117 



NERITINA. Lamarck. 



Clithon, Velates and Theocloxis. Montfort. 



Gen. Char. A slender, semiglobose or oval 

 shell, flat beneath, not iimbilicate ; aperture 

 semicircular ; columella lip flattened, with 

 a sharp straightish edge ; outer lip neither 

 toothed, nor crenulated within ; operculum 

 furnished with a lateral process. 



J- HE 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 Avhich 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 

 hinge. TheNeritinashavea distinct coriaceous epidermis, 

 and are often ornamented with black stripes spots or bands 

 beneath it; the sj)ire 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 ])laced oblio^uely u\)Ox\ 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 ISeiitacea. 



