BRITISH MOLLUSCA. 



GASTEROPODA PROSOBRANCHIATA. 



NERITID^. 



Among the inhabitants of our British fresh waters, is a 

 single representative of a tribe which in warmer climates 

 plays a conspicuous part along the shores of the ocean, 

 and in their rivers and lakes. Nerita and its allies have 

 shells of considerable solidity, more or less ovate, often 

 expanded, with lunate mouths, bounded on their colu- 

 mellar side by an expanded and flattened lip. The 

 animals have broad muzzle-shaped heads, with subulate 

 tentacles, and prominent sus-tentacles bearing the eyes 

 at their external bases. The foot is oblongo-triangular ; 

 its sides are not furnished with cirrhi. An hemispheric, 

 few-whorled operculum, sometimes corneous, sometimes 

 calcareous, is always present, and furnishes important 

 generic characters. 



Although this family ranges far back in time, even 

 to the palseozoic epoch, its chief development is in the 

 present era. Its affinities appear to be with the Tro- 

 cMdt£, on the one hand, and the Paludinid(E on the other. 

 The genus Natica has been associated with it by most 

 writers on Conchology, though, in truth, it is far removed 

 from Nerita. 



