4 66 



HISTORICAL GEOLOGY 



(Fig. 444). Typical ganoids were the most abundant and character- 

 istic fish of the Triassic and Jurassic. 



Teleosts or Bony Fish. — The typical fish of to-day, such as the 

 trout, perch, cod, and mackerel, were absent and do not appear until 

 the Mesozoic. 



Comparison of Devonian and Modern Fish. — The teeth of Devo- 

 nian and Carboniferous fishes were adapted for crushing and few had 



the sharp, rending teeth 

 possessed by fish to-day. 

 The fishes of the De- 

 vonian and Carbonif- 

 erous were, as a class, 

 massive and clumsy as 

 compared with those of 

 the present, and their 

 bodies were probably less 

 flexible. In Devonian 

 fishes the backbone runs 

 through to the end of the 

 tail, and the fin is formed 

 by vertical rays extend- 

 ing from above and below 

 (Fig. 445 D). In some, 

 the resulting fin is sym- 

 metrical, but in others, 

 as in the modern shark, 

 it is unsymmetrical, the 

 backbone turning up- 

 wards with an unequal 

 lobe formed of rays on 

 the under side. The 

 tails of all Devonian 

 fishes were either sym- 

 metrical or unsymmetrical (Fig. 445 C, D), but none had the homo- 

 cercal tail (Fig. 445 E) of modern bony fish (Teleosts), in which the 

 backbone ends in a broad plate from which diverging rays spread 

 to form a symmetrical tail of a type very different from that of 

 Devonian fish. 



It is interesting in this connection to note that the heavily armored 

 fish were the first to become extinct. They were admirably suited 



Fig. 445. — Evolution of the tail fin of fishes. 

 • Vfter Dean.) A, embryonic tail fin; C, uneven- 

 lobed tail fin of the Port Jackson shark; D, uneven- 

 lobed tail fin of a Devonian shark, Cladoselache ; 

 />', even-lobed tail fin of the fringe-finned ganoid; 

 .ind E, the tail fin of a modern Teleost, such as the 

 t rout. 



