DEVONIAN ANIMALS. 



341 



slightly so; in Devonian Ganoids the tail-fins are always distinctly 

 vertebrated. All Placoids, both living and extinct, have vertebrated 



Fig. 444.— Homocercal Tail-fin : A, 

 form; B, structure. 



Fig. 445.— Heterocercal or Vertebrated Tail-fin: A, 

 form; B, structure. 



tail-fins. Therefore, all Devonian fishes, without exception, had verte- 

 brated tail-fins — sometimes asymmetric (Figs 431, 434), and sometimes 

 symmetric (Figs. 432, 433). 

 5. In all Teleosts, and 

 in nearly all living fishes, 

 the paired fins (corre- 

 sponding to limbs) are 

 simply fins. The bones of 

 the limbs are buried in the 

 body. But there is one 

 very characteristic Devoni- 

 an family (Crossopterygi- 

 ans) in which the limb is a 

 lobe of the body running through the fin. The relation between the 

 two styles of paired fins is similar to that of the two styles of tail- 



Fig. 446. 



-Vertebrated bnt Symmetrical Fin: A, form ; 

 B, structure. 



Fig. 447.~Head and fore limb of a Ceratodus. 



Fig. 448.— Hind limb of same (after Gunther). 



fins. If, in the other case, we had a vertebrated tail-fin, in this we have 

 paired fins. This style of paired fins is still found in some fishes, 



