120 



LAND AND FRESH-WATER SHELLS OF X. A. [PAIIT IT. 



in figure 9, when the left posterior angle of the aperture advances along 



the carina ; but the syni- 



Fig. 201. 



Planorhis trivolvix, \ar.falkiar. 



metry of the cup disap- 

 pears, when the inner por- 

 tion of the last whirl re- 

 volves to the right of' tlie 

 carina, as in figure 3 ; in 

 this case, the right margin 

 of the aperture is nearly 

 level with this side of the 

 shell, but it is frequently thrown below, or to the left of it, when it bears 

 some resemblance to figjire 5 : aperture slightly compressed anteriorly, the 

 left margin extending beyond the plane of the shell. Color light brown, 

 sometimes greenish. 



Massachusetts, Lake Erie, Indiana ? 

 Monstrosity : Posterior extremity of the foot divided. 

 In color and consistency, the ova resemble those of P. hicariunfu.';. Dr. 

 Gould has expressed an opinion, that if this be not P. lent us, it must be an 

 Tincharacterized species. He remarks that it is " a darker shell than P. 

 trivolvis, and is distinguished from it by its left side and its aperture. The 

 cup of the left side is less smooth and regular, and is not bounded by the 

 sharp, elevated line ; when this shell is laid upon its right or upper side, 

 the lip of that side will scarcely touch the plane on which it lies ; while, 

 in P. trivolvis, the shell would be lifted by the lij) ; the aperture has not 

 the sha7-p angle of the left side jiroduced by the termination of the carina, 

 but in the young stages it is difBcult to distinguish the two." 



Professor Adams remarks that "P. Jentus, P. corpulentns, and P. trivolvis, 

 of Say, are undoubtedly varieties of one species ;" but he sent me large 

 specimens of P. trivolvis (pi. 2, fig. 6) as P. corpulentus ; and believed the 

 shell now under consideration to belong to /'. lentiis. I have figured it 

 upon the same plate with the latter, to afTord a ready comparison between 

 them ; and have thought best to describe it at large, under a distinct head- 

 ing. I have seen it living in the vicinity of Boston, but have examined 

 .«iO small a number of individuals, that I do not feel myself comjjetent to 

 make- a final decision between two authors whose location gives them 

 facilities which I do not enjoy. {Haldtman.) 



