1881.] OF THE PAIRED FINS OF ELASMOBRANCHS . 661 



exhibits may be derived from a ground-form which persists in a few 

 cases only, and which represents the first, and consequently the 

 lowest, stage of the skeleton in the fin — the archipterygium. This 

 is made up of a stem which consists of jointed pieces of cartilage, 

 which is articulated to the shoulder-girdle and is beset on either side 

 with rays which are likewise jointed. In addition to the rays of the 

 stem there are others which are directly attached to the limb- 

 girdle. 



" Ceratodus has a fin-skeleton of this form ; in it there is a stem 

 beset with two rows of rays. But there are no rays in the shoulder- 

 girdle. This biserial investment of rays on the stem of the fin may 

 also undergo various kinds of modifications. Among the Dipnoi, 

 Protopterus retains the medial row of rays only, which have the 

 form of fine rods of cartilage ; in the Selachii, on the other hand, the 

 lateral rays are considerably developed. The remains of the medial 

 row are ordinarily quite small, but they are always sufficiently dis- 

 tinct to justify us in supposing that in higher forms the two sets of 

 rays might be better developed. Rays are still attached to the stem 

 and are connected with the shoulder-girdle by means of larger plates. 

 The joints of the rays are sometimes broken up into polygonal plates 

 which may further fuse with one another ; concrescence of this kind 

 may also affect the pieces which form the base of the fin. Bv re- 

 garding the free rays, which are attached to these basal pieces, as 

 belonging to these basal portions, we are able to divide the entire 

 skeleton of the fin into three segments — pro-, meso-, and metaptery- 

 gium. 



" The metapterygium represents the stem of the archipterygium 

 and the rays on it. The propterygium and the mesopterygium are 

 evidently derived from the rays which still remain attached to the 

 shoulder-girdle." 



Since the publication of the memoirs of Thacker, Mivart, and 

 myself a pupil of Gegenbaur's, M. v. DavidofF ^, has made a series of 

 very valuable observations, in part directed towards demonstrating 

 the incorrectness of our theoretical views, more especially Thacker's 

 and Mivart's view of the genesis of the skeleton of the limbs. 

 Gegenbaur' has also written a short paper in connexion with 

 Davidoif's memoir, in support of bis own as against our views. 



It would not be possible here to give an adequate account of 

 DavidofPs observations on the skeleton, muscular system, and nerves 

 of the pelvic fins. His main argument against the view that the 

 paired fins are the remains of a continuous lateral fin is based on 

 the fact that a variable but often considerable number of the spinal 

 nerves in front of the pelvic fin are united by a longitudinal com- 

 missure with the true plexus of the nerves supplying the fin. from 

 this he concludes that the pelvic fin has shifted its position, and 

 that it may once therefore have been situated close behind the 



' M. V. David off, " EeitrJige z. vergleich. Anat. d. hinteren Gliedmaassen d. 

 Pische, I.," Morphol. Jabrbuch, vol. v. 1879. 



^ "Zur Gleidiuaassenfrage. Au die Untersuchungen von DaTidofTs ange- 

 kniipfte Bemerkungeii," Morphol. Jalirbuch, vol. v. 1879. 



