Synopsis Frksii-Water Moli^usca 



Kig. 20 



Section PLANORBINA Haldeman, 1842. 



Shell like Planorhis, s. s., but vertically compressed, 

 with smaller and more numerous whorls and a very 

 oblique aperture. 

 Type : P. o!i7'aceus Spix. 

 Example: P. glahratns Say, fig. 20. 



Subgenus HELISOMA Swainson, 1840. 



Shell dextral or sinistral, few whorled, the whorls 

 carinate above and rapidly enlarging ; base funicular ; aperture suddenly ex- 

 panding and thickened. 



Key to the sections of Helisoma. 



1. Shell dextral, carinated above and below, spire and base 



funicular Helisoma s. s. 



2. Shell sinistral, early wliorls flattened and carinate above, 



base funicular Pierosonia. 



Section of HELISOMA s. s. 



Shell dextral, moderate size, few whorled, the whorls 

 carinated above and below and rapidly enlarging ; spire and 

 base funicular, aperture suddenly expanded, with a thick- 

 ened peritreme. 



Fig. 21 



Fig. 22 Fig. 23 



Type: P. bicarinatus Say, fig. 21. Radula, fig. 22. Animal, fig. 23. 



Section PIEROSOMA Dall, 1905. 



Shell sinistral, large, high, with few transversely sculptured whorls ; the 

 early whorls carinate and flattened above, funicular below ; in the adult the 

 flattened apex is usually depressed below the upper level of the body whorl ; 

 the aperture is suddenly expanded and somewhat thickened. 



Fig. 24 Fig. 26 



Type : P. trivolvCs Say, fig. 24. Radula, fig. 25. Animal, fig. 26. 



