[Illustration: FIG. 13.--Illustrating the development of the fresh-water genus _Navicella_, through the brackish-water _Neritina_, from the marine _Nerita_, with corresponding changes in the operculum. 1. _Nerita_; 2, 3. _Neritina_; 4. _Neritina_, intermediate form; 5, 6. _Navicella_.] The great family of the Melaniidae, which occurs in the rivers of warm countries all over the world, and that of the Pleuroceridae, which is confined to North America, are, in all probability, derived from some form or forms of _Cerithium_. The origin of the Paludinidae, Valvatidae, and Ampullariidae is more doubtful. Their migration from the sea was probably of an early date, since the first traces of all three appear in the lower Cretaceous, while Melaniidae are not known until Tertiary times. _Ampullaria_, however, shows distinct signs of relationship to _Natica_, while the affinities of _Paludina_ and _Valvata_ cannot as yet be approximately affirmed. (2) _Pulmonata._--Intermediate between the essentially fresh-water and the essentially marine species come the group sometimes known as Gehydrophila, consisting of the two families Auriculidae and Otinidae. These may be regarded as Mollusca which, though definitely removed from all marine species by the development of a true lung or lung cavity in the place of a gill, have yet never become, in respect of habitat, genuine fresh-water species. Like _Potamides_, they haunt salt marshes, mangrove swamps, and the region about high-water mark. In some cases (_Otina_, _Melampus_, _Pedipes_) they live on rocks which are moistened, or even bathed by the spray, in others (_Cassidula_, _Auricula_) they are immersed in some depth of brackish water at high tide, in others again (_Scarabus_) they are more definitely terrestrial, and live under dead leaves in woods at some little distance from water. Indeed one genus of diminutive size (_Carychium_) has completely abandoned the neighbourhood of the sea, and inhabits swampy ground almost all over the world. [Illustration: FIG. 14.--Examples of the _Auriculidae_: =A=, _Auricula Judae_ Lam., Borneo; =B=, _Scarabus Lessoni_ Blainv., E. Indies; =C=, _Cassidula mustelina_ Desh., N. Zealand; =D=, _Melampus castaneus_ Mühlf., S. Pacific; =E=, _Pedipes quadridens_ Pfr., Jamaica.] [Illustration: FIG. 15.--An example of _Amphibola_ (_avellana_ Chem.), the only true Pulmonate which possesses an operculum.]