lo Bryant Wai^ker 



Subfamily PLANORBIN.^ H. and A. Adams, 1858. 



Shell discoidal, ultra-dextral or sinistral. Animal sinistral, having 

 the pulmonary, genital and excretory orifices on the left side. Tentacles 

 long, slender and cylindrical. Jaw in three segments. Radula with the 

 numerous teeth arranged in nearly horizontal rows, central small and bi- 

 cuspid, marginals tricuspid, laterals multicuspid. 



In the formation of the keys and descriptions of the various subdivisions 

 of the family, the shells are treated with reference to their apparent mode of 

 spiral growth. 



Key to the genera of Planorbince. 



1. x-\perture dentate within Segmcntina, 



2. Aperture without internal teeth Planorhis. 



Genus PLANORBIS Mriller, 1774. 



Shell discoidal, dextral or sinistral, spire very much depressed, not 

 usually rising above the margin of the body whorl ; no real columella ; aper- 

 ture oblique; outer lip simple and sharp or thickened. Animal smooth^ 

 head short; tentacles long, slender and cylindrical; eyes sessile on the inner 

 bases of the tentacles; foot short, narrow, obtuse at both ends; jaw and 

 radula as in the subfamily. 



Ke}' to the subgenera of Planorhis. 



f Aperture simple, lip thickened HeUsoma. 



I. \ Aperture simple, lip sharp 2. 



[ Aperture campanulate, lip thickened Planorbella. 



f Shell sinistral Planorhis s. s. 



[ Shell dextral 3. 



f Base of body-whorl flattened Tropidiscns. 



[ Base of body-whorl convex 4. 



4-^ 



f Bodv-whorl rounded or angulated . Gyranhts. 



J 

 1 



[ Body- whorl acutely carinated or lenticular Hippeufis. 



Subgenus PLANORBIS s. s. 



Shell sinistral, large, with a moderate number of gradually increasing 

 whorls, rounded above and below ; aperture slightly and gradually expanded, 

 with its margin simple and sharp. 



Represented in our fauna by a single section. 



