14 



ORIGIN OF FRESH-WATER BIVALVES 



lationsliips, nob 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 lind this 

 process of conversion still going on, and that we should be 

 iible 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 Ije 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 Fotamides, Assiminea, 

 S'qolionaria, Melmiqyus, Hydrohia, Truncatella, among the uni- 

 valves, and many species of Cyrena and Area among the 

 l»ivalves. 



Origin of the Fresh-water Fauna 



(a) Pelecypoda. — Estuarine species, which have l)ecome 

 accustomed to a certain admixture of fresh water, have gradually 

 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, Tlie common Mytilus edulis 

 L. , a marine genus and species. B, 

 Dreissensia, a fresh- water genus, closely 

 allied to Mytilus. 



Fk;. 9. — A, Area naricella Reeve, 

 Philippines, a marine species. B, 

 Area {Scaphula) pinna Bens., R. 

 Tenasserim, a fresh - water species 

 which lives many miles above the 

 tide-way. 



Thus Brcisse^isia (rivers a,nd canals throughout X. Europe 

 and X. America) and llytilojjsis (rivers of America) are 

 scarcely modified My till (Fig. S) ; Sccvpluda is a modified Area, 



