152 THE OLDEST IISHES 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 a last home in far Australia. 



The existing Australian lung-fish lives mainly or entirely 

 on leaves, ami 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 1 lelonged to the herbivorous type, 

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

 mating more or less mai-lcedly to tin; Ceratodus type really 

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

 in which the mouth is covei'ed with a conqdete pavement 

 of flattened teeth adajited 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 tlie relies of former epochs. 



