MR. GRAY ON TESTACEOUS MOLLUSCA. 309 



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These anomalies are not restricted to the univalves : bivalves have also their share. 

 Thus, the genus Solen 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 No- 

 vamlina. 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 

 Domheyi 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 

 brackish water on the coast of America. In hke manner M. Nilsson has found his 

 Tellina Balthica, which appears to be little more than a variety of the Tellina solidula 

 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 armaria 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 spe- 

 cies of the genus are found only where the water is quite salt. 



By far the greater part of the species of Corhulce are truly marine ; but there is a 

 large species of the genus, called by Dr. Maton* Mya lahiata, brought with fresh- 

 water 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 additional 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 brack- 

 ish w^ater twice in each day. It is even affirmed that oysters can exist in water abso- 

 lutely fresh ; for in the Museum of the Bristol Institution there is a large group said 

 to have been dredged up in a river on the coast of Africa where the stream was so 

 sweet as to have been used to water the ship. To these shells are attached specimens 

 of Cerithium armatum ; and the person by whom they were presented to the collection 

 stated that Cardlum ringens was found abundantly in the same situation. 



The genus Cucullcea, again, is universally considered as truly marine ; but Mr. Ben- 

 son has found in the Ganges a small shell belonging to it, regarded by him as an 

 Area, but on account of its freshwater origin formed into a new genus under the 

 name of Scaphula. 



On this subject I may observe, that I was some time ago informed that Area senitis 

 was found in the rivers of Africa in company with Galatea radiata : M. Cailliaud, 

 * Linnean Transactions, vol. x. p. 326, t. 24, f. 3. 



