43 6 



The Evolution of Fishes 



upper Silurian. The earliest in Europe are found in the Lud- 

 low shales, both of these localities being in or near the horizon of 

 the Niagara rocks, in the Upper Silurian Age. 



It is, however, certain that these Lower Silurian remains do 

 not represent the beginning of fish -life. Probably Ostracophores, 

 and Arthrodires, with perhaps Crossopterygians and Dipnoans, 



FIQ. 247. Fossil fish remains from Ordovician rocks, Canon City, Colo. (After 

 (Walcott.) a. Scale of Eriptychius americanus Walcott. Family Holopty- 

 chiidoe? b Dermal plate of Asteraspis desideratus Walcott. Family 

 Asterolepidce. c. Dictyorhabdus priscus Walcott, a fragment of uncertain 

 nature, thought to be a chorda! sheath of a Chimcera, but probably part 

 of a Cephalopod (Dean). Chimceridcef 



existed at an earlier period, together perhaps with unarmed, 

 limbless forms without jaws, of which no trace whatever has 

 been left. 



The Earliest Sharks. The first actual trace of sharks is 

 found in the Upper Silurian in the form of fin-spines (Onchus), 

 thought to belong to primitive sharks, perhaps Acanthodeans 

 possibly to Ostracophores. With these are numerous bony 

 shields of the mailed Ostracophores, and somewhat later those 

 of the more highly specialized Arthrodires. Later appear the 

 teeth of Cochliodontida, with Chimaeras, a few Dipnoans, and 

 Crossopterygians. 



Devonian Fishes. In the Devonian Age the Ostracophores 

 increase in size and abundance, disappearing with the beginning 



