flow to obtain tt. 217 
rivers unless they are deep and slow flowing. It has a long pointed 
spire to the shell, and is one of the largest of the genus, attaining, 
under favourable conditions, a length of one-and-three-quarter 
inches. There are a number of others of this class that are 
both beautiful and interesting to collectors, but for practical 
importance the first mentioned (ZL. fereger) is decidedly the most 
Fig. 34, Limuc@us stagnalis. Fig. 35. Planorbis corneus. 
useful. The round flat shell fish (P/avorbis) are useful and in 
some waters abundant, but they do not grow so fast nor breed 
so plentifully as the Zimnea. There is a useful little limpet, that 
when once established multiplies rapidly in shallow cold water, 
where the other snails do not thrive so well. This is Avevlus 
Jiuviatilts. Fish have often been taken in brooks with these 
molluscs in their stomachs. 
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Fig. 36. Ancy/us 
fluviatilis 
Fig. 37. Paludina vivipera. 
The Paludina are larger snails, but are ovoviviparous ; that 
is, they keep their eggs within their shells until they are hatched. 
They are also possessed of a hard horny plate to close the entrance 
of the shell, called an operculum, and the shell itself is thick and 
strong, and fish will probably not eat them, at least when mature. 
