112 MOLLUSCA. 
neritella, (Lister, Conch. tab. 61. fig. 59), is the eo 
tive of the genus. 
In the genus Mexanra of Lamark, the shells are turret- 
ed, longer than broad, effuse at the base, with a twisted 
solid pillar. The H. amarula is the type. 
The genus MELAnopsis was instituted by Daudebard to 
include the shells termed melanie by Olivier in his voyage 
to the Levant. The mouth is lanceolate, the pillar trun- 
cated and emarginated above with a callosity at the base. 
29. Nerita.—This genus has been subdivided by Ad- 
anson and Bruguiére into Nerrra and Natica. In the 
former there is no umbilicus as in the WV. eruvia, and, in the 
latter, there is an umbilicus, as in the 1. canrena. Of the 
restricted genus nerita, we possess two species; the /éttoralis, 
common on our shores, and V. virginea. There are several spe- 
cies of natica of British growth, the largest of which is the 
glaucina. The fresh water species have been formed by 
Lamark, with great propriety, into a distinct genus, under 
the title Neritina. The WV. fluviatilis occurs in the English 
rivers. 
30. Hatyotis.—This genus has been dismembered of 
those species which are destitute of the perforations on the 
disc. These have been formed into a new genus termed 
STOMATIA. 
31. Paretia.—This genus, which at first sight appears 
so very natural, contains shells which exhibit considerable 
differences, both in form and structure, when narrowly ex- 
amined. Geoffroy, with great propriety, separated. the flu- 
viatile species under the generic title ANCyLUs, a genus af- 
terwards employed by Miiller. The animal is essentially 
distinct from the marine patella. There are two species — 
