ORIGIN OF FRESH-WATER UNIVALVES 



17 



Chem.) ascends rivers as far as 25 miles from their mouth, and 

 others haunt marshes of brackish water. Neritma 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 (^Neritochi/as} actually 

 occurring in the Philippines on trees of some height, at a dis- 

 tance 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 Nerltlna, from the marine NeHta, with corresponding changes 

 in the operculum. 1. Nerita ; 2, 3. Keritina ; 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 CeritJiium. 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. Ampidlaria, however, shows distinct signs of relation- 

 ship to Natica^ while the affinities of Paludina and Valvata can- 

 not as yet be approximately affirmed. 



(2) Pulmonata. — 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 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, 



VOL. Ill C 



