=Origin of the Fresh-water 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 have at last become habituated to water which is perfectly fresh. [Illustration: FIG. 8.--=A=, The common _Mytilus edulis_ L., a marine genus and species. =B=, _Dreissensia_, a fresh-water genus, closely allied to _Mytilus_.] [Illustration: FIG. 9.--=A=, _Arca navicella_ Reeve, Philippines, a marine species. =B=, _Arca (Scaphula) 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_, and lives in the Ganges, the Jumna, and the Tenasserim at a distance of 1600 miles from the sea (Fig. 9). _Pholas rivicola_ is found imbedded in floating wood on the R. Pantai many miles from its mouth. _Cyrena_, _Corbicula_, and probably _Sphaerium_ and _Pisidium_ are derived, in different degrees of removal, from the exclusively marine _Veneridae_; _Potamomya_ (rivers of S. America), and _Himella_ (R. Amazon) are forms of _Corbula_. The Caspian genera derived from _Cardium_ (_Adacna_, _Didacna_, _Monodacna_), have already been referred to. _Nausitora_ is a form of _Teredo_, which lives in fresh water in Bengal. _Rangia_, _Fischeria_, and _Galatea_ probably share the derivation of the Cyrenidae, while in _Iphigenia_ we have one of the Donacidae which has not yet mounted rivers, but is confined to a strictly estuarine life. The familiar _Scrobicularia piperata_ of our own estuaries is a _Tellina_, which lives by preference in brackish water. [Illustration: FIG. 10.--_Trigonia pectinata_ Lam., Sydney, N.S.W.] The great family of the Unionidae is regarded by Neumayr[22] as derived from _Trigonia_, the points of similarity being the development of a nacreous shell, the presence of a strong epidermis, and the arrangement of the muscular scars. It is remarkable, too, that on many Uniones of Pliocene times there is found shell ornamentation of such a type as occurs elsewhere among the Pelecypoda only on _Trigonia_.