At the Frisches Haff[21] _Mya arenaria_ is the only marine species, and lives in company with 6 sp. _Limnaea_, 1 _Physa_, 9 _Planorbis_, 1 _Ancylus_, 4 _Valvata_, 2 _Sphaerium_. Were the Sound to become closed, and the waters of the Baltic perfectly fresh, it would be inevitable that _Mya arenaria_, and such other marine species as continued to live under their changed conditions, should in course of time submit to modifications similar in kind to those experienced by the quondam marine species of the Caspian. It seems probable, however, that the origin, at least in a great part, of the land and fresh-water Mollusca need not be accounted for by such involuntary changes of environment as the enclosure of arms of the sea, or the possible drying up of inland lakes. These cases may be taken as illustrations of the much more gradual processes of nature by which the land and fresh-water fauna must have been developed. The ancestry of that fauna must be looked for, as far as the Gasteropoda are concerned, in _the littoral and estuarine species_; for the Pelecypoda, in _the estuarine alone_. The effect of the recess of the tide, in the one case, and the effect of the reduced percentage of salt, in the other, has tended to produce a gradual adaptation to new surroundings, an adaptation which becomes more and more perfect. It may be safely asserted that no marine species could pass into a land or fresh-water species except after a period, more or less prolonged, of littoral or estuarine existence. Thus we find no land or fresh-water species exhibiting relationships with such deep-sea genera as the _Volutidae_, _Cancellariidae_, _Terebridae_, or even with genera trenching on the lowest part of the littoral zone, such as the _Haliotidae_, _Conidae_, _Olividae_, _Capulidae_. The signs of connexion are rather with the _Neritidae_, _Cerithiidae_, and above all the _Littorinidae_, which are accustomed to live for hours, and in the case of _Littorina_ for days or even weeks, without being moistened by the tide. Similarly the fresh-water Pelecypoda exhibit relationships, not with genera exclusively marine, but with genera known to inhabit estuaries, such as the _Mytilidae_, _Corbulidae_, _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 intermediate 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_, _Hydrobia_, _Truncatella_, among the univalves, and many species of _Cyrena_ and _Arca_ among the bivalves.