42 COOKE : ORIGIN OF THE GENERA OF L. AND F. W. MOLLUSCA. 



(Mieromelania, Caspia, Clessinia, Nematurella), all of which are 

 modified forms of Rissoidce. The characteristic 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, combined 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 freshwater 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 freshwater 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 Papenwickt 

 Mytilus edulis, Cardium edule, Tellina balthica, Mya arenaria, 

 Littorina rudis, and Hydrobia balthica are the only true marine 

 species ; with these live Unio, Cyclas, Neritina, Liuuma, and 

 Bithynia. The marine species and Neritina live up to 15-20 fath., 

 the rest only up to 3 fath. Under stones close to the shore of the 

 Skargard at Stockholm} are found young Cardium and Tellina, and 

 at 3 to 6 fath. Limntza peregra, and Physa fontinalis. Near Gothland 

 Litnnaa is found in the open sea at 8-12 fath., and with it occur 

 Cardium and Tellina. At the Frischen Haff || Mya arenaria is the 

 only marine species, and lives in company with 6 sp. Lim/uea, 

 1 Physa, 9 Planorbis, 1 Ancylus, 4 Valvata, 2 Sphwrium. Were the 

 Sound to become closed, and the waters of the Baltic perfectly fresh, 

 it would be inevitable that Mya arenaria, and such other marine 

 species as continued to live under their changed conditions, should 

 in course of time submit to modifications similar in kind to those 

 experienced by the quondam marine species of the Caspian. 



It seems probable, however, that the origin, at least in a great 

 part, of the land and freshwater mollusca need not be accounted for 

 by such involuntary changes of environment as the enclosure of arms 

 of the sea, or the possible drying up of inland lakes. These cases 

 may be taken as illustrations of the much more gradual processes of 

 nature by which the land and freshwater fauna must have been 

 developed. The ancestry of that fauna must be looked for, as far as the 

 Gasteropoda are concerned, in the littoral and estuarine species, for the 

 Pelecypoda, in the estuarine alone. The effect of the recess of the 



* See Pelseneer, Bull. Mus. Belg. iv., 1886. 

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

 J Lindstrom, Of. K. Vet. Akad. Forh. Stockh. 1855, p. 49. 

 Mendthal, Schr. Ges. Konigsb. xxx, p. 27. 



