TUT AS LAKES 



197 



The next great lacustrine period began with the 

 Permian and continued through the Trias. During 

 this interval extensive lakes covered a large part of 

 our northern continents : some were evidently salt, but 



FIG. 68. Air-breathing Shells from the Carboniferous 

 System in North America. From White, " Non- 

 marine Fossil Mollusca." 



1 and 2. Pupa vetusta, Dawson, after Dawson, from 

 the coal measures of South Joggins, Nova Scotia. 



3. Dawsonella Meeki, Bradley, after Whitfield, regarded 

 by Dawson as allied to Helix, but according to 

 Whitfield it approaches Helicina. 



4 and 5. Z writes priscus, Dawson, found in association 

 with Pupa vetusta, and referred to the Arionidae. 

 After Dawson. 



presumably not all. Unfortunately, except in Texas, 

 where four species of Unio have been described from 

 the Trias, these have not left behind sediments con- 

 taining freshwater fossils. But in the immediately 



