374 



HISTORICAL GEOLOGY 



queer ostracoderms had disappeared, and the heavily protected 

 sluggish types of the true fishes were replaced by more active 

 varieties which relied upon swiftness rather than upon armor. 



The place of prominence was 

 occupied by the sharks and 

 their relatives, but the Missis- 

 sippian forms of sharks (Fig. 

 392) were by no means so for- 

 midable as their modern rep- 

 resentatives. In those days 

 many of them were provided 

 only with flat, corrugated teeth 

 suitable for grinding mollusks and other small animals. As 

 weapons of defense against predaceous fishes, such teeth were 

 evidently not effective ; and perhaps to offset this lack, further 

 protection was added to some varieties in the form of sharp 

 spines on the outside of the body. 



Advent of the amphibians. The vertebrates now show 

 a distinct advance in the advent of a class, some members of 



FIG. 392. The Port Jackson shark. 

 One of the nearest living relatives 

 of some of the Paleozoic sharks. 

 Like them its mouth is paved 

 with grinding teeth. 



FIG. 393. A modern salamander or tailed amphibian. 



Kellogg.) 



(Jordan and 



which, at least, were equipped with legs and toes, and were 

 able to live on land and breathe air. As the vertebrates are 

 now predominately land animals, this was a notable step 

 toward the realization of the future destiny of the group. 

 The fossil remains of amphibians are very rare in the Mis- 

 sissippian rocks, and little is known about them. They were 

 long, salamanderlike animals (Figs. 393 and 394), which 

 doubtless spent most of their time in the water. The rela,- 



