r 



The Nautilus. 



Voj.. XV. JULY, 1901. No. 3. 



THE AMEEICA PHYSJE. 



BY O. A. CRAXDALL, SEDALIA, MO. 



The members of this genus are the most beautiful shells of 

 all of our fresh-water snails, and, were they properly understood, 

 would be the most sought for by collectors. It lias been the custom 

 of authors to attempt to establish species upon slight variations in 

 form until the multiplicity of varieties and synonyms lias caused 

 such confusion that even our most experienced conchologists hesitate 

 to venture an opinion as to many of the described species, while 

 amateurs, being unable to obtain satisfactory information upon which! 

 to correctly label their "finds," have become so careless about col- 

 lecting them that they are rarely offered for exchange. 



To remedy this evil, and as far as possible to simplify the deter- 

 mination of species, I have given my spare time for several years 

 past to the study of this genus, and now give to the public the result 

 of my observations. 



In determining the species of this genus it is not only necessary to 

 observe the characteristic form of the shell, but something of its 

 life-history must also be known. Some species grow to maturity in 

 one season, while others require two or more seasons of growth to 

 mature. 



In central Missouri, P. gyrina Say is the first to appear in the 

 spring and the last to disappear in the fall. I have seen them 

 crawling on the ice in March and depositing the ova as late as the 

 first of October. They begin depositing eggs early in May and con- 

 tinue throughout the season ; and as the young are continually 



