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POPULAR SCIENCE MONTHLY. 



of direction, then into fan-shaped and later paddle-shaped organs of 

 locomotion. Finally, from the jointed paddle, which Gegenbaur has 

 called the archipterygium, there has developed, on the one hand, the 

 ra} r ed pectoral and ventral fins of ordinary fishes, and on the other, in 

 iand-creeping animals, jointed legs and arms. As to this the evidence 

 of paleontology is conflicting. An early shark of the Devonian, Cladose- 



Cladoselache fyleri. After Dean. 



lache, has fan-shaped paired fins so formed and placed. Another shark 

 almost as old, PI eur acanthus, of the Carboniferous, has fins which fit 

 best a totally different theory of origin. Its jointed or archipterygial 

 fin has no resemblance to a fold of skin, but accords better with Gegen- 

 baur's theory that the pectoral limb was at first a modified septum or 

 gill arch. Sharks still older than either (Heterodontidse) in the 

 Silurian, so far as we can judge by their teeth, are closely related to 

 forms bearing the more specialized type of fin found in the typical 



Pleurocanthus decheni Restored. After Brogniart. 



Hypothetical. 



The Anterior Anal Very 



sharks of to-day. Evidently none of these three, as seen in the rocks, 

 represents the real beginning of paired fins in the life of the past. As 

 we shall see, the evidence of comparative anatomy may be consistent 

 with either of these theories, while that of ontogeny or embryology is 

 apparently inconclusive, and that of paleontology seems contradictory. 



Development of the Paired Fins in the Embryo. 

 According to Dr. John A. Ryder ( ' Embryography of Osseous 

 Fishes,' 1882) "the paired fins in Teleostei, like the limbs of the higher 



