ORIGIN OF FRESH-WATER UNIVALVES 



17 



rivers as far as 25 miles from their mouth, and others liauiit 

 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 Huviatile, one group {Neritodryas) actually occurring 

 in tlie 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). 



Fig. 13. — Illustrating the development of the fresh-water genus Navicella,, through the 

 brackish-water Nentina, from the marine Nerita, with corresponding changes in the 

 operculum. I. Nerilu ; 2, 3. Neritma; 4. iVenYM«(, intermediate form; 5, 6. 

 yavicella. 



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 Ceritliium. The 

 origin of the Paludinidae, Valvatidae, and Ampullariidae is more 

 douljtful. 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, liowever, shows distinct signs of relationship to 

 Natica, while the aftinities of Faludiiia and Vcdvata cannot as 

 yet be approximately aflirmed. 



(2) rvlmonata. — Intermediate between the essentially fresh- 

 water and the essentially marine species come the group some- 

 times known as Gehydrophila, consisting of the two families 

 Auriculidae and Otinidae. These may l)e regarded as Mollusca 

 which, though definitely removed from all marine species by the 

 development of a true lung or lung cavity in place of a gill. 

 VOL. Ill c 



