274 On the Tertiaries of the Island of Cos. 



such cause had previously been in action, is probable from 

 the fact, that the pulmoniferous testacea found in this forma- 

 tion are confined to the lowest series of horizons. One of 

 the authors has elsewhere shewn, that no species of mollusc 

 can live for any length of time on the same ground. A 

 change of ground is necessary for its prosperity, otherwise 

 it dies off. But, as the fry of even the most sedentary testa- 

 cea are active creatures of a different form, organised for 

 swimming, when all the adult animals upon a ground are de- 

 stroyed, their descendants may survive their destruction, and 

 replace them, providing the ground be sufficiently changed 

 during the interval. 



Now these two facts, /rs^, of the nature and causes of 

 the variations among such testacea as present such curious 

 changes of form in the Cos fresh-water beds , and, second^ 

 of the necessity of a change of ground for the well-being of 

 a 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 pro- 

 blem. 



The lowest series of horizons were deposited in the basin 

 when it was purely a fresh-water one, and in it we found the 

 Paludin(B, &c., in their normal condition, associated with or- 

 dinary 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 mol- 

 luscs of the uppermost horizon in the lowest series have ex- 

 hausted 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 develop themselves 

 under the aspect of distinct species. A second revolution of 

 the same kind brings about a third, still more remarkable, 

 and apparently equally sudden, change, and the continued 

 inroads of the sea at length revolutionise the character of 

 the fauna, introduce marine testacea in the place of the fresh- 

 water species, and destroy the latter altogether. Such an ex- 

 planation is consistent with what we now know of the modes 



