102 MOLLUSCA, INSECTS, ETC. 



bodies. Paludina is a genus geologically older than 

 either of the foregoing ; for the remains of certain 

 species of this shell completely make up the bulk of 

 the fresh-water marbles of the Wealden and Purbeck 

 beds. Like most fresh-water genera, the forms have 

 not greatly varied, owing, perhaps, to the similarity 

 of fresh-water conditions in the most remote periods 

 to those of the present time. We have two species 

 extant in our ponds and rivers, of which one, Paludina 

 vivipara, has always been a great favourite with 



Fig. 50. 



Planorbis 

 marginatus. 



Paludina vivipara. 



aquarium keepers. As its specific name implies, it 

 generally brings forth its young alive ; that is to say, 

 it keeps the eggs within its body until they are hatched 

 there. This, however, is not always the case. The 

 males are said to be usually smaller than the females. 

 Both are tolerably active, and look very pretty with 

 their colour bands running up the whorls of the shell. 

 Both in this genus and another nearly related to it, 

 which is even commoner in our ponds and streams, 



