. The Cambridge natural history. Zoology; Zoologia Geral. 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 peculia
. The Cambridge natural history. Zoology; Zoologia Geral. 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 RisBoidae. The character- istic bivalves belong to the genera Adacna^ Didacna, and Monodacna, all of which can be shown to be derived from the common Ca7'dium edide. We have here a case where complete isolation from the sea, com- bined no doubt Avith 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 balthica, Mya arenaria^ Littorina rudis^ and Hydrohia haltliica are the only true marine species; with these live Unio^ Cyclas^ Neritina^ Limnaea, and Bithynia. The marine species and Nei^itina 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, Naturk. Liv. (2), x. p. 102
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