PLAXORBIS. 



131 



specimens, but no description, may possibly be the young of tliis, but at 

 all events is a closely allied species. (^DeKay.') 



PJanorhis deformis, Lam., figured in Delessert's Recueil, very 

 much resembles this species in the characteristic deflection of the 

 Last whirl at the aperture. 



Fig. 218. 



ca§) 



Planorhis 

 dilatatus. 



Planoi'liis dilafaflis, Gould. — Shell small, circumference cari- 

 nated, flat above, convex below, and with a small, deep umbilicus ; whirls 

 three ; aperture large, expanded. 



State Coll. No. 75, Soc. Cab. No. 2399. 



Shell small, of a yellowish-green color, minutely wrinkled by the lines 

 of growth ; spire flat, composed of not more than three whirls, 

 separated by a well-defined suture ; the outer whirl has a sharp 

 margin on a level with the spire, diminishing near, but still 

 modifying, the aperture ; below this line, the whirl is very 

 convexly rounded so as to encircle a small, . deep, abruptly 

 formed umbilicus. This whirl rapidly enlarges, and termi- 

 nates in a very large, not very oblique aperture, with the lip 

 expanded so as to make it trumpet-shaped. Largest diameter 

 three-twentieths inch, breadth one-twentieth inch. 



This curious little shell was found several years since on the Island of 

 Nantucket, clinging to some damp moss, and was communicated by Mr. J. 

 M. Earle, of Worcester. Specimens of it have also been sent to me by 

 Professor Foreman, of Baltimore. But its characters were not fully ascer- 

 tained from these few specimens. In July, 1840, Mr. T. J. Whittemore 

 found it in great numbers at Hingham, in a small pool, southeast of the 

 Old Colony House. 



It has a miniature resemblance to P. bicarinatus, as to its two sides, but 

 it has only a single carina, which encircles the shell, instead o£one on each 

 side. Its large, expanded aperture, and small, deeply sunken umbilicus 

 readily distinguish it from any of the small species hitherto known. The 

 surface is rather rough, and perhaps a little hispid when viewed under 

 the microscope. The P. lens of Lea (Amer. Philos. Trans., New Series, VI, 

 68, pi. xxiii, f. 83), which he received from near Cincinnati, is probably 

 the same as this shell. His name, however, is pre-occupied by a fossil 

 species. {Gould.) 



