CHAPTER XIII 



SHELLS OF LAKES AND STREAMS 



By Harold Hannibal 



The tresh- water mollusks include both Gastero- 

 pods and Pelecypods. They may be distinguished 

 from most other shells by having a greenish or 

 brownish horny epidermis covering the shell, to pro- 

 tect it from the corrosive acids in the waters in which 

 they live. Unlike the land and marine shells, they 

 have been little studied, and many interesting 

 things can be learned by anyone who takes the 

 trouble to observe them. Most of the species are 

 not common or are found at but tew localities. For 

 this reason the writer has selected for description 

 only some of the more wide-spread and particularly 

 interesting forms, but representing nearly all the 

 genera. 



For convenience the distribution is given by drain- 

 age basins or systems. The Columbia System in- 

 cludes the entire Columbia, Fraser, and Umpqua 

 Basins. The Utah System includes the eastern por- 

 tion of the Great Basin which drains into the former 

 Lake Bonneville. The Nevada System includes the 

 western portion of the Great Basin draining into 

 the former Lake Lahontan, and the Owens Basin and 

 Mojave Desert in California. The Klamath System 

 includes the Klamath and Rogue Basins, the Sierras 



