108 



LIFE ON THE EARTH. 



influence declined in the Tertiary Seas, the skates and 

 rays, furnished with a suitable pavement of teeth, may 

 be supposed to have assumed some of their duties. 



I 



ERESHWATEK LIFE. 



The stratified rocks on which we base the general 

 scale of elapsed time being marine, and the occur- 

 rence of freshwater deposits among such being ne- 

 cessarily limited, it is not surprising that our history 

 of freshwater life is marked by many lacunae. 

 Rather may we be surprised that the nature of the 

 movements of the earth in ancient times was such 

 that marine deposits were covered by lakes, or estu- 

 aries, at several epochs, and sometimes for long 

 periods of time. In the following brief catalogue, 

 suited to the British Isles, we find what may be 

 termed Freshwater zones in all the great systems 

 of Strata. 



