NERITID^E. 81 



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 fresh- 

 water 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 (Ne- 

 ritina viridis) is only found in the sea. ( See Phil. Trans. 

 1835.) 



M. Deshayes and several other conchologists, espe- 

 cially those who only study the external form of shells, 

 have proposed to unite this genus to the Nerites, be- 

 cause 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 the Nerites is horny, 

 covered on both sides with a hard shelly coat. 

 The position of the horny operculum is shown by a 

 groove in the edge between the two coats ; and if a 

 knife is inserted, the coats can be separated from the 

 operculum. 



As the periostraca is essential to the structure of the 

 shell, and is always present, some shells being formed 

 of scarcely any thing else, so it is with the operculum, 

 the horny part similar to the periostraca of shells 

 being always present, and forming its essential part, 

 and a shelly coat being in some instances added to 

 the outer surface, as in Turbo and Phasianella, or to 

 e 5 



