80 STUDIES ON THE ELASMOBRANCH SKELETON, 



first to have been brought to bear on this question by Gegenbaur,* 

 who came to the conclusion that the primitive limb-skeleton 

 or Archipterygium resembled generally that of Ceratodus.j From 

 this primitive form he traced the various modifications of the 

 fiu of fishes and the limbs of the higher vertebrates. The 

 Elasmobranch fin, with which we are immediately concerned, 

 he regards as having been developed from the Archipterygium 

 by the suppression of the post-axial series of fin-rays — the 

 metapterygium representing the axis of the archipterygium, 

 and the rnesopterygium and propterygium, together with the 

 rays connected with them, having been derived from certain 

 rays which he supposes to have been directly connected with the 

 shoulder-girdle in the primitive fin. The archipterygium itself he 

 regards as having been developed from a branchial arch and its 

 connected rays. A similar view of the relations of the Elasmo- 

 branch fin to that of Ceratodus and the Crossopterygian Ganoids 

 was embraced by Professor Huxley, and maintained by him as 

 late as 1876, J: but Huxley regarded the axis of the archipterygium 

 as being represented by the mesopterygium, not the metapterygium, 

 and does not give his adherence to the theory that the limbs are 

 modified branchial arches. 



Balfour § has shown more recently (1878) that the paired fins 

 of Scylliuiyi originate in two pairs of lateral ridges of epiblast, the 

 embryonic limbs of each side being connected for a time by a low 

 continuous ridge, which, however, soon disappears. In the 

 messoblast subsequently growing into these folds of epiblast is 

 developed a longitudinal bar of cartilage. The outer side of this 

 is connected with a plate which extends into the fin and becomes 

 segmented to form a series of parallel rays situated at right angles 

 to the longitudinal bar. In front this longitudinal bar is 

 continuous with the limb-arch. From these observations has been 



* Grundriss der Versrieichenden Anatornie. 



t See Giinther, Description of Ceratodus, Phil. Trans.. 1871. 



I On Ceratodus Forsteri, with Observations on the Classification of Fishes, P.Z.S., ISfG, 

 pp. 24-59. 



§ Monograph on the Development of Elasmobranch Fishes, pp. 101-104 ; Comparative 

 Embryology, Vol. II., p. 49, (1881.) 



