MR. GRAY ON TESTACEOUS MOLLUSCA. 303 



in these respects agrees with the equally marine genus Amphihola, confounded by 

 Lamarck with the Ampullariw. 



About fifteen years since, I first observed, in the marshes near the banks of the 

 Thames between Greenwich and Woolwich, in company with species of Valvata, 

 Bithynia, and Pisidium, a small univalve shell, agreeing with the smaller species of the 

 littoral genus Littorina in every character both of shell and operculum ; yet this very 

 peculiar and apparently local species has an animal which at once distinguishes it 

 from the animal of that genus, and from all other Ctenobranchous Mollusca. Its 

 tentacles are very short and thick, and have the eyes placed at their tips ; while the 

 Littorince, and all the other animals of the order to which they belong, have their 

 eyes placed on small tubercles on the outer side of the base of the tentacles, which 

 are generally more oi* less elongated. The shell in question and its animal were 

 described and figured by Dr. Leach, in his hitherto unpublished work on British 

 Mollusca, under the name oi Assiminia Qrayana ; and as this name has been referred 

 to by Mr. Jeffries and other conchologists, it may be regarded as established, and 

 that of Syncera hepatica, proposed by myself in the Medical Repository, vol. x, p. 239, 

 will take the rank of a synonym. A second species of this genus, has lately been 

 made known by Mr. Benson, by whom it was found in ponds in India. Its shell is 

 banded like that of Littorina A-fasciata and several others of the smaller Littorince, 

 and had been figured in the Supplement to Wood's Catalogue, t. 6. f. 28, under the 

 name of Turbo Francesice. 



Taking this in conjunction with the preceding, we have here two instances of uni- 

 valve 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 show that a similar fact has been ob- 

 served among the bivalves. 



The Mytilus polymorphus of Chemnitz is truly a freshwater species, having been 

 first observed in the Wolga by the illustrious Pallas. It has recently been intro- 

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

 pullarice, Paludince, and Neritince of fresh water, and the Littorince, Monodontce, and 

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

 Lake of Haarlem 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 an- 

 other 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 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 



MDCCCXXXV. 2 R 



