THE MOLLUSCA 



983 



(37) Shell sinistral; early whorls flattened and carinate above; base funic- 

 ular. . Section Pierosoma Dall. 



This group includes nearly all the larger North American Planorbes 

 and is represented by numerous species found in all parts of the country. 

 Type, Planorbis trivolvis Say, (Fig. 1428). 



FIG. 1428. 



39 (40) Shell sinistral; aperture campanulate; lip thickened. 



Subgenus Planorbella Haldeman. 



The typical form, Planorbis campanulatus Say, (Fig. 1429; X ii), is of 

 common occurrence and wide distribution in eastern Canada and the 



United States north of Tennessee. 



^ 



FIG. 1429. 



40 (41) Shell dextral, much depressed; upper surface convex, base flattened; 



body- whorl carinate; lip simple. 



Subgenus Tropidiscus Stein. 



A single species, Planorbis cultratus Orb., (Fig. 1430; X 2), of this (in 

 America) tropical group has been collected in Texas and Florida. 



FIG. 1430. 



41 (42) Shell small, dextral; periphery carinated; base convex; aperture 

 oblique; lip simple ...... Subgenus Hippuetis Agassiz. 



A group of small species of general distribution through the Northern 

 States and Canada. All of our species belong to the section Menetus H. 

 & A. Adams, of which the type is Planorbis opercularis Gld., (Fig. 1431; 

 X 3), from California. 



4 2 (35) Shell small, depressed; body- whorl rounded or obtusely angulated; 

 lip simple Subgenus Gyraulus Agassiz . . 43 



43 (44, 45) Surface spirally striate and hispid. 



Section Gyrai4us s.s. 



A few, small species, not uncommon in the eastern Northern States 

 and Canada. The group is not represented on the Pacific coast. Ex- 

 ample, Planorbis hirsutus Gld., (Fig. 1432; X 3). 



FIG. 1432. 



