204 EFFECT OF CHANGE OF WATER. 



us in referring the remarkable peculiarities of 

 the fossils therein found to a similar cause. 

 That an influx of salt-water changed the charac- 

 ter of the basin in which they lived towards 

 the close of its existence, is evident from the 

 presence of the Cardium edule in its uppermost 

 part. That some such cause had previously been 

 in action is probable from the fact, that the 

 pulmoniferous testacea found in this formation 

 are confined to the lowest series of horizons. 

 One of the authors has elsewhere shown that 

 no species of mollusk 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 

 testacea are active creatures of a different 

 form, organized for swimming, — when all the 

 adult animals upon a ground are destroyed, their 

 descendants may survive their destruction and 

 replace them, providing the ground be sufficiently 

 changed during the interval. 



Now these two facts, first 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 



