liVIIAlUT DIFFKREiVT SITUATIONS. 293 



ascertain (rom the descriptions, of the animal also. These 

 are the Paludina fusca of Pfeiffer, and the P. naticoides of 

 De Ferussac : they are truly fluviatile. 



These anomalies are not restricted to the univalves : 

 bivalves have also their share. Thus, the genus Solcn is 

 generally and properly considered as marine ; but Mr. Ben- 

 son has lately discovered a species inhabiting the mud on the 

 banks of the Ganges ; and conceiving, from the nature of its 

 habitation, that it ought to be separated from the common 

 species, he has formed a genus for its reception under 

 the name of Novaculina. On comparing, however, some 

 specimens of the shell presented to the British Museum by 

 Mr. Royle, I can scarcely distinguish it as a species from the 

 Solen dombeyi of Lamarck, which is found on the coast of 

 Peru ; and I have two other species, very nearly related, one 

 from the rivers of China, and the other from pools of brack- 

 ish water on the coast of America. In like manner M. 

 Nilsson has found his Tellina balthica, which appears to be 

 little more than a variety of the Tellina sohdula of our coast, 

 in the brackish water of the shores of the Baltic. Avicula 

 margaritifera, the mother-of-pearl shell, commonly found in 

 the ocean, has been taken by M. Rang in marshes in the 

 Isle of Bourbon, in the neighbourhood of the sea in which 

 the water is nearly fresh. Specimens of Mya arenaria also 

 are often found so high up the rivers that the water in which 

 they live is brackish only during high tides. They are 

 found, moreover, with freshwater shells on the coasts of the 

 Baltic, while all the other species of the genus are found 

 only where the water is quite salt. 



By far the greater part of the species of Corbulse are truly 

 marine ; but there is a large species of the genus, called by 

 Dr. Maton* Mya labiata, brought with freshwater shells 

 from the mouth of the Rio de la Plata ; and this agrees in 

 many respects with the fossil Corbula gallica, which occurs 

 in what are called the upper freshwater strata of the Isle of 

 Wight. 



The transitions to which the oysters intended for the 

 London market are exposed may be mentioned as an ad- 

 ditional illustration. Many of these are collected in the sea 

 on the coasts of Guernsey and of France, and are brought 

 to situations in the mouth of the river where the water is 

 merely brackish during the ebb of the tide, and where they 

 are consequently subjected to the alternate action of salt 

 and brackish water twice in each day. It is even affirmed 



* Liniican Tiausaclions, vol. x. p. 32(5, t. 24, f. 3. 



