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Cincinnati Society of Natural History. 



tral, while most of the smaller forms are dextral. In regard 

 to the position of the genus, much has been said and written. 

 By some authors it is considered nearly related to Physa, and 

 by others to Lim?icea. My observations have convinced me 

 that the genus is closely related to both, its sinistral shell, as 

 seen in dissected specimens, shows that it is closely related to 

 Physa, while its dentition and jaw show it to be nearer 

 Lhmicea. Planorbis has subquadrate teeth, which are bi- and 

 tricuspid for the most part, save the marginals which are 

 serrated. Physa, on the other hand, has a small central tooth 

 and numerous comb-like lateral teeth. When a revision of 

 the genus is made, all of the species should be subjected to an 

 examination, similar to that which I have given the forms 

 spoken of in this paper. 



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