at present hold the field. If untrue it is so dangerously plausible as to surely deserve more consideration than it appears to have had. One of the main differences between it and the other two hypotheses is that, instead of deriving 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 difficulties which have beset those who have endeavored to establish 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 evidence of embryology in this matter is inadequate, that of morphology is inconclusive and perhaps 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 that in Neo-Ceratodus and its Dipnoan and Crossopterygian allies have been derived independently from the archipterygium in the primitive sharks. 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 descendants and without special significance in phylogeny.
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 specialized serrated spine, evidence of a far from primitive structure.
