OF THE PAIRED FINS OF ELASMOBRANCHS. 57 



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 modi- 

 fications. Among the Dipnoi, Profopierus 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 

 distinct 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 joint of the rays are 

 sometimes broken up into polygonal plates, which may 

 further fuse with one another ; concrescence of this kind 

 may also afi'ect the pieces which form the base of the fin. 

 By regarding 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 metapterygium. 



" The metapterygium represents the stem of the archi- 

 pterygium and the rays on it The propterygiura and the 

 mesopterygium are evidently derived from the rays which 

 still repaain attached to the shoulder-girdle.^' 



Since the publication of the memoirs of Thacher, 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 Thacher's and Mivart's view of the 

 genesis of the skeleton of the limbs. Gegenbaur^ has also 

 written a short paper in connection with Davidoff's memoir, 

 in support of his own as against our views. 



It would not be possible here to give an adequate account 

 of Davidoff's 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 commissure with the 

 true plexus of the nerves supplying the fin. From this he 



' M. V. DavidofF, " Beitriige z. vergleicb. Anat. d. hinteren Gliedmaassen 

 d. Fische I.," 'Morpho!. Jahrbuch,' vol. v, 1879. 



2 " Zur Gliedmaassenfrage. An die Uiitersuchungen von DavidofiTs 

 angekniipfte Bemerkungen," 'Morphol. Jahrbuch,' vol. v, 1879. 



