3. Ampullaria. Eleven species. This genus is evidently intermediary to the Paludina and the Natica. Its species are probably all fluviatile; some attain a great size. Shell thin, globular, ventricose; umbilicus small, forming a compressed funnel-shaped aperture, without interior callosity; spire very short, the last whorl much larger than all the others together; aperture ovate, longer than broad, with margins united; right margin smooth and sharp; columellar lip thickened, projecting, and reflected over the umbilicus; operculum horny, rarely calcareous, thin, oval, not spiral, with concentric elements; summit sub-marginal, inferior, passing obliquely by the right edge of the aperture, but attached to the left. Ampullaria Guyanensis. A. rugosa. A. fasciata. A. canaliculata. A. effusa. A. Guinaica. A. virens. A. carinata. A. avellana. A. intorta. A. fragilis. _A. Guyanensis._ The Guiana Ampullaria. Globular, thick, with unequal longitudinal striæ; covered with brown epidermis; inside golden colour. _A. rugosa._ The rough Ampullaria. Pl. 21, fig. 3. Species dextral. _A. Guinaica._ The Guinea Ampullaria. Species sinistral. _A. carinata._ The carinated Ampullaria. Species sinistral, with a very large umbilicus, spirally carinated. FAMILY XI. NERITACEA. Five genera. 1. Neritina. Twenty-one species. Formerly classed with the Nerita, but separated from it because the Nerita is a marine shell, and those of this genus are fluviatile. They are generally thin, smooth, or very finely striated; the right side of the aperture is not crenulated or dentated, and the animal dissolves the interior of the spire. Shell thin, ovate, not umbilicated; aperture semilunar; inner lip reflected on the columella, and sometimes crenated; outer lip without teeth internally; operculum with a lateral tooth.