SEGMENTINA. 499 



small, deep, abruptly formed imibilicus. This whorl rapidly en- 

 larges, and terminates in a very large, not very oblique aperture, 

 with the lip expanded so as to make it trumpet-shaped. Largest 

 diameter, three twentieths of an inch ; breadth, one twentieth of an 

 inch. 



This curious little shell was found several years since on the 

 island of Nantucket, clinging to some damp moss, and was com- 

 municated by Mr. J. M. Earlc, of Worcester. Specimens of it have 

 also been sent to me Ijy Professor Foreman, of Baltimore. But its 

 characters were not fidly ascertained from these few specimens. In 

 July, 1840, Mr. T. J. Wliittemore found it in great numbers at 

 Hingham, in a small pool, southeast of the Old Colony House. 



It has a miniature resemljlance to P. bicarinatus as to its two 

 sides, but it has only a single carina, which encircles the shell, in- 

 stead of 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. 23, fig. 83), which he received from near Cincinnati, is probably 

 the same as this shell. His name, however, is preoccupied by a 

 fossil species. 



[New England to Maryland. 



Ocnus SEGITIElVTIiVA, Fleming. 1817. 



Tentacles filiform. Foot narrow anteriorly, larger behind. 



Shell dextral, discoidal, spire depressed, horn colored ; whorls 

 few, visible on both sides, furnished internally with transverse, tes- 

 taceous partitions or teeth ; aperture transversely oval or circular ; 

 outer lip simple. 



Jaws (of S. lacustris') very narrow, very much arched, flexible, 

 scarcely brown, greatly attenuated, pointed. Vertical striae or mar- 

 ginal denticulations hardly apparent. 



Lingual membrane — ? 



'O' 



Segmentina armigera. 



Fig. 138. 



Shell flat on the right side, and concave on the left; whorls four, with minute 

 revolving lines on the concave side ; throat with five unequal teeth far within the 

 aperture. 



