The Conchologist: 



Jl {ihictrfertg journal °f /Ifcalacologg. 



Vol. II. SEPTEMBER 29th, 1892. No. 3. 



ON THE ORIGIN OF THE GENERA 

 OF LAND AND FRESHWATER MOLLUSCA. 



By the Rev. A. H. COOKE, M.A., F.Z.S., 

 Fellow and Tutor of Kings College, Cambridge. 



The ultimate derivation of the whole of the land and freshwater 

 molluscan fauna must, in common with that of all other forms of 

 life, be looked for in the sea. All the great families of mollusca can 

 be referred, with more or less distinctness, to a marine origin, and 

 all are the modified descendants of an ancestry originally marine. 

 In certain cases the process of conversion, if it may be so termed, 

 from a marine to a non-marine genus, is still in progress, and can be 

 definitely observed ; in others the conversion is complete, but the 

 modification of form has been so slight, or the date of its occurrence 

 so recent, that the connection is unmistakable, or at least highly 

 probable ; in others again, the modification has been so great, or the 

 date of its occurrence so remote, that the actual line of derivation 

 is obscured or at best only conjectural. 



This passage from a marine to a non-marine life — in other words, 

 this direct derivation of non-marine from marine genera — is illustrated 

 by the faunal phenomena of an inland freshwater sea like the 

 Caspian, which is known to have been originally in connection with 

 the Mediterranean, and therefore originally supported a marine fauna. 

 The mollusca of the Caspian, although without exception freshwater 

 species, are in their general facies distinctly marine. Of the 

 2 j univalve species which inhabit it* 19 belong to 4 peculiar genera 



Dybowski, Mat. Blatt, N.F., x, if. 

 Conchologist, vol. ii., pt. 3. 



