1881.] OF THE PAIRED FINS OF ELASMOBRANCHS. 669 



by Gegenbaur and Huxley, both of whom, as stated above, consider 

 the primitive type of fin to be most nearly retained in Ceratodus, 

 and to consist of a central multisegmented axis with numerous 

 lateral rays. 



Gegenbaur derives the Elasmobranch pectoral fin from a form 

 which he calls the archipterygium, nearly like that of Ceratodus, 

 with a median axis and two rows of rays — but holds that in addition 

 to the rays attached to the median axis, which are alone found in 

 Ceratodus, there were other rays directly articulated to the shoulder- 

 girdle. He considers that in the Elasmobranch fin the majority of 

 the lateral rays on the posterior (or median according to his view 

 of the position of the limb) side have become aborted, and that the 

 central axis is represented by the metapterygium ; while the pro- 

 and mesopterygium and their rays are, he believes, derived from those 

 rays of the archipterygium which originally articulated directly with 

 the shoulder-girdle. 



This view appears to me to be absolutely negatived by the facts 

 of development of the pectoral fin in Scylliuin — not so much be- 

 cause the pectoral fin in this form is necessarily to be regarded as 

 primitive, but because what Gegenbaur holds to be the primitive 

 axis of the biserial fin is demonstrated to be really the base, and it 

 is only in the adult that it is conceivable that a second set of lateral 

 rays could have existed on the posterior side of the metapterygium. 

 If Gegenbaur's view were correct, we should expect to find in the 

 embryo, if anywhere, traces of the second set of lateral rays ; but 

 the fact is that, as may easily be seen by an inspection of figs. 6 and 7, 

 such a second set of lateral rays could not possibly have existed in 

 a type of fin like that found in the embryo. With this view of 

 Gegenbaur's it appears to me that the theory held by this anatomist 

 to the effect that the limbs are modified gill-arches also falls, in that 

 his method of deriving the limbs from gill-arches ceases to be ad- 

 missible, while it is not easy to see how a limb, formed on the type 

 of the embryonic limb of Elasmobranchs, could be derived from a 

 gill-arch with its branchial rays. 



Gegenbaur's older view, that the Elasmobranch fin retains a pri- 

 mitive uuiserial type, appears to me to be nearer the truth than his 

 more recent view on this subject ; though I hold the fundamental 

 point established by the development of these parts in Scyllium 

 to be that the posterior border of the adult Elasmobranch pectoral 

 fin is the primitive base-line, i.e. line of attachment of the fin to the 

 side of the body. 



Huxley holds that the mesopterygium is the proximal piece of 

 the axial skeleton of the limb of Ceratodus, and derives the Elasmo- 

 branch fin from that of Ceratodus by the shortening of its axis and 

 the coalescence of some of its elements. Tlie entirely secondary 

 character of the mesopterygium, and its total absence in the young 

 embryo Scyllium, appear to me as conclusive against Huxley's view 

 as the character of the embryonic fin is against that of Gegenbaur ; 

 and I should be much more inclined to hold that the fin of Ceratodus 

 has been derived from a fin like that of the Elasmobranchs by a 

 series of steps similar to those which Huxley supposes to have led 



