44 NERITID^E. 



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 confined 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 Neritina is solid, shelly, and 

 furnished with a thin flexible outer edge; that of 



