Mr Gray on Testaceous Mollusca. 91 



lated, one from the rivers of China, and the other from pools of 

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

 Nilsson has found his Tellina Balthica^ which appears to be lit- 

 tle more than a variety of the Tellma solidula of our coast, in 

 the brackish waters of the shores of the Baltic. Avicula mar^ 

 garitifera^ 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 brack- 

 ish only during high tides. They are found, moreover, with 

 fresh-water 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 CorhulcB are truly 

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

 Maton * Mya labiata, 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 illustra- 

 tion. 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 that oysters can exist in water absolutely 

 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 pre- 

 sented to the collection, stated that Cardium ringens was found 

 abundantly in the same situation. 



The genus Cuculloea^ again, is universally considered as truly 

 marine ; but Mr Benson has found in the Ganges a small shell 

 belonging to it, regarded by him as an Arca^ but, on account of 

 its fresh-water origin, formed into a new genus under the name 

 of Scaphnlo, 



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



