422 OSBURN 



(3) In those Teleosts which show a migration of the pelvic 

 to a thoracic position, the nerves of the fin are carried forward 

 during the process, while no nerves are picked up on the way 

 and no new collector is formed as a result of the migration. 



The hypothesis of migration is therefore very far from meet- 

 ing all the conditions. The only adequate explanation (Mollier 

 '93) is that all the fins have shortened up at the base and have 

 formed the collectors by bringing together nerves which once 

 entered separately to innervate the longer fin. In favor of this 

 view we have direct evidence that the fins of modern sharks 

 do shorten up at the base during ontogeny, and we know also 

 that the fins of the oldest fossil sharks (Pleuropterygidas, Acan- 

 thodidae, and Diplacanthidae) were of the fin-fold character, 

 broadest at the base and without any posterior indentation or 

 notch such as modern selachian fins possess. 



III. The facts of the degeneration of the most anterior 

 muscle-buds, and of the backward extension of those buds 

 immediately posterior to these to enter the fin, can not be 

 accepted as proof of the migration of the fin, as has been so 

 strongly urged by the gill-arch theorists (Braus '98), for: 



(1) The same process occurs at the posterior border of the 

 same fin. The observations of Braus ('98) demonstrate that 

 in Spinax the last pelvic muscle-bud degenerates without enter- 

 ing the fin, while the buds immediately in front of it are com- 

 pelled to reach forward to enter. My preparations of Spinax 

 show that four such buds extend forward to attain their posi- 

 tions in the fin, and the same condition is observed in 

 Cestracion. 



(2) But, most significant, the same process, I find, occurs 

 in the unpaired fins. In the dorsal fins of Cestracion the most 

 anterior buds extend backward while the most posterior reach 

 forward to enter the fin. Again, in Paul Mayer's work ('86) 

 published more than twenty years before this objection to the 

 fin- fold theory was raised, we learn from his description of 

 the unpaired fins of Scyllium and Pristiurus that there are 

 abortive muscle-buds both before and behind the dorsal fins, 

 and we may observe from his plates that the most posterior 



