INTRODUCTION. 15) 
My views on the classification of our older fossil fishes and their rela- 
tion to living forms are there given at such length, that it has seemed 
unnecessary to enter on any general discussion of these subjects here. 
The new material described on the following pages has an important bear- 
ing upon some of the questions of a general nature relating to the origin 
and development of fish life on the earth, and reference will be found to 
such general questions in the descriptions given of genera and species as 
they occur in the chronological arrangement which follows, where the 
fishes of the different geological systems are treated of in order, beginning 
with the oldest. This review has been carried to the top of the Coal 
Measures and stops there, as no Permian fishes from this country have come 
under my observation. I have already prepared a monograph on the fossil 
fishes of the American Trias,! so far as they are known, and almost no 
Jurassic fishes have yet been found in this country. With the great 
ichthyic revolution which took place in the Cretaceous age the fishes were 
brought into much closer relation to those of the present day, and those 
collected in this country have been made the subject of elaborate and 
important memoirs by Prof. E. D. Cope, who was specially qualified for 
this work by his great familiarity with living fishes. 
1 Mon. U.S. Geol. Survey No. XIV. 
