The Evolution of Fishes 445 



but with sharp, notched, turtle-like jaws quite different from 

 those of the fish or those of any animal alive to-day. These 

 creatures appear in Silurian rocks and are especially abundant 

 in the fossil beds of Ohio, where Newberry, Claypole, Eastman, 

 Dean and others have patiently studied the broken fragments 

 of their armor. Most of them have a great casque on the head 



FIG. 255. An Arthrodire, Dinichthys intermedius Newberry, restored. Devonian, 

 Ohio. (Family after Dean.) 



with a shield at the neck and a movable joint connecting the 

 two. Among them was almost every variation in size and form. 



These creatures have been often called ganoids, but with 

 the true ganoids like the garpike they have seemingly nothing 

 in common. They are also different from the Ostracophores. 

 To regard them with Woodward as derived from ancestral 

 Dipnoans is to give a possible guess as to their origin, and a very 

 unsatisfactory guess at that. In any event these have all passed 

 away in competition with the scaly fishes and sharks of later 

 evolution, and it seems certain that they, like the mailed Ostra- 

 cophores, have left no descendants. 



The Sharks. Next after the lampreys, but a long way after 

 them in structure, come the sharks. With the sharks appear 

 for the first time true limbs and the lower jaw. The upper 

 jaw is, however, formed from the palate, and the shoulder- 

 girdle is attached behind the skull. "Little is known," says 

 Professor Dean, "of the primitive stem of the sharks, and even 

 the lines of descent of the different members of the group can 

 only be generally suggested. The development of recent forms 

 has yielded few results of undoubted value to the phylogenist. 

 It would appear as if paleontology alone could solve the puzzles 

 of their descent." 



