Page:Natural History, Mollusca.djvu/199

 the most part they are marine, but some inhabit rivers and lakes of fresh water; and M. Lesson affirms of one species which is found in Australia, that it lives abundantly on trees. Some of the Neritinœ are ascertained, on indubitable authority, to live indifferently in the sea and in fresh-water.

Handsome globose shells, with a smooth surface as if varnished, and with a very wide aperture, compose this genus, which is technically distinguished by having the inner lip smooth, not depressed; the umbilicus open, with a central gibbous ridge or prominence, and the operculum formed of shelly substance.

The animal is large, slimy, and slug-like, with the foot so enormously developed as to communicate to the creature a most remarkable aspect when seen crawling. It is a long oval plate of soft flesh, commonly squared in front, and rounded or notched behind. The forepart, where it is widest, forms a thick sort of shield, somewhat eared, which is turned up on the front of the shell, partially covering it, and hiding the whole head of the animal, except the tentacles, which project from behind it. The hind part of the foot, and the sides also, partially envelope the shell, of which only the summit is seen, apparently embedded in an immense mass of white flesh. The operculum, notwithstanding its large size, is not visible when the animal is in motion, being hidden by the extremity of the shell. When one beholds the great volume of fleshy substance which the animal displays when crawling,