THE MOLLUSCA 



983 



38 (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, Flanorbis Irivolvis Say, (Fig. 1428). 



Fig. 1428. 



39 (40) She ll sinistral; aperture campanulate; Hp thickened. 



Subgenus Planorhella Haldeman. 



The typical form, Planorbis campanulatus Say, (Fig. 1429; X li), 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 Cahfornia. 



Fig. 143 I. 



42 (35) SheU smaU, depressed; body- whorl rounded or obtusely angulated; 

 lip simple Subgenus Gyraulus Agassiz . . 43 



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



. . Section Gyraulus 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; X3)- 



