306 MR. GRAY ON TESTACEOUS MOLLUSCA. 



2. Of Species belonging to the same natural Genus, inhabiting essentially different 



situations. 



The general belief that all the species of the same genus inhabit the same kind of 

 situation, undoubtedly holds good with reference to most of the genera of shells ; 

 but many exceptions have already been observed, and we may anticipate that many 

 more will be discovered as the natural habits of the different species become better 

 known. In bringing together a number of these exceptions, I have been under the 

 necessity of placing considerable reliance on the observations of others, who have 

 noted in foreign countries facts similar to those which I have myself witnessed at 

 home ; but these observations have been chiefly collected from the works of Pro- 

 fessor NiLssoN of Sweden, of Mr. Say of the United States of North America, and 

 of MM. Lesson, Quoy, and Rang of Paris, writers who, from their extensive know- 

 ledge of conchology, are fully capable of accurately recording their observations, and 

 whose statements may therefore be received as deserving of the most implicit confi- 

 dence. It is moreover to be observed, that all their observations on this subject were 

 made simply with the view of extending the knowledge of the history of the species 

 to which they refer, and without reference to the establishment of any preconceived 

 theory. 



These observations may be classed under the four following subdivisions : 1 st, where 

 species of the same genus are found in more than one kind of situation, as on land, 

 in fresh and in salt water ; 2nd, where one or more species of a genus, most of whose 

 species inhabit fresh water, are found in salt or brackish water ; 3rd, where, on the 

 contraiy, one or more species of a genus, whose species generally inhabit the sea, are 

 found in fresh water ; and 4th, where the same species is found both in salt and fresh 

 water. 



Of the first of these classes the genus Auricula, as defined by Lamarck, may be 

 quoted as a striking example. Of its species, A. Scarahus and A. minima are found 

 in damp places on the surface of the earth ; A. Judce lives in sandy places overflowed 

 by the sea ; A. Myosotis, A. coniformis, A. nitens, &c. (separated by De Montfort 

 under the name of Conovulus,) are found only in the sea in company with Chitons, 

 Littorince, and other truly marine shells ; and the South American species which I 

 distinguished some time since under the name of Chilina, including A. Dombeyi of 

 Lamarck, and A. Jluviatilis of Lesson, inhabit freshwater streams, having most of 

 the habits of the Lymncece. This disparity of habitation has been in some degree 

 overcome by dividing the genus into several, as noticed above ; but the characters 

 employed for their distinction are very slight, and species apparently intermediate 

 between them are constantly occurring. 



The genus Lymnaea has usually been considered as confined to fresh water ; but 

 M. NiLssoN describes a species under the name of L, Balthica, which is found " in 

 aqua par^m salsa Maris Balthici ad littora Gothlandise et Scaniae, &c. In maris juxta 

 Esperod fucis et lapidibus adhserens frequenter obvenit simul cum Paludind Balthicd 

 et Neritind Jiuviatili ;" and a second under the name of Lymnoea succinea, which is 



