=Origin of the Land and Fresh-water Mollusca.=--The ultimate derivation of the whole of the land and fresh-water molluscan fauna must, as has already been remarked, be looked for in the sea. In certain cases the process of conversion, if it may be so termed, from a marine to a non-marine genus, is still in progress, and can be definitely observed; in others the conversion is complete, but the modification of form has been so slight, or the date of its occurrence so recent, that the connexion is unmistakable, or at least highly probable; in others again, the modification has been so great, or the date of its occurrence so remote, that the actual line of derivation is obscured or at best only conjectural. [Illustration: FIG. 7.--=A=, the common cockle (_Cardium edule_ L.). =B=, _Adacna plicata_ Eichw., Caspian Sea. =C=, _Didacna trigonoides_ Pall., Caspian Sea.] This passage from a marine to a non-marine life--in other words, this direct derivation of non-marine from marine genera--is illustrated by the faunal phenomena of an inland brackish-water sea like the Caspian, which is known to have been originally in connexion with the Mediterranean, and therefore originally supported a marine fauna. The Mollusca of the Caspian, although without exception brackish- or fresh-water species, are in their general facies distinctly marine. Of the 26 univalve species which inhabit it 19 belong to 4 peculiar genera (_Micromelania_, _Caspia_, _Clessinia_, _Nematurella_), all of which are modified forms of the marine _Rissoidae_. The characteristic bivalves belong to the genera _Adacna_, _Didacna_, and _Monodacna_, all of which can be shown to be derived from the common _Cardium edule_. We have here a case where complete isolation from the sea, combined no doubt with a gradual freshening of the water, has resulted in the development of a number of new genera. The singularly marine facies of several of the fresh-water genera now inhabiting Lake Tanganyika, has given rise to the belief, among some authorities, that that lake was at one time an inlet of the Indian Ocean. In the upper waters of the Baltic, marine and fresh-water Mollusca flourish side by side. So complete is the intermixture, that an observer who had lived on no other shores would probably be unable to separate the one set of species from the other.[18] Thus between Dagö and Papen-Wiek[19] _Mytilus edulis_, _Cardium edule_, _Tellina balthica_, _Mya arenaria_, _Littorina rudis_, and _Hydrobia balthica_ are the only true marine species; with these live _Unio_, _Cyclas_, _Neritina_, _Limnaea_, and _Bithynia_. The marine species and _Neritina_ live down to 15–20 fath., the rest only down to 3 fath. Under stones close to the shore of the Skärgård at Stockholm[20] are found young _Cardium_ and _Tellina_, and at 3 to 6 fath. _Limnaea peregra_, and _Physa fontinalis_. Near Gothland _Limnaea_ is found in the open sea at 8–12 fath., and with it occur _Cardium_ and _Tellina_.