SJMILAll SHELLS. 405 



will show that a similar fact has been observed among the 

 bivalves. 



" The Mytilns polymor})hus of Chemnitz is truly a fresh- 

 water species, having been first observed in the Wolga by 

 the illustrious Pallas. It has recently been introduced, 

 doubtless witli the Russian timber (for this species, in com- 

 mon with the Ampullariye, Paludina?, and Neritinae of fresh 

 water, and the Littorina?, Monodonta;, and Cerithia of salt, 

 has the faculty of living for a very long time out of water), 

 into the Lake of Haarlem and the Connnercial Docks at 

 Rotherhithe ; in both of which it appears to increase with 

 great rapidity. 1 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 li\dng out of water, 1 cannot hesi- 

 tate in giving the preference to the suggestion advanced 

 above, rather than supposing it to have made its passage 

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

 bottom of a vessel. The shell in question differs from the 

 shells of other Mytili 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 circumference, 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 rejection 

 of the water, from the contents of which the animal derives 

 its sustenance. This shell must consequently 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 of the mantle 

 so essential a distinction as to fovnid on it his division of the 

 bivalves into families. In his system, therefore, the genus 

 Dreissena would be placed with the family of Chamacees, 

 while the genus Mytilus forms the type of the preceding 

 family of Mytilacees. The genus Iridina, however, and one 

 or two others, show that this character cannot be implicitly 

 relied on for the natural classification of animals of this class, 

 although it forms a very good generic mark of distinction. 



" The genus Iridina f above referred to affords a second 

 instance of this anomaly ; for though the animals of the 



* Inslitiit., 183o, p. 130 ; and Ann. dcs Sc. Nat. N.S. iii. ]!)3, 

 t Lamarck loimod tliis genus on a specimen which had its hingc-niaigin 

 accidentally tubercular and slightly crenated ; but this character is not found 



