110 Description of Eight new Species of Shells. 



the distinctive mark of that curious and interesting genus. I 

 have only met with it on the Wissahiccon, where it does not 

 seem to be very common. 



Genus Pasithea. — Lea. 

 P. sordida. PL I, fig. 6. 



P. testa ovato-conoidea, perforata, tenui, albida, diaphana, lasvi, 

 poHta; spira obtusa ; anfractibus quaternis, convexis ; suturis 

 sub-profandis ; apertura eUiptica, intus alba ; colnmella tevi. 



Shell ovately conical, thin, perforated, whitish, diaphanous, 

 smooth, polished ; spire obtuse ] whorls four, convex ; sutures some- 

 what deep ; mouth elliptical, white within ; columella smooth. 



Length -075. Breadth -025 of an inch. 



Hab. Near Boston. Cabinet of I. Lea. 



Remarks. — I found this shell among a number of specimens of 

 Cerithiuni Sayii, sent to my father by Mr. Adams from Boston. 

 It might be mistaken for a very young specimen of Actoion irifi- 

 dus, Totten, but that species has the fold on the columella, the 

 same shape and the transverse striae, even in its youngest state. 

 In the present species, the mouth is acute above and slightly 

 rounded below, and is -037 of an inch in length. It may perhaps 

 be considered as the smallest of our marine shells yet described. 



There has been great confusion among conchologists respect- 

 ing the group of shells to which this species belongs. Lamarck 

 placed some marine shells in the genus Melania, but the differ- 

 ence which mast exist between species breathing fresh and salt 

 water, would in itself warrant their separation. The genus Eu- 

 lima, Risso, may perhaps embrace the Pasithece, but in the for- 

 mer the mouth is often not effuse, the labrum slightly thickened, 

 there are non-secund varices, and the spire is frequently carved. 

 Lowe has lately made a genus Parihenia, which only differs 

 from Eulima in being white, and having cancellations. This 

 does not seem sufficient to vi^arrant a generic distinction. The 

 genus Rissoa, Fremenville, closely resembles the Eulima, and 

 wall also take in part of the Cingula, Fleming, which however 

 may be distinguished from others, by having the lip continuous 

 posteriorly. The Hydrohia, Hartmann, according to Dr. Gould, 

 seems to be the same as the Cingula. The Tiirritella, La- 

 marck, having the mouth non-effuse, is easily separated from the 

 rest. The Pasithea differs from these in its effuse mouth and 

 acute outer lip. The Niso, Risso, only differs from it in the 

 large umbilicus. The Pyramis, Brown, judging from the refer- 



