FOSSIL VERTEBRATES — FISHES 397 



The development of the fins seems to have followed a very 

 definite line that has served as a great aid in making out the 

 classification of the various fossil and recent forms. Undoubtedly 

 the first stage of the development of the fins was the forma- 

 tion of the long folds of the skin that were without any internal 

 support, and capable of very complex, wavelike motion, and 

 without any great power of resistance to impressed forces. Of 

 this stage we do not have any trace in the preserved fossil forms 

 for the reason already assigned, that soft parts are not preserved 

 except under the most exceptional circumstances. We should 

 expect it to occur in the remains of the Marsupiobranchii, if at 

 all, but the single specimen preserved, Paleospo?idylus, does not 

 show any evidence of such a fold. The function of the fin fold, 

 to preserve the equilibrium of the fish, would demand some 

 degree of resistance in the fold, and the next stage of the fin 

 must have been the appearance of fine, hair-like rays of horny 

 material, confined to the dermal part of the fold and not joined 

 to the body proper. These were many in number, and only 

 served to stiffen the fin without strengthening its attachment to 

 the body. These fine rays have been called the actinotrichia. 

 The second stage in the development of the fins was the fusion 

 of certain of these actinotrichia at points of the greatest strain 

 in the fins into larger and more solid elements that afforded a 

 much greater power of resistance at those points. The compara- 

 tively large and strong cartilaginous rods thus formed have 

 received the name of radials. The same necessity of resisting 

 outside forces that caused the union of the actinotrichia to form 

 the radials demanded a stronger attachment of the radials to the 

 body wall of the fish, and this was accomplished by the separa- 

 tion of the proximal portion of the radial as a separate element, 

 which became elongate and penetrated into the body wall, afford- 

 ing a very strong support to the radials. This proximal section 

 is called the basal (Fig. 2). 



• Up to this point in the development of the fins the history of 

 the paired and the unpaired fins is regarded as practically the 

 same, for the paired fins were as yet but undifferentiated parts 



