14 INTRODUCTION. 
Hence, from their number, size, and prowess, the Mollusca have given a 
name to this age, and it is everywhere known as the Age of Mollusks. 
Careful search in many countries has failed to discover anywhere in 
the Ordovician rocks any unmistakable traces of vertebrate life. It is true 
that the ‘“conodonts,” discovered by Pander in the strata underlying St. 
Petersburg, were considered by him and have been thought by others to be 
the teeth of cartilaginous fishes, but there is little probability that they are 
such. I have discussed this question elsewhere’ at greater length than I 
can do here, and have shown that they cannot be the teeth of Elasmo- 
branchs. Very diverse opinions have been expressed on the nature of these 
organs, but Professors Rohon and Zittel have recently (1886) carefully 
reviewed the entire subject, and have published in the Sitzungsberichte of 
the Bavarian Academy of Sciences their conclusion, that the ‘‘ conodonts ” 
are the teeth of annelids. Probably no one now believes that they are the 
teeth of fishes, and therefore, as the evidence stands, fish life began on the 
earth in the Silurian (Upper Silurian) age. Even then fishes were very 
feebly represented in the life of the globe. In the next succeeding age, 
however, they exhibited enormous development, and their history becomes 
more and more varied, interesting, and dramatic through the Paleozoic 
ages. In the Carboniferous and Permian their rule was disputed by the 
Amphibians, and in the revolution which occurred at the beginning of the 
Mesozoic the scepter which they held so long passed to the reptiles, and 
thenceforward they played a subordinate part in the world’s history. 
The Paleozoic ages, then, formed the culminating period in fish life, 
when the whole world of waters was theirs, and they expanded rapidly in 
every direction; early developing a variety of structure and a nice adapta- 
tion to their diverse surroundings, which, when fully displayed, can not fail 
to excite surprise and to be instructive as well as interesting. 
In the following pages I shall endeavor to convey some idea of the 
progress of fish life in North America during their golden age, as illus- 
trated by the large amount of material which has come into my hands. 
I have given a historical review of the subject of American Pale- 
czoic fishes in the introduction to my memoir on the fossil fishes of Ohio.” 
1 Paleontology of Ohio, vol. 2, 1875, p. 41. 2 Paleontology of Ohio, vol. 1, 1873, p. 245. 
