68 



Morphology of the Fins 



derived from a series of gill-rays attached to a branchial arch. 

 The baclavard position of the ventral fin is due to a succession 

 of migrations in the individual and in the species. 



As to this theory, Mr. J. Graham Kerr observes: 

 ' ' The Gegenbaur theory of the morphology of vertebrate 

 limbs thus consists of two very distinct portions. The first, 

 that the archipterygium is the ground-form from which all other 

 forms of presently existing fin skeletons are derived, concerns 

 us only indirectly, as we are dealing here only with the origin 



Fig. .54. — Skull and .shoulder-girdle of Neoceratodus forsteri (Giinther), showing the 



archipterygium. 



of the limbs, i.e., their origin from other structures that were 

 not limbs. 



"It is the second part of the view that we have to do with, 

 that deriving the archipterygium, the skeleton of the primitive 

 paired fin, from a series of gill-rays and involving the idea that 

 the limb itself is derived from the septum between two gill-clefts. 



" This view is based on the skeletal structures within the fin. 

 It rests upon (i) the assumption that the archipterygium is 

 the primitive type of fin, and (2) the fact that amongst the 

 Selachians is found a tendency for one branchial ray to become 

 larger than the others, and, when this has happened, for the 

 base of attachment of neighboring rays to show a tendency 

 to migrate from the branchial arch on to the base of the larger 

 or, as we may call it, primary ray; a condition coming about 

 which, were the process to continue rather farther than it is 

 known to do in actual fact, would obviously result in a struc- 



