HYDROBIA. 69 



H. ventrosa, bear such a close and suspicious resem- 

 blance to a Cape of Good Hope species of Hydrobia, 

 that I cannot venture to include it among the British 

 Mollusca. It does not appear to have been described by 

 any author; but Mr. G. B. Sowerby has named it Rissoa 

 castanea, on my authority, in his * Illustrations of British 

 Conchology.' Both Mr. Pickering and myself have 

 failed to rediscover this species in the spot where he 

 originally found it, although we have at different times 

 carefully searched for it. The fact of Litiopa bombyx, 

 which is peculiar to thq Gulf-weed, having been taken 

 with it, leads to the supposition that both of these shells 

 might have been accidentally brought into the Thames, 

 attached to the keel, rudder, or anchor of an inward- 

 bound vessel, and carried by the tide into the ditch where 

 they were discovered by Mr. Pickering. Many other 

 modes of introduction will doubtless occur to my readers. 



I received some years ago from the late Mr. G. B. Sow- 

 erby two specimens of Hydrobia Ferussina, which he said 

 had been found in Hampshire. I do not propose to add 

 this species to the British list on such slight and insuffi- 

 cient grounds; but as Helix obvoluta has only been found 

 in the same county, as a British shell, and both these 

 species inhabit the greater part of France, I merely call 

 the attention of conchologists to the circumstance, it 

 being not improbable that the H. Ferussina may also 

 turn up in the South of England. 



The Hydrobia marginata (Paludina marginata of Mi- 

 chaud) inhabited this country a long time ago, but ap- 

 pears to have become extinct as a British species. Sir 

 Charles Lyell first, I believe, recorded it as occurring in 

 the Mundesley bed, where I have since found it ; it also 

 occurs in the well-known upper tertiary strata at Grays, 

 Stutton, Clacton, and Cropthorn ; and I lately detected it 



