PALUDINA.
"Some of the Valvata genus are found in fresh-water in Britain and other European countries. The shells are small; they are discoid or conoid, and have an operculum. In the shells of this family the margin of the aperture is carried completely round. In Paludina the whorls are convex. They generally inhabit fresh-waters.
"P. vivipara is found in rivers. Quantities of empty shells may be taken from the sand of the Thames.
"Fossil species abound—Petworth marble is full of them."
NERITACEA.
NERITINA.
"The family of the Neritacea," said Mr. Elliot, as he renewed his lessons to Charles and Lucy, "are remarkable in their form. Their left-margin is truncated, without any appearance of a columella. They possess an operculum, and are either marine or fluviatic. The genera are,