72 PALUDINA. 



water in Britain and other European countries. 

 The shells are small ; they are discoid or conoid, 

 and have an operculum. In the shells of this 

 family the margin of the aperture is carried com- 

 pletely round. In Paludina the whorls are 

 convex. They generally inhabit fresh-waters. 



" P. riripara is found in rivers. Quantities of 

 empty shells may be taken from the sand of the 

 Thames. 



" Fossil species abound — Petworth marble is 

 full of them." 



