The Evolution of Fishes 



441 



to date from the Eocene times. Doubtless each of them had 

 its origin at an earlier period, and the simultaneous appearance 

 is related to the fact of the thorough study of the Eocene shales, 

 which have in numerous localities (London, Monte Bolca, Licata, 

 Mount Lebanon, Green River) been especially favorable for 

 the preservation of these forms. Practically fossil fishes have 

 been thoroughly studied as yet only in a very few parts of 

 the earth. The rocks of Scotland, England, Germany, Italy, 

 Switzerland, Syria, Ohio, and Wyoming have furnished the 

 great bulk of all the fish remains in existence. In some regions 

 perhaps collections will be made which will give us a more just 



FIG. 253. Decurrent Flounder, Pleuronichthys decurrens Jordan & Gilbert. 



San Francisco. 



conception of the origin of the different groups of bony fishes. 

 We can now only say with certainty that the modern families 

 were largely existent in the Eocene, that they sprang from 

 ganoid stock found in the Triassic and Jurassic, that several of 

 them were represented in the Cretaceous also, that the Berycoids 

 were earliest of the spiny-rayed fishes, and forms allied to herring 

 the earliest of the soft-rayed forms. Few modern families arose 

 before the Cretaceous. Few of the modern genera go back to 

 the Eocene, many of them arose in the Miocene, and few species 



