70 



THE DOCTRINE OF DESCENT. 



The Molluscs were mainly represented by Brachiopoda 

 and Cephalopoda. However, as Bivalves and Gaster- 



opoda were also in existence, the appearance of this, the 

 most ancient molluscous fauna known, differs from the 

 present one only in its numerical proportions, and in the 

 circumstance, certainly very important, that of the 

 Cephalopoda the Nautilus alone is found. The Brachio- 

 poda soon attain to their highest development, and have 

 lingered on till now in a greatly reduced state. Among 



