'11,, Cnitiimj of Vertebrates <m<l I tine of 



Another group of fishes which arose during 

 the Devonian period is that termed Crossop- 

 terygii, or fringe-finned, because the members 

 of the group have their side fins formed by a 

 fringe of rays arranged about a central portion. 

 These fishes, according to Dr. Dean, combine 

 the characters of a shark, lung-fish, and ganoid, 

 and among their curious features is what may 

 be called their attempts to construct a tail-fin. 

 The most commonly accepted theory of the ori- 

 gin of fins is that they have been derived from 



The development of fins. The light shading indicates the hypo- 

 thetical original fold, the dark shading shows the present fins. 



a fold of membrane running along either side 

 and uniting beneath the body to form a single 

 fold, which continued around the tail and along 

 the back to the head. It is easy to see that if 

 pieces were cut from such a fold the various 

 fins of fishes would be left, and some of the 

 fishes under consideration look as if in doing 



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