NERITS. 205 
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 with- 
out endangering the strength of the shell, as only 
a very small part of the whorl is exposed on the 
outside. 
Our single native species (Neritina fluviatilis) 
is about three-eighths of an inch long, and two- 
eighths broad; usually of a pur- 
plish 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. Itisfound chiefly in slow- “*277*4 FUUVtAtTIEIS: 
running streams, adhering to stones. The shells are 
