HYDROBIA. 65 



above, thin and flat, having a lateral and indistinct spire of 

 only 2 whorls, and resembling that of the marine genus Lit- 

 torina ; it is marked with strong, but remote, irregular and 

 flexuous Knes of increase. L. 0*15. B. O'l. 



Habitat : Muddy ditches which are occasionally^ but 

 seldom^ 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 embank- 

 ment^ which is provided at distant intervals with sluices 

 to drain off the surface water. It lives there in company 

 with Bythinia tentaculata and other freshwater shells, 

 as well as with the more marine and peculiar mollusk, 

 Assiminia Gray ana, and it is gregarious. Its food 

 appears to consist of decaying vegetable matter ; and its 

 habits are rather active, creeping and floating with tole- 

 rable 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 con- 

 sidered one of our upper tertiary fossils. This species is 

 widely diffused in France, and extends south to Corsica. 

 The Palud'ma meridionalis of Risso appears to be only a 

 rather longer and stouter form of this species, judging 

 from typical specimens in the Museum at the Jardin des 

 Plantes. 



No one can, I think, take the trouble of carefully 

 comparing specimens of this shell with the description 

 and figure given by Draparnaud of his Cyclostoma simile, 

 without being satisfied of their specific identity ; and the 

 general consent of continental conchologists is in favour 

 of this view. In France H. similis inhabits fresh water. 

 Morelet states that in the South of Portugal it is found 

 both in running water and marshes, and that the shells 

 of the males have a longer spire than those of the other 



