202 PLANORBIS. 



B. Species inhabiting the Atlantic and 

 Middle States. 



16. PLANORBIS TRIVOLVIS, Say. 



Plate 6, fig. 16. 



Hald. Monog. p. 13, t. 2, f. 4—7. 



P. trivolvis, Y&r.fallax, Hald., p. 15, t. 3, figs. 1, 2, 3. 



As previously remarked under PI. Binneyi, 

 Mr. Say's PI. corpulentus is the same as the 

 species now under consideration. In the 

 waters of the Northern border States and in 

 Canada, PI. ti^ivolvis sometimes attains a very 

 large size and is furnished with a somewhat 

 reflected lip. I have figured such a specimen 

 (fig. 16), being a shell described by Mr. Whit- 

 eaves as PI. macrostomus. Until recently I 

 considered Mr. Lea's PI. regularis distinct, but 

 a close examination leads me to the conclu- 

 sion that it is not entitled to specific rank 

 apart from trivolvis. 



PI. prohoscideus, Potiez, ascribed to North 

 America, and included by Binney in the syn- 

 onymy of this species, is, I think, of very 

 doubtful position, and I incline to believe it is 



not North American. 



(49) 



