LIMNEANA. MOLLUSCA. PLANORBIS. 203 



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. 

 ///Yo/r/.v, the shell would be lifted by the lip ; the aperture has not the 

 sharp angle of the left side, produced by the termination of the carina, 

 but in the young stages it is difficult to distinguish the two. It is very 

 closely allied to P. corneas of Europe ; but in that shell the left side 

 is scarcely concave, and the suture is deep ; the aperture is nearly or- 

 bicular, being almost equally rounded on both sides. 



This shell has hitherto generally borne the name of P. trivolvis in 

 New England ; but it is not the trivolvis of Say, and is either his 

 P. lentus or a new species. 



i 



PLANORBIS BICARINA'TUS. 



Shell deeply concave on both sides ; whorls three ; strongly cari- 

 nated on both sides ; aperture abruptly arched at the carina of the 

 left side, its lip extending far beyond the plane of the preceding 

 whorl. 



FIGURE 134. 

 State Coll., No. 81. Soc. Cab., No. 1268. 



Helix angulatus, RA.CKETT; Lin. Trans., xiii. 42, pi. 5, f. 1. WOOD; Index, 



Suppl., pi. 7, f. 12. 

 Planorbis bicarinatus, SAY; Nicholson's Encyc., (Amer. ed.,) iv. pi. 1, f. 4. Amer. 



Conch., pi. 54, f. 3. SOWERBY; Genera. 



Shell orbicular, its tube rapidly increasing, deeply excavated 

 on both sides, color brownish-yellow on the carina. Whorls 

 rather more than three, as seen on both sides, forming on the 

 right side a large and deep concavity, bounded by a sharp, raised 

 line or carina, and on the left side a still deeper, inversely conic 

 cavity, bounded by a similar carina, but of smaller circuit ; sur- 

 face rather smooth, with faint, irregular lines of growth, most 

 distinct on the right side ; aperture ovate, right side broadest, 

 and on the general plane of that side of the shell ; left margin 

 strongly modified by the carina, and extending far beyond the 

 plane of the preceding whorl ; lip slightly expanded, white ; 

 interior brownish, with white lines in the grooves answering to the 

 carina. Longest diameter J inch, shortest diameter T 3 T inch. 



Animal light russet-color, beautifully dotted with amber ; foot 



