1881.] OF THE PAIRED FINS OF ELASMOBRANCHS. 667 



The pelvic fin thus retains in all essential points its primitive 

 structure. 



The Pectoral Fin. — The earliest stage of the pectoral fin differs, 

 as I have shown, from that of the pelvic fin only in minor points 

 (Plate LVIIl. fig. 6). There is the same longitudinal or basipterygial 

 bar {bp), to which the fin-rays are attached, which is continuous in 

 front with the pectoral girdle {p g)- The changes which take place 

 in the course of the further development, however, are very much 

 more considerable in the case of the pectoral than in that of the 

 pelvic fin. 



The most important change in the external form of the fin is 

 caused by a reduction in the length of its attachment to the body. 

 At first (Plate LVIII. fig. 6), the base of the fin is as long as the 

 greatest breadth of the fin ; but it gradually becomes shortened by 

 being constricted off from the body at its hinder end. In connexion 

 with this process the posterior end of the basipterygial bar is gra- 

 dually rotated outwards, its anterior end remaining attached to the 

 pectoral girdle. In this way this bar comes to form the posterior 

 border of the skeleton of the fin (Plate LVIII. figs. 8 & 9), consti- 

 tuting the metapterygium {mp). It becomes eventually segmented off 

 from the pectoral girdle, simply articulating with its hinder edge. 



The plate of cartilage, which is continued outwards from the basi- 

 pterygium, or, as we may now call it, the metapterygium, into the fin, is 

 not nearly so completely divided up into fin-rays as the homologous 

 part of the pelvic fin ; and this is especially the case with the basal 

 part of the plate. This basal part becomes, in fact, at first only 

 divided into two parts (Plate LVIII. fig. 8) — a small anterior part at 

 the front end (^nie.p), and a larger posterior along the base of the 

 metapterygium ("i/)) ; and these two parts are not completely seg- 

 mented from each other. The anterior part directly joins the pec- 

 toral girdle at its base, resembling in this respect the anterior fin-ray 

 of the pelvic girdle. It constitutes the (at this stage undivided) rudi- 

 ment of the mesopterygium and propterygium of Gegenbaur. 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 prolon- 

 gations outwards of the edge of the plate continuous with the meta- 

 pterygium. These rays are at the stage figured more or less trans- 

 Tersely 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, mor- 

 phologically speaking, not important, and are easily understood by 

 reference to Plate LVIII. fig. 9 (representing the skeleton of the limb 

 of a nearly ripe embryo). The front end of the anterior basal carti- 

 lage becomes segmented off as a propterygium (pp), 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 

 home by the metapterygium. 



General Conclusions. — From the above observations, conclusions 

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



43* 



