I 2 ORIGIN OF THE LAND AND FRESH-WATER MOLLUSCA cii. 



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

 — is illnstrated hj the faunal plienomena of an inland l^ackish- 

 water sea like the Caspian, which is known to have 1)een 

 originally in connexion with the Mediterranean, and therefore 

 originally supported a marine fauna. The Mollusca of the 

 Caspian, although without exception lirackish- or fresh-water 

 species, are in their general facies distinctly marine. Of the 2(5 

 univalve species which inhabit it 19 Ijelong to 4 peculiar genera 

 {Micromelania, C'asjna, Clessiiiia, Nematurellct), all of whicli are 



modified forms of the marine 

 Bissoidae. The character- 

 istic bivalves belong to the 

 genera Adacna, Didocna, 

 and Monodacna, all of 

 whicli can l)e shown to he 

 derived from the common 

 Cardium edulr. ^Ve have 

 here a case where complete 

 isolation from the sea, 

 combined no doulit witli 

 a gradual freshening of tlie 

 water, lias resulted in the 

 development of a number 

 of new genera. The singu- 

 larly marine facies of several 

 of the fresh-water genera 

 las uiven rise to the belief, 



Fig 



-A, the common cockle (Cardium 

 edide L.). B, Adacna 2}licata Eichw., 

 Caspian Sea. C, Didacna trigonoldcH 

 Pall., Caspian Sea. 



now inhaljiting Lake Tanganyika, 

 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 oljserver who had lived 

 on no other shores would prol;)al)ly Ije unal;)le to separate the 

 one set of species from the other.^ Thus between Drago and 

 Papenwick ^ Mytilus edulis, Cardium rdidc, TeUina lied fit lea, 

 Mya arcnaria, Littorina rudis, and Hydrohia ludthica are the 

 only true marine species ; with these live Unio, Cyclas, Xeritina, 

 Limnaca, and Bithynia. The marine species and jSferitina live 



^ Beudaiit, by very gradually changing the water, accustomed marine species to 

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

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



