190S] Jackson, List of Wisconsin Mammals. 23 
Microtus ochrogaster (Wagner). 
Prairie Meadow Mouse. 
Microtus ochrogaster is not so generally distributed as Mic- 
rotus pennsylvaniciis; our present knowledge limits its distribu- 
tion in the state to the extreme southern and western counties, 
where it is locally common in upland grassy meadows and corn- 
fields. 
This species is often mistaken for the more common Microtus 
pennsylvanicus; in summer pelage the species are very similar, 
and a study of cranial characteristics is generally necessary; in 
winter pelage adult specimens of ochrogaster can always be dis- 
tinguished from pennsylvanicus by the cinnamon color of the ven- 
tral portions of the former. Immature specimens are more diffi- 
cult to determine. 
Fiber zibethicus (Linnaeus). 
Muskrat. 
Muskrats are distributed over the entire state wherever there 
are streams with clay or loam banks, and, more abundantly, around 
the lakes and the open swamps and marshes. The familiar musk- 
rat houses are much more common in the southern part of the state 
than in the northern regions, but this does not necessarily indicate 
that the species is less plentiful in the northern sections; where 
sedge, rushes and flags are found the animals usually build houses 
in which to dwell, but where vegetation is lacking muskrats make 
their homes in holes in the banks of streams and lakes. It is 
generally believed that the large houses are used for winter head- 
quarters, but from my own observations, and from what I can 
learn from old trappers, I am inclined to believe that these are 
chiefly summer houses, the smaller mud or sod houses being used 
for winter homes. 
