Shell. This genus appears to partake of the characteristics, both of the Paludina and the Natica, being fluviatile and frequently attaining a large size; spire very short, the last whorl much larger than all the others together; globular, very ventricose, umbilicus small, forming a compressed funnel-shaped aperture, without interior callosity; aperture longer than broad, with margins united; columellar lip thickened, projecting and reflected over the umbilicus; operculum horny. Inhabits the rivers of Europe. Thirteen species.

FAMILY XI.
Neritacea. Four genera.

1. Genus Neritina. Pl. [X].

Animal. Globular; foot circular, thick, without a furrow anteriorly, or a lobe for the operculum posteriorly, with a bipartite columellar muscle; conical tentacula; eyes subpedunculate at their external side; mouth without labial tooth, but with a denticulated tongue prolonged into the visceral cavity; a single large pectiniform branchia.

Shell. This genus was formerly classed with the Nerita, which in form it much resembles, but from the circumstance of the latter inhabiting the sea, and Neritinæ fresh water, it now forms a separate genus. Thin, smooth, or finely striated, in all the species known, the right side of the aperture does not possess any crenulations or teeth; the operculum is provided with a lateral tooth on one side. Some species are armed with spines; aperture semicircular; left margin smooth and sharp; spire imperfect, generally dissolved by the animal. Inhabits the fresh water rivers of Europe. Twenty-seven species.

2. Genus Nerita. Pl. [X].

Animal. See Neritina.

Shell. Marine, but never spined, solid, semiglobular, flattened beneath, not imbilicated; aperture large, entire, semilunar, the external margin much hollowed; the columella sharp and often dentated, operculum horny, subspiral, with a projecting tooth. Inhabits the seas of S. America and W. Indies. Twenty species.