GENUS FIBER 149 



cisor extending 1 back to third molar, displacing the base of that 

 tooth ; molars not rooted, the crowns with irregular triangles or 

 loops ; soles of feet with four to LUX tubercles. 



Dental formula I. 1/1; M. 3/3=16. 

 Key to Species of Microtus. 



1. Total length more than 5.50 inches; tail vertebrae more 



than 1.10 inches long 2 



Total length less than 5.50 inches; tail vertebrae 1.00 inch 

 or less long; upper parts dull chestnut; under parts 

 plumbeous tinged with cinnamon rufous ; mammae 4 



Woodland Vole. 

 Microtus pinetorwm nemoralis. p. 91 



2. Tail usually less than 1.60 inches long; five tubercles on 

 soles of feet ; upper parts mixed light and dark brown ; 

 under parts buffy gray, or pale brownish gray ; mammae 

 6 



Prairie Meadow Mouse. 

 Microtus ochrogaster ochrogaster. p. 87 



Tail usually more than 1.60 inches long; six tubercles on 

 soles of feet; upper parts brownish, but without the 

 ' ' grizzled ' ' appearance of M . o. ochrogaster; under parts 

 grayish white, sometimes tinged with brown ; mammas 8. 



Meadow Mouse. 

 Microtus pennsylv (miicus pennsylvamcus. p. 78 



Genus Fiber Cuvier. 



Size large, form stout; legs short, feet large and modified for 

 swimming; toes of hind feet partly webbed; tail long, verti- 

 cally compressed, covered by thin scales, and scantily haired; ex- 

 ternal ears small, almost hidden in the fur, which is dense and 

 waterproof; a series of long, glossy overlying hairs intermingled 

 with the shorter pelage; well developed perineal glands secreting 

 a strong odor; mammaa 6; five tubercles on soles of feet; upper 

 incisors without grooves; molars rooted; auditory bullae large, the 

 edges protruding. 



