H 



ORIGIN OF FRESH-WATER BIVALVES 



CHAP. 



fresh-water Pelecypoda exhibit relationships, not with genera 

 exclusively marine, but with genera known to inhabit estuaries, 

 such as the Mytilidae^ Corhulidae^ Cardiidae. 



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 intermediate between a marine and a 

 land or fresh-water species, and difficult to classify distinctly 

 as one or the other. Cases of Mollusca occupying this interme- 

 diate 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^ Melampus^ Hydrohia^ Truncatella^ among the uni- 

 valves, and many species of Cyrena and Area among the 

 bivalves. 



Origin of the Fresh-water Fauna 



(a) Pelecypoda. — Estuarine species, which have become 

 accustomed to a certain admixture of fresh water, have gradu- 

 ally ascended the streams or been cut off from the sea, and have 

 at last become habituated to water which is perfectly fresh. 



Fig. 8. — A, The common Mytilus edulis 

 L., a marine genus and species. B, 

 Dreissensia, a fresh-water genus, closely 

 allied to Mytilus. 



Fig. 9. — A, Area navicella Reeve, 

 Philippines, a marine species. B, 

 Ai'ca (Scaphiila) pinna Bens., R. 

 Tenasserim, a fresh-water species 

 which lives many miles above the 

 tide-way. 



Thus Dreissensia (rivers and canals throughout N. Europe 

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

 scarcely modified Mytili (Fig.' 8) ; Scaphula is a modified Arca^ 



