The genera of fresh-water Pelecypoda are comparatively few in number, and their origin is far more clearly discernible than that of any other group. This is perhaps due to the fact that the essential changes of structure required to convert a marine into a fresh-water bivalve are but slight. Both animals “breathe water,” and both obtain their nutriment from matter contained in water. Similar remarks apply to fresh-water operculate Gasteropoda. But the passage from a marine to an aerial life involves much profounder changes of environment, which have to be met by correspondingly important changes in the organism. This may be in part the reason why the ancestry of all Pulmonata, whether land or fresh-water, is so difficult to trace. [Illustration: FIG. 11.--=A=, _Cominella_, a marine genus, which lives between tide marks, and from which is probably derived =B=, _Clea_, a genus occurring only in fresh water.] [Illustration: FIG. 12.--=A=, _Cerithium columna_ Sowb. (marine). =B=, _Potamides microptera_ Kien. (brackish water). =C=, _Io spinosa_ Lea, one of the _Pleuroceridae_ (fresh water).] (_b_) =Gasteropoda.=--(1) _Operculate._ _Canidia_ and _Clea_ are closely allied, with but little modification, to the marine _Cominella_[23] (Fig. 11), as is also _Nassodonta_ to _Nassa_. They occur (in fresh water) in the rivers of India, Indo-China, Java, and Borneo, associated with essentially fresh-water species. _Potamides_, with its various sub-genera (_Telescopium_, _Pyrazus_, _Pirenella_, _Cerithidea_, etc.), all of which inhabit swamps and mud-flats just above high-water mark in all warm countries, are derived from _Cerithium_ (Fig. 12); _Assiminea_, _Hydrobia_, and perhaps _Truncatella_, from _Rissoa_. It is a remarkable fact that in _Geomelania_ (with its sub-genera _Chittya_ and _Blandiella_) we have a form of _Truncatella_ which has entirely deserted the neighbourhood of the sea, and lives in woody mountainous localities in certain of the West Indies. _Cremnoconchus_, a remarkable shell occurring only on wet cliffs in the ghâts of southern India, is a modified _Littorina_. _Neritina_ and _Nerita_ form a very interesting case in illustration of the whole process. _Nerita_ is a purely marine genus, occurring on rocks in the littoral zone; one species, however, (_N. lineata_, Chem.) ascends rivers as far as 25 miles from their mouth, and others haunt marshes of brackish water. _Neritina_ is the fresh-water form, some species of which are found in brackish swamps or even creeping on wet mud between tide marks, while the great majority are fluviatile, one group (_Neritodryas_) actually occurring in the Philippines on trees of some height, at a distance of a quarter of a mile from any water. _Navicella_ is a still further modified form of _Neritina_, occurring only on wet rocks, branches, etc., in non-tidal streams (Fig. 13).