12 ORIGIN OF THE LAND AND FRESH- WATER MOLLUSCA chap. 



words, this direct derivation of non-marine from marine genera 

 — is illustrated by the faunal phenomena of an inland brackish- 

 water sea like the Caspian, which is known to have been origi- 

 nally in connexion with the Mediterranean, and therefore origi- 

 nally supported a marine fauna. The Mollusca of the Caspian, 

 although without exception brackish- or fresh-water species, are 

 in their general facies distinctly marine. Of the 26 univalve 

 species which inhabit it 19 belong to 4 peculiar genera (^Micro- 

 melania, Oaspia, Clessinia^ Nematurella)^ all of which are modi- 

 fied forms of the marine 

 Rissoidae. The character- 

 istic bivalves belong to the 

 genera Adacna^ Didacna^ 

 and Monodacna^ all of 

 which can be shown to be 

 derived from the common 

 Cardium edule. We have 

 here a case where complete 

 isolation from the sea, com- 

 bined no doubt with a 

 gradual freshening of the 

 water, has resulted in the 

 development of a number of 

 new genera. The singularly 

 marine facies of several of 

 the fresh- water genera now 

 inhabiting Lake Tanganyika, has given rise to the belief, among 

 some authorities, that that lake was at one time an inlet of the 

 Indian Ocean. In the upper waters of the Baltic, marine and 

 fresh-water Mollusca flourish side by side. So complete is the 

 intermixture, that an observer who had lived on no other shores 

 would probably be unable to separate the one set of species 

 from the other.^ Thus between Drago and Papenwick^ Mytilus 

 edulis, Cardium edule, Tellina halthica, Mya arenaria, Littorina 

 rudis, and Hydrohia halthica are the only true marine species ; 

 with these live Unio, Cyclas, Neritina, Limnaea, and Bithynia. 

 The marine species and JVeritina live down to 15-20 fath., the 



1 Beudant, by very gradually changing the water, accustomed marine species 

 to live in fresh, and fresh-water species to live in salt water. 



2 Braun, Arch.f. Naturk. Liv. (2), x. p. 102 f. 



Fig. 7. — A, the common cockle {Cardium 

 edule L.). B, Adacna pZicato Eichw., 

 Caspian Sea. C, Didacna trigonoides 

 Pall., Caspian Sea. 



