OF THE PAIRED FINS OF ELASMOBRANCHS. 729 



pterygium and propterygium of Gcgenbaur. It bears in^ my 

 specimen of this age four fin-rays at its extremity, the anterior 

 not being well marked. The remaining fin-rays are prolonga- 

 tions outwards of the edge of the plate continuous with the 

 metapterygium. These rays are at the stage figured more or 

 less transversely segmented; but at their outer edge they are 

 united together by a nearly continuous rim of cartilage. The 

 spaces between the fin-rays are relatively considerably larger 

 than in the adult. 



The further changes in the cartilages of the pectoral limb are, 

 morphologically speaking, not important, and are easily under- 

 stood by reference to PI. 33, fig. 9 (representing the skeleton of 

 the limb of a nearly ripe embryo). The front end of the anterior 

 basal cartilage becomes segmented off as a propterygium (//), 

 bearing a single fin-ray, leaving the remainder of the cartilage as 

 a mesopterygium (mes). The remainder of the now considerably 

 segmented fin-rays are borne by the metapterygium. 



General Conclusions. From the above observations, conclu- 

 sions of a positive kind may be drawn as to the primitive 

 structure of the skeleton ; and the observations have also, it 

 appears to me, important bearings on the theories of my pre- 

 decessors in this line of investigation. 



The most obvious of the positive conclusions is to the effect 

 that the embryonic skeleton of the paired fins consists of a 

 series of parallel rays similar to those of the unpaired fins. 

 These rays support the soft parts of the fins, which have the 

 form of a longitudinal ridge ; and they are continuous at their 

 base with a longitudinal bar. This bar, from its position at 

 the base of the fin, can clearly never have been a median axis 

 with the rays on both sides. It becomes the basipterygium 

 in the pelvic fin, which retains its embryonic structure much 

 more completely than the pectoral fin ; and the metapterygium 

 in the pectoral fin. The metapterygium of the pectoral fin is 

 thus clearly homologous with the basipterygium of the pelvic 

 fin, as originally supposed by Gegenbaur, and as has since been 

 maintained by Mivart. The propterygium and mesopterygium 

 are obviously relatively unimportant parts of the skeleton as 

 compared with the metapterygium. 



B. 47 



