312 



WEST COAST SHELLS 



Fig. 295 



that there is but one species, Pkysa heterostropha^ 

 Say, the American Pocket-shell, Fig. 295, which 

 has slightly impressed sutures and a 

 curved pillar. In the south it is re- 

 placed by P. //. osculans^ Haldeman, 

 the Southern Physa, PI. Ill, Fig. 17, 

 which has a straight pillar and im- 

 pressed sutures. There are a great 

 many forms some of which are shown in PL I, Fig. 

 5, and PI. Ill, Fig. 16, 18. They are due to physi- 

 ological causes. In general specimens from streams 

 are slender, while those from lakes usually have a 

 shorter spire and more inflated whorls. They are 

 also liable to be distorted and develop shouldering, 

 irregularities of the whorls, and vertical costse. The 

 thickness of the shell varies a great deal, for it 

 depends on whether the snail obtains sufficient lime. 

 This species is found from Alaska to Central Mexico. 

 Heterostropha is found in the Columbia, Colorado, 

 Utah, Nevada and Klamath Systems, while osculans 

 occurs in the Colorado, Utah, Nevada, and Klamath 

 Systems, and farther south. Occasionally specimens 

 of each are found beyond their range. 



Aplexa hypnorum^ Linne, the Moss Physa, Fig- 

 ure 296, differs from the true Physas mostly 

 in anatomy. The shell is heavy and glossy, 

 with the spire equal in length to the aper- 

 ture, and the sutures not impressed. This 

 is a boreal species, occurring in Eurasia and 

 Canada and the Columbia, Utah, and 

 Colorado Systems. 



Fig. 296 



