The animal has two slender tentacles; a rather short foot, three-sided in outline, with the angles rounded. The tongue is armed with rows of teeth, differing much in form and size. The operculum is semi-oval, slightly shelly, with a sharp, flexible outer edge, and a tooth on its lower margin.
Many species are assigned to this genus, the greater number of which inhabit fresh-water rivers, especially of tropical countries, where they adhere to stones with considerable tenacity. Yet there is a species which inhabits one of the great North American rivers, through a range of two hundred miles, from the mouth, where the water is quite salt, to beyond the reach of the tide, where it is perfectly fresh. Another species is found only in the sea.
When the animals have arrived at their full size, they have the power of absorbing the shelly matter of the divisions which separate the whorls of the spire, so as to allow more room for the spiral body, without increasing the size of the shell. This reduction of substance is accomplished without endangering the strength of the shell, as only a very small part of the whorl is exposed on the outside.
NERITINA FLUVIATILIS.Our single native species (Neritina fluviatilis) is about three-eighths of an inch long, and two-eighths broad; usually of a purplish hue, banded or chequered with spots of white; but the colouring of different specimens varies exceedingly. The animal is white, with the head and neck black. It is found chiefly in slowrunning streams, adhering to stones. The shells are
