Introduction 



not merely so many specimens of fossil shells 

 and pieces of petrified bones: they represent 

 the life of their day, and this life was quite as 

 real as that now about us. 



There are several ways in which this ancient 

 natural histoiy might be written : one would be 

 to start with the animals found in the lowest 

 rocks and mention the various species or groups 

 of animals which occur in the formations as 

 we come upward. This, it has been thought, 

 would give a somewhat mixed and disconnected 

 view of the life of our continent, and would 

 result, moreover, in the frequent repetition of 

 names and the unavoidable scattering of infor- 

 mation. Still another method would be to tell 

 the history of each group as a whole ; of its 

 origin, rise, period of supremacy, and final de- 

 cline ; and this has many things to recommend 

 it. But the plan finally adopted has been to 

 treat the history of the past by periods, and 

 endeavor to sketch the characteristic or more 

 striking features of the life of well-marked 

 epochs ; to tell something of the habits, appear- 

 ance, and relationships of the more conspicuous 



3 



