Mr Gray 07i Testaceous Molhisca, 88 



Supplement to Wood's Catalogue, t. 6. f. 28, under the name 

 of Turbo FrancesicB. 



Taking this in conjunction with the preceding, we have here 

 two instances of univalve shells, apparently belonging to the 

 same genus, the one found in fresh and the other in salt water, 

 but proving, when their animals are examined, to belong to 

 genera essentially distinct. My next illustration will shew that 

 a similar fact has been observed among the bivalves. 



The Mytilus 'polymorphus of Chemintz is truly a fresh-water 

 species, having been first observed in the Wolga by the illus- 

 trious Pallas. It has recently been introduced, doubtless with 

 the Russian timber (for this species, in common with the Am- 

 pullarioe^ Pahcdince, and Neritina of fresh water, and the lAt- 

 torina, Monodonta, and Cerithia of salt, has the faculty of 

 living for a very long time out of water), into the Lake of Haar- 

 lem and the Commercial Docks at Rotherhithe ; in both of which 

 it appears to increase with great rapidity. I am aware that Mr 

 Lyell has given another explanation of the mode of introduction 

 of this remarkable species ; but from experiments which I have 

 myself made on the animal's power of living out of water, I 

 cannot hesitate in giving the preference to the suggestion advan- 

 ced above, rather than supposing it to have made its passage 

 from one river to the other, across the sea, attached to the bot- 

 tom of a vessel. The shell in question differs from the shells 

 of other MytiU in no character of more than specific importance ; 

 but the animal is essentially distinct. In the genus Mytilus the 

 lobes of the mantle are free throughout nearly their whole cir- 

 cumference, as in Unio^ Cardita, Pecten, Ostrea^ &c. ; while in 

 the animal of Mytilus polymorphus they are united through 

 nearly their whole extent, leaving only three small apertures, 

 one for the passage of the foot and beard, and the other two for 

 the reception and ejection of the water, from the contents of 

 which the animal derives its sustenance. This shell must con- 

 sequently form a new genus, to which the name of Dreissena 

 has been appropriated by Van Beneden.* As a proof of the 

 importance attached to this character, it may be observed that 

 Cuvier considered the adherence or non-adherence of the lobes 



• Institut. 1835, p. 130 ; and Annales des Sciences Naturelles, S. N. torn, iii 

 p. 193. 



r2 



