Morphology of the Fins 79 



the swimming-fin from the walking and supporting limb, it goes 

 the other way about. That this is the safer line to take seems 

 to me to be shown by the consideration that a very small and 

 rudimentary limb could only be of use if provided with a fixed 

 point d'appui. Also on this view, the pentadactyle limb and 

 the swimming-fin would probably be evolved independently 

 from a simple form of limb. This would evade the great diffi- 

 culties which have beset those who have endeavored to estab- 

 lish the homologies of the elements of the pentadactyle limb 

 with those of any type of fully formed fin." 



Uncertain Conclusions. In conclusion we may say that the evi- 

 dence of embryology in this matter is inadequate, though possibly 

 favoring on the whole the fin-fold theory ; that of morphology 

 is inconclusive, and probably the final answer may be given by 

 paleontology. If the records of the rocks were complete, they 

 would be decisive. At present we have to decide which is the 

 more primitive of two forms of pectoral fin actually known among 

 fossils. That of Cladoselache is a low, horizontal fold of skin, 

 with feeble rays, called by Cope ptychopterygium. That of 

 Pleuracanthus is a jointed paddle-shaped appendage with a 

 fringe of rays on either side. In the theory of Gegenbaur and 

 Kerr Pleuracanthus must be, so far as the limbs are concerned, 

 the form nearest the primitive limb-bearing vertebrate. In 

 Balfour's theory Cladoselache is nearest the primitive type from 

 which the other and with it the archipterygium of later forms 

 may be derived. 



Boulenger and others question even this, believing that the 

 archipterygium in Pleuracanthus and other primitive sharks and 

 that in Neoceratodus and its Dipnoan and Crossopterygian allies 

 and ancestors have been derived independently, not the latter 

 from the former. In this view there is no real homology between 

 the archipterygium in the sharks possessing it and that in the 

 Dipnoans and Crossopterygians. In the one theory the type of 

 Pleuracanthus would be ancestral to the other sharks on the 

 one hand, and to Crossopterygians and all higher vertebrates 

 on the other. With the theory of the origin of the pectoral 

 from a lateral fold, Pleuracanthus would be merely a curious 

 specialized offshoot from the primitive sharks, without descend- 

 ants and without special significance in phylogeny. 



