152 THE OLDEST FISHES AND THEIR FINS. 



Secondary rocks of Europe, Africa, India, and North 

 America, and it is an interesting subject of speculation 

 why the group should have totally disappeared from all 

 those regions, to find n last home in far Australia. 



The existing Australian lung-fish lives mainly or entirely 

 on leaves, and we may therefore conclude that the fossil 

 teeth likewise pertained to fishes which subsisted on some- 

 what similar nutriment. If, however, we were to infer 

 that all the teeth of fossil fishes which have a ridged or 

 flattened grinding surface belonged to the herbivorous type, 

 we should be sadly in error, since many of them approxi- 

 mating more or less markedly to the Ceratodus type really 

 indicate fishes allied to the well-known Port Jackson shark, 

 in which the mouth is covered with a complete pavement 

 of flattened teeth adapted for crushing shell- fish and other 

 hard animal substances. In all such investigations, the 

 truth can, indeed, only be found out by careful induction, 

 and by availing ourselves of every scrap of information 

 left to us among the relics of former epochs. 



