32 



Neritid^. 



The curious family of Neritidas is almost exclusively 

 a tropical fresh-water, or marine shell. England, how- 

 ever, possesses one solitary branch, the Neritina fluvia- 

 tilis. The shell is of considerable thickness and 

 strength, and is the most solid of all the British 

 mollusca. The animal is possessed of an operculum. 



Neritina fluviatilis (The River Neritine) Linncsus. 



Figures 12 and 13. 



Generally an inhabitant of slow rivers, but occasion, 

 ally, although not frequently, to be found in lakes and 

 canals. 



The River Neritine is to be met with adhering to 

 stones, and less frequently to other shells, more 

 especially to the valves of dead ones, yet sometimes to 

 those of living mollusca. I have found them on live 

 specimens of Dreissena polymorpha and Paludina 

 vivipora, and on valves of dead AnodontJi cygnea and 

 Unio pictorum. 



This species is gregarious, stones being discovered 

 completely covered over with them. In winter they 

 abandon these stones, and in all probability lie dor- 

 mant, with closed opercula, in the mud or gravel at the 

 bottom of the water. 



Neritina fluviatilis is rather a difficult shell to des- 

 cribe in plain terms, the illustrations giving a much 

 clearer idea than any lengthened description ; it has 

 therefore been considered more advisable to refer the 



