158 THE STORY OF THE EARTH AND MAN. 



Palceoniscus and Atnblypterus. Pursuing and feeding 

 on these were larger ganoids, armed with strong bony 

 scales, and formidable conical or sharp-edged teeth. 

 Of these were Ehizodus and Acrolepis. There were 

 besides multitudes of sharks whose remains consist 

 almost wholly of their teeth and spines, their cartila- 

 ginous skeletons having perished. One group was 

 allied to the few species of modern sharks whose 

 mouths are paved with flat teeth for crushing shells. 

 These were the most abundant sharks of the Carboni- 

 ferous slow and greedy monsters, haunting shell 

 banks and coral reefs, and grinding remorselessly all 

 the shell-fishes that came in their way. There were 

 also sharks furnished with sharp and trenchant teeth, 

 which must have been the foes of the smaller mailed 

 fishes, pursuing them into creeks and muddy shallows ; 

 and if we may judge from the quantity of their re- 

 mains in some of these places, sometimes perishing 

 in their eager efforts. On the whole, the fishes of the 

 Carboniferous were, in regard to their general type, a 

 continuation of those of the Devonian, but the sharks 

 and the scaly ganoids were relatively more numerous. 

 They differed from our modern fishes in the absence 

 of the ordinary horny-scaled type to which all our 

 more common fishes belong, and in the prevalence of 

 that style of tail which has been termed " heterocer- 

 cal," in which the continuation of the backbone forms 

 the upper lobe of the tail, a style which, if we may 

 judge from modern examples, gives more power of 

 upward and downward movement, and is especially 



