80 Morphology of the Fins 



As elements bearing on this decision we may note that the 

 tapering unspecialized diphycercal tail of Pleuracanthus seems 

 very primitive in comparison with the short heterocercal tail 

 of Cladoselache. This evidence, perhaps deceptive, is balanced 

 by the presence on the head of Pleuracanthus of a highly special- 

 ized serrated spine, evidence of a far from primitive structure. 

 Certainly neither the one genus nor the other actually repre- 

 sents the primitive shark. But as Cladoselache appears in 

 geological time, long before Pleuracanthus, Cladodus, or any 

 other shark with a jointed, archipterygial fin, the burden of 

 proof, according to Dean, rests with the followers of Gegenbaur. 

 If the remains found in the Ordovician at Caflon City referred 

 to Crossopterygians are correctly interpreted, we must regard 

 the shark ancestry as lost in pre-Silurian darkness, for in sharks 

 of some sort the Crossopterygians apparently must find their 

 remote ancestry. 



Forms of the Tail in Fishes. In the process of develop- 

 ment the median or vertical fins are, as above stated, older than 



FIG. 63. Heterocercal tail of Sturgeon, Acipenser sturio (Linnaeus). (After 



ZitteL) 



the paired fins or limbs, whatever be the origin of the latter. 

 They arise in a dermal keel, its membranes fitting and accentuat- 

 ing the undulatory motion of the body. 



In this elementary fin-fold slender supports (actinotrichia), 

 the rudiments of fin-rays, appear at intervals. In those fins of 

 most service in the movement of the fish, the fin-rays are 

 strengthened, and their basal supports specialized. 



Dean calls attention to the fact that in fishes which swim, 



