. The Cambridge natural history. Zoology; Zoologia Geral. 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^ Corhididae^ 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
. The Cambridge natural history. Zoology; Zoologia Geral. 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^ Corhididae^ 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 Potaniides^ Assimiiiea^ SipJionaria, Melampiis, 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 Fig. 8. — A, The common Mytilus eduUs L., a marine genus and species. B, Dreissensia, a fresh-water genus, closely allied to Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Harmer, S. F. (Sidney Frederic), Sir, 1862-; Shipley, A. E. (Arthur Everett), Sir, 1861-1927. [London, Macmillan and co. , limited; New York, The Macmillan company
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