COOKE : ORIGIN OF THE GENERA OF L. AND F. \V. MOLLUSCA. 45 



able shell occurring only on wet cliffs in the ghats of South 

 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 (lineata, Chem.) ascends rivers as far as 25 miles from their 

 mouth, and others haunt marshes of brackish water.* Neritina is 

 the freshwater 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 (A r eritodryas) 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, &c, 

 in non-tidal streams. 



The great family of the Melaniidaz, which occurs in the rivers of 

 warm countries all over the world, and that of the Pteuroceridee, 

 which is confined to North America, are, in all probability, derived 

 from some form or forms of CeritJiium. The origin of the Paludinida, 

 ValvatidcB, and Ampullariidce 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, f while Melaniidee. are not 

 known until tertiary times. Ampullaria, however, shows distinct 

 signs of relationship to Nalica, while the affinities of Paludina and 

 Valvata cannot as yet be approximately affirmed. 



(2) Inoperculate. 



Intermediate between the essentially freshwater and the essentially 

 marine species come the group known as Gehydrophila, consisting of 

 the two great divisions Auriculidce and Otinidce. These may be 

 regarded as mollusca which, though definitely removed from all 

 marine species by the development of a true lung in place of a gill, 

 have yet never become, in respect of habitat, genuine freshwater 

 species. Like Potawides, 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 ( Cassidu/a, 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. 



* e.g. Boettger (Ber. Senckenb. Gesell. , 1891), classifies several as brackish water species. 

 t It is curious, however, that while Ampullaria. has developed a lung, it at the same time 

 breathes water through the gilU. See, in particular, Fischer and Bouvier, Comptes-Rendus 

 cxi, p. 200, for a full description of the process. 



