COOKE : ORIGIN OF THE GENERA OF L. AND F. W. MOLLUSCA. 43 



tide, in the one case, and the effect of the reduced percentage of 

 salt, in the other, has tended to produce a gradual adaptation to new 

 surroundings, an adaptation which becomes more and more perfect. 

 It may be safely asserted that no marine species could pass into 

 a land or freshwater species except after a period, more or less 

 prolonged, of littoral or estuarine existence. Thus we find no land 

 or freshwater species exhibiting relationships with such deep-sea 

 genera as the Volutidee, Cancellariidce, Terebndee, or even with genera 

 trenching on the lowest part of the littoral zone, such as the Ha/iotidee, 

 Cotiidoe, Olividee, Capulidee. The signs of connection are rather with 

 the Neritidec, Cerithiidee, and above all the Littorinida;, which are 

 accustomed to live for hours, and in the case of Littorina for days or 

 even weeks, without being moistened by the tide. Similarly the 

 freshwater Pelecypoda exhibit relationships, not with genera exclusively 

 marine, but with genera known to inhabit estuaries, such as the 

 Mytilidee, Corbulidce, Cardiidee. 



It would be natural to expect that we should find this process of 

 conversion still going on, and that we should be able to detect 

 particular species or groups of species in process of emigration from 

 sea to land, or from sea to fresh water. Such species will be inter- 

 mediate between a marine, and a land, or freshwater species, and 

 difficult to classify distinctly as one or the other. Cases of mollusca 

 occupying this intermediate position occur all over the world. They 

 inhabit brackish swamps, damp places at high-water mark, and rocks 

 only at intervals visited by the tide. Such are Potamides, Assiminea, 

 Siphonaria, Mela/npus, Hydrobia, Truncate/la, among the univalves 

 and many species of Cyrena and Area among the bivalves. 



ORIGIN OF THE FRESHWATER FAUNA. 



(a) PELECYPODA. 



Estuarine species, which have become accustomed to a certain 

 admixture of fresh water, have gradually ascended the streams or 

 been cut off from the sea, and at last have become habituated to 

 water which is perfectly fresh. 



Thus Dreissena (rivers and canals throughout N. Europe and 

 N. America) and Mytilopsis (rivers of America) are scarcely modified 

 Mytili ; Scaphula is a modified Area, and lives in the Ganges, the 

 Jumna, and the Tenasserim at a distance of 1,600 miles from the sea. 

 Pholas rivicola is found imbedded in floating wood on the R. Pantai 

 many miles from its mouth. Cyrena, Corbieu/a, and probably 

 SpJicerium and Pisidium are derived, in different degrees of removal, 

 from the Veneridce; Potamomya (rivers of S. America), and Hitnella 

 (R. Amazon) are forms of Corbula. The Caspian genera, derived 



