lemmus 



Middle part of zygoma only slightly expanded. 



Rostrum about ^ total length of skull Synaptomys 



Rostrum more than total length of skull, 



Postorbital process of squamosal peg-like Dicrostonyx 



Postorbital process of squamosal shelf-like Microlus 



2. KEY TO THE GENERA OE MICROTIX^. 



[Based primarily on tlie teeth.] 



Length of maxillary tooth row in adult more than 14 mm Fiber 



Length of maxillary tooth row in adult less than 13 mm. 



Roots of lower incisors ob inner (lingual) side of molar roots. 



Upper incisors grooved Syna2)iomys 



Upper incisors not grooped. 



m 1 with 3 closed triangles Lemmus 



m 1 with 7 closed triangles Dicrostonyx 



Roots of lower incisors on outer side of molar roots. 

 Molars rooted. 



Teeth weak; triangles tending to remain open; salient angles 



rounded Evotomys 



Teeth strong; triangles closed; salient angles sharp Pkenacomys 



Molars rootless Microtiis 



3. KEY TO THE GENERA OF MICROTIN^. 



[Based primarily on external characters.] 

 Tail flattened laterally : Fiber 



Tail terete. 



Tail shorter than hind foot. 



Thumb with strap-shaped nail Lemmus 



Thumb without strap-shaped nail. 



External ear rudimentary Dicrostonyx 



External ear well developed Microtus (Asiatic species 



of subgenus Lagurus) 



Tail longer than hind foot. 



Upper incisors grooved Synaptomys 



Upper incisors not grooved. 



Color usually reddish; molars weak, with triangles tending 



to remain open and with salient angles rounded Evotomys 



Color brownish, grayish, or yellowish; very seldom reddish; 

 molars strong, with closed triangles and sharp salient angles. 



Molars rooted Phenacomys 



Molars rootless Microtus 



