HISTORY OF THE COS BEDS. 205 



species, and the manner in which, owing to the 

 nature of the larva, such change may be effected 

 on the same spot, have led us to propose the 

 following solution of the Cos problem. 



The lowest series of horizons were deposited 

 in the basin when it was purely a fresh-water one, 

 and in it we found the Paludince, &c, in their 

 normal condition, associated with ordinary fresh- 

 water mollusca. These latter are killed off by 

 an influx of salt-water, sufficient to render the 

 basin slightly brackish. This influx takes place 

 at a time when the mollusks of the uppermost 

 horizon in the lowest series have exhausted their 

 ground, whilst at the same time their fry are 

 swimming in the manner of Pteropods through 

 the waters. 



The adults are destroyed, but their descendants 

 survive, so affected, however, by the change in 

 the condition of the element, as to assume a new 

 form, and develope themselves under the aspect 

 of distinct species. A second revolution of the 

 same kind brings about a third, still more re- 

 markable and apparently equally sudden, change, 

 and the continued inroads of the sea at length 

 revolutionise the character of the fauna, in- 

 troduce marine testacea in the place of the 



