OVOVIVIPAROUS POND-SNAILS. 55 



The two species of PaliuUna, which, as above 

 stated, are closely allied to Bythinia, are vivijmra 

 and contecta (PI. VII., figs. 4, 5). They are pretty 

 generally dispersed, inhabiting rivers, canals, and 

 large sheets of water, and their distinguishing cha- 

 racters have already been pointed out (p. 13). In 

 the timber docks on the Thames, and in the various 

 canals about London, these shells are very abundant. 

 Specimens from a pond on Hampstead Heath 

 were found to have their apices much eroded, which 

 was due, no doubt, to the action of sulphuretted 

 hydrogen given off from decomposing animal and 

 vegetable matter. The variety unicolor, without 

 bands, has been obtained by Mr. Gwyn Jeffreys in 

 the Thames at Eichmond. 



In waters which have a stony or gravelly bottom 

 may be found a pretty little shell about the size of a 

 pea, with a very short spire and semilunar mouth, 

 solid and glossy, and of a purple colour spotted 

 with white. This is Neritina fluvlatilis (PI. VII., 

 fig. 3), the only fresh- water species of the genus 

 to be found in this country, although there are 

 several marine forms which are also met with in 

 brackish water. It may sometimes be found on the 



