OF THE PAIRED FINS OF ELASMOBRANCHS. 731 



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 poste- 

 rior (or median according to his view of the position of the limb) 

 side have become aborted, and that the central axis is repre- 

 sented by the metapterygium ; while the pro- and mesoptery- 

 gium 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 Scy Ilium not so 

 much because 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 

 admissible, 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 primitive uniserial 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 Scyllimn 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. 



472 



