117 
NERIT1NA. Lamarck. 
Clithon, Velates and Theodoxis . Montfort. 
Gen. Char. A slender, semiglobose or oval 
shell, flat beneath, not umbilicate ; aperture 
semicircular ; columella lip flattened, with 
a sharp straightish edge; outer lip neither 
toothed, nor crenulated within ; operculum 
furnished with a lateral process. 
The Genus Nerita as established by Linneus, contains 
many shells whose animals live in fresh water, besides 
such as inhabit the sea, and it has been discovered that 
teeth or small plaits inside their outer lips, are possessed 
exclusively by the latter; a circumstance that will serve, 
as well in fossil as in recent shells, to distinguish them 
by; the fresh water species are now classed under a dis- 
tinct Genus, of which two fossil species are before us. 
Neritina differs from Nerita very little in the general 
form of the shell; both genera have a peculiarly formed 
inner lip, that gives the aperture a semicircular form ; 
and the operculum opens against it as a door upon a 
hing-e. TheNeritinaehave a distinct coriaceous epidermis, 
and are often ornamented with black stripes spots or bands 
beneath it; the spire is very variable, sometimes being 
conspicuous and even very prominent; at others very 
small, and even concealed. The inner lip* of the aper- 
ture is often toothed ; it is placed obliquely upon the 
base of the columella or axis of the spire. This axis, 
together with the inner part of the spire, and even a part 
of the lip is removed by the animal in proportion as it 
proceeds in the enlargement of its shell, whence it ap- 
pears to have no columella. Like many fresh water shells, 
some of the species are liable to erosion, particularly at 
their apices, which are providently thickened to pre~ 
vent injury to the animal. The recent species are very 
numerous, the fossil ones all occur in the formations 
above the London Clay. 
* Sometimes called the columella by Lamarck, although he denies a 
columella to the Neritacea. 
