74 



THE SILURIAN PERIOD. 



CHAP. 



the occurrence of each is marked in the five great divisions of these 

 strata already referred to. A sixth column is added to shew how 

 many of the genera have been found to occur again in strata of 

 more recent date. Thus may readily be seen how large a pro- 

 portion of the early generic groups has been continued through 

 long succeeding periods. To the consideration of this subject 

 attention will be called again. 



