NERITIDiE. 



NERITINA, Lamarck. 



Shell hemispherically ovate, oblique, imperforate, with 

 a spire of few whorls, last whorl very large; columella 

 flattened, smooth, or slightly denticulated, forming a nearly 

 straight sharp-edged border to the inner side of the 

 hemispherical mouth ; surface smooth, or striated, or spi- 

 nous, protected by an epidermis ; operculum obliquely 

 lunate, solid, corneo-calcareous, with a tooth on its lower 

 margin. 



Animal with two slender tentacula, with detached eye- 

 bearing sus-tentacula at their external bases. Foot rather 

 short, triangularly oblong. Structure of the tongue (as 

 observed by Loven) compHcated ; each transverse series 

 of teeth has a minute central denticle, flanked by three 

 laterals, of which the first is largest, transversely ex- 

 panded and laminar, the second and third minute, and 

 bounded by numerous uncini, the first of them very 

 large, broadly hooded and denticulated, the remainder 

 narrow and linear. 



The Neritin<B adhere to stones, and, with very few 

 exceptions, inhabit fresh water. The greater number of 

 species are tropical. We find unquestionable examples 

 of this genus in fresh-water deposits of the oolitic period. 

 The animals of Neritina and Nerita are so nearly allied, 

 and the shells in essential features so similar, that the 

 propriety of their separation is questioned by many ma- 

 lacologists. The opercula of Nerita are entirely corne- 

 ous, but it is yet to be seen whether this character be 

 constant. 



