198 MARINE AND FRESH-WATER SHELLS. 



native element. When the lagoons become 

 very fresh, Neritina^ Melania and Melanopsis, 

 with Limneus and Cyclas, join its company, 

 but perish on the destruction of the barrier and 

 the invasion of salt-water, bringing with it marine 

 creatures to take their places. The Cerithium, 

 however, survives. Not so in case of a change 

 of level ; if there be elevation, though but very 

 slight, the lagoon will be drained, and the 

 shell-fish inhabiting it perish ; if depression, the 

 Cerithium itself is so littoral in its habits, that 

 a very inconsiderable sinking of the bed of the 

 lagoon would bring it into a region of depth 

 unfitted for its existence. In the former case, 

 the whole fauna would be cut off ; in the latter, 

 many of the marine associates of the Cerithium 

 would continue to live, and be joined by a 

 numerous assemblage of new companions. A 

 new elevation would cause a new change, and if 

 the Cerithium had continued to live in the neigh- 

 bourhood, fresh lagoons for its residence might 

 be formed, occupying hollows in a marine for- 

 mation, charged with the organic remains of its 

 destroyed contemporaries. 



Such changes have taken place in the neigh- 

 bourhood of Macri within the historical period, 



