OF NORTH AMERICA. 



279 



and from Mackenzie Territory and Quebec south to Arizona and 

 northern Mexico. 



Obrussa covers a wide range, including all of North America 

 excepting the Labradorian, Yukonian, Alaskan, Central American and 

 Carolinian regions. It is well distributed throughout the Boreal, Tran- 

 sition and Upper and Lower Austral life zones. Like Galba humilis 

 modicella, it is absent from that portion of the Lower Austral (Austro- 

 riparian) which borders the Atlantic Ocean and the Gulf of Mexico. 

 It ranges from the sea level to a height of some 10,000 feet in Colo- 

 rado. It is quite probable that additional records will greatly extend, 

 besides adding to, the records from British America. Records are also 

 needed from the area of the Great Plains. 



Geological 



Distribution 



OBRUSSA/ 



FIG. 29. 



The range of this species is remarkable in that it covers a terri- 

 tory 35 degrees long and about 60 degrees wide. It is evident that 

 the species has taken advantage of all the post-glacial water connec- 

 tions to extend its range. Next to palustris, it is the most universally 

 distributed of the American Lymnseas. 



GEOLOGICAL RANGE (Figure 29) : Pliocene to Pleistocene. As 

 in the case of Galba humilis modicella, the records of the occurrence 

 of this species in geological strata are not numerous enough to indicate 

 with any degree of exactness the past distribution of obrussa. Records 

 from the Loess give it a wide range in northern Illinois, Iowa and 

 Nebraska, while the marl records include the territory from Maine 

 to Wisconsin. The marl records are in the Alleghanian division of 



