212 THE CARBONIFEROUS SYSTEM. 



Consequently they must have been capable of subsisting in the brack- 

 ish and impure water of the coal creeks and lagoons. The smaller 

 ganoid species would find in these abundance of worms, small Crus- 

 tacea, and larvaj of insects on which to feed ; and if, like the modern 

 ganoids of our North American rivers, they were provided with a. 

 lung-like air-bladder, they could subsist in stagnant water deprived of 

 its free oxygen by decomposing vegetable matter, conditions under 

 which the ordinary ctenoid and cycloid fishes, had they existed in the 

 Coal formation period, would have perished. The larger ganoids and 

 the shark-like Diplodonts no doubt preyed upon the smaller fishes, as 

 the abundant scales seen in their coprolites prove. The flat-toothed, 

 shark-like Psammodus and Conchodus may have ground up the 

 shells of Naiadites, which probably hung in countless multitudes on 

 the floating and sunken timber of these coal lagoons and creeks. 

 Lastly, when these fish died, the millions of little Cytheres and 

 Bairdias, by removing every particle of flesh and ligament, would 

 scatter the scales and bones over the bottom of the waters, to be 

 embedded in the black ooze. 



