198 PLANORBIS. 



Subgenus HELISOMA, Swainson. 



jA. Species inhabiting the Pacific States. 



12. PLANORBIS AMMON, Gould. 



Plate 6, figs. 3, 4, 5. 



PL TrasJcii, Lea. Plate 6, fig. 6. 



Shell large, discoidal, subconic, finely or 

 coarsely striate ; left side broadly and deeply 

 concave, showing four obtusely carinated 

 whorls; right side concave, showing two-and 

 a-half rounded whorls ; aperture ovate-trian- 

 gular, expanded on each side when fully 

 grown. 



Diam. 25, alt. 15 mill. 



California, Oregon, Washington Territory, 



The specimens at different stages of growth 

 vary greatly in the development of the aper- 

 ture ; but all agree in the peculiar slope of the 

 outer volution, giving them a conical or dome- 

 shaped form when lying on the left side. 

 (45) 



