WATER SNAILS. 53 



are, more correctly speaking, brackish- water shells. 

 Hydrobia similis resembles Bythinia leachii, but 

 may be distinguished by its smaller size and grooved 

 suture ; the operculum is horny, concentric, and the 

 nucleus lateral ; whereas in Bythinia it is somewhat 

 shelly, and marked by concentric ridges having the 

 nucleus central. Mr. Gwyn Jeffreys states that 

 this species is found in muddy ditches, occasionally 

 overflowed by the tide, by the side of the Thames 

 from Greenwich to below Woolwich. These ditches 

 are separated from the river by a high and broad 

 embankment, which is provided at distant intervals 

 with sluices to drain off the surface water. It lives 

 there in company with Bythinia tentaculata and 

 other fresh-water shells, as well as with the more 

 marine and peculiar mollusk Assiminia grayana ; 

 and it is gregarious. Its food appears to consist of 

 decaying vegetable matter ; and its habits are rather 

 active, creeping and floating with tolerable rapidity. 

 Mr. Prestwich and Mr. Pickering found specimens 

 of it in peat, in the main drainage cutting between 

 Woolwich Arsenal and the exit to the Thames, 

 through Plumstead Marshes ; but it can scarcely be 

 considered one of our upper tertiary fossils. 



