Mr Gray on Testaceous Molltisca, 87 



subject were made simply with the view of extending the know- 

 ledge of the history of the species to which they refer, and with- 

 out reference to the establishment of any preconceived theory. 



These observations may be classed under the four following 

 subdivisions : — 1st, where species of the same genus are found 

 in more than one kind of situation, as on land, in fresh and in 

 salt water ; 2d, where one or more species of a genus, most 

 of whose species inhabit fresh water, are found in salt or brackish 

 water ; 3d, where, on the contrary, 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, Scarabus and A. minima are found in damp places on the 

 surface of the earth ; A. JudcB lives in sandy places overflowed 

 by the sea ; A. Myosotis, A. coni/brmis, A. nitens, &c. (sepa- 

 rated by De Montfort under the name of Conovulus), are found 

 only in the sea in company wuth Chitons, Littorince, and other 

 truly marine shells; and the South American species which I 

 distinguished some time since under the name of Chilinay inclu- 

 ding A. Domheyi of Lamarck, and A. Jluviatilis of Lesson, in- 

 habit fresh-water streams, having most of the habits of the Lym* 

 ncecB. 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 

 occurringji 



The genus Lymncea 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 parum salsd 

 Maris Balthici ad iittora Gothlandiae et Scaniae, &c. In maris 

 juxta Esperod fucis et lapidibus adhaerens frequenter obvenit 

 simul cum Paludind Balthicd et Neritina Jluviatili ; " and a 

 second under the name of Lymncea sucdnea^ which is found on 

 the shores of the sea near Trelleborg. All the species of Po/m- 

 ^ma and Bithynia which have fallen under my own observation 

 are essentially fluviatile; but M. Nilsson refers in the paragraph 

 above quoted to a species of the former genus inhabiting the 



