44 
NERITID^. 
nished with a sharp flexible outer edge ; foot 
short, rounded at each end. 
This genus is separated from the marine Nerita 
by the pillar being sharp, only slightly denticulated, 
and the outer lip not being toothed within. 
Neritina is the diminutive of Nerita^ the ancient 
name of a sea-shell. 
The greater part of the species are conflned to 
freshwater streams, but one of the North American 
species is found for 200 miles up a river, from the 
mouth where it is quite salt, to beyond the reach of 
the tide, where the water is perfectly fresh. One 
species {Neritina viridis) is only found in the sea. 
(See Phil. Trans. 1835.) They live on vegetable 
substances, crawl slowly, and have not been ob- 
served to swim on the surface of the water. 
They deposit their eggs on the surface of their 
shells ; these are roundish, and provided with a 
thick opaque coriaceous case ; and when the animal 
hatches, the upper part separates from the lower 
by a circular slit, like the lid of a box, the base 
remaining adherent to the shell. 
M. Deshayes and several other conchologists, es- 
pecially those who only study the external form of 
shells, have proposed to unite this genus to the 
Nerites^ because some of the species are marine, and 
some of the fluviatile species have a tooth on the 
pillar lip. The genera are, however, very distinct ; 
and they may be well characterised by the structure 
of the operculum. (See Phil. Trans. 1833, p. 814.) 
The operculum of the Neritince is solid, shelly, and 
furnished with a thin flexible outer edge; that of 
