18(3 



INTRODUCTION. 



time between the two periods, the immense extent of 

 which we have no means of estimating, very few, and 

 perhaps none of the then existing species (as indicated 

 bj the fossil deposits) have become extinct ; and that 

 consequently, the term of their specific existence is of 

 great length. That, although the numbers contained in 

 the Wabash deposit indicate that, at the time of their 

 deposition, the species had been a long time in existence, 

 none of their remains have yet been discovered in ear- 

 lier formations. 



