M r. W. Clark on Assiminia Grayana ffwfi? Rissoa anatina. 119 



The animal continues lively for some days in a mixture of salt, 

 as well as in the fresh, or scarcely brackish water. These crea- 

 tures were received from Greenwich in company with the Trun- 

 catella Grmjana ; they are neither shy nor apathetic. 



Though the shell of R. anatina resembles the Bithynia ventri- 

 cosa [Leachii, nonnulL), and is found in company with it and 

 the Limneus pahstris, L. truncatuius and B. impura, all of fresh- 

 water habitat, I am of opinion that the R. anatina, R. ulvce and 

 R. ventrosa are essentially of marine organization, and inhabit 

 the salt and brackish estuaries as their natural localities, and 

 that the Bithynia and Limnei are mixed with them fortuitously, 

 — perhaps impelled into their districts by floods, and agreeably to 

 a well-known habit and law, live and even nmltiply and become 

 acclimated in localities that are not strictly natural to them. 



In other words, I believe that both these tribes are eminently 

 distinct, the one being of marine, the other of freshwater habitat, 

 and that there is no connexion between them beyond that of 

 accident, chance acclimation, and the alliance which is conse- 

 quent on the proximity of their genera in most malacological 

 methods. It results from these views of ours, that the R. ana- 

 tina, R. ventrosa and R. ulv^e are in their right places as the 

 Ulvan group of the Rissoideans. 



It has been proposed to remove the above species to Litturina, 

 or Hydrobia, or Paludinella ; the two latter of these genera would 

 only be useless synonyms of Rissoa ; for the variations of the 

 malacology of the three species from the typical Rissoa are not 

 greater, perhaps not so great, as those of many of the animals 

 that form part of that genus, and if such differences are to be 

 held as of generic value, every admitted Rissoa must have its 

 particular genus and be itself the only species. 



I have merely mentioned casually the localities of Truncatella 

 Grayana and Rissoa anatina, but the deficiency of the detail of 

 the habitats will be well supplied by presenting, by way of con- 

 clusion, a valuable letter from Mr. Jeffreys, which contains many 

 excellent remarks on these points. 



" 9 Montague Place, Bryanstone Square, 

 London, 25tli June 1855. 



" My dear Sir, — Our friend Mr. Barlee told me that he had 

 not been successful in procuring for you specimens of the Assi- 

 minia Grayana, the animal of which you were desirous of exami- 

 ning. I went to Greenwich last Saturday, and have the pleasure 

 of sending you some lively examples of this curious mollusk, as 

 well as a few Littorinu (?) anatina. The shell of the latter is 



