Tertiaries of the Island of Cos, 273: 



throughout the vertical thickness of the section. These ho- 

 rizons form three series, each of which is characterised by a 

 peculiar form of Paludina and of Neritina^ not present in the 

 other two ; and in the two lower horizons, there are two spe- 

 cies of Melanopsis peculiar to each. 



So very different are the several fossils of one zone from 

 those of another, that at first examination, we appear to have 

 before us very distinct and well-marked species ; and that 

 each series of horizons was characterised by FaludincB^ Neri- 

 tincB, and Melanopsides, peculiar to itself, and representative 

 of those inhabiting the other divisions. This would be a 

 very startling phenomenon to occur within so limited an area. 

 If the successive species be considered distinct, we must re- 

 gard them as mutually representative, and hold that a suc- 

 cession of creations and extinctions took place, in this pro- 

 bably limited basin, during a brief geological period, or else 

 that a transmutation of species took place. 



A careful study of the forms in question, and an inquiry 

 into the modes and capacity of variation of species in the 

 genera to which they belong, and among their allies, have 

 convinced us, however, that these curious changes of form 

 may be accounted for otherwise ; and that in the successive 

 Paludince, Neritina, &c., we have before us only the same 

 species assuming protean variations. By reference to the 

 figures in Plate III., it will be seen that the PaludiruB and 

 NeritimB of the first or lowest zone, have smooth and un- 

 wrinkled shells ; that those of the second have their shells 

 belted by a strong fold or corrugation ; whilst those of the 

 uppermost zone are deeply sulcated, and surrounded by strong 

 spiral ridges. 



Such changes of form take place among Littorinse and Ne- 

 ritinse, even now, in places where alternations of fresh and 

 salt water affect the mollusca, and in brackish water locali- 

 ties. There are phenomena in the Cos beds, which warrant 

 us in referring the remarkable peculiarities of the fossils 

 therein found to a similar cause. That an influx of salt 

 water changed the character of the basin in which they lived 

 towards the close of its existence, is evident from the pre- 

 sence of the Cardium edtUe in its uppermost part. That some 



