186 
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 
by 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 hi existence, 
none of their remains have yet been discovered in ear- 
lier formations. 
