OF THE PAIRED FINS OF ELASMOBRANCHS. 719 



shortening of the axis of such a fin-skeleton as that of Ceratodus, 

 and the coalescence of some of its elements." Huxley does not 

 enter into the question of the origin of the skeleton of the pelvic 

 fin of Elasmobranchii. 



It will be seen that Huxley's idea of the primitive structure 

 of the archipterygium is not easily reconcilable with the view 

 that the paired fins are parts of a once continuous lateral fin, in 

 that the skeleton of such a lateral fin, if it has existed, must 

 necessarily have consisted of a series of parallel rays. 



Gegenbaur 1 has done more than any other living anatomist 

 to elucidate the nature of the fins ; and his views on this subject 

 have undergone considerable changes in the course of his in- 

 vestigations. After Giinther had worked out the structure of 

 the fin of Ceratodus, Gegenbaur suggested that it constituted the 

 most primitive persisting type of fin, and has moreover formed a 

 theory as to the origin of the fins founded on this view, to the 

 effect that the fins, together with their respective girdles, are to 

 be derived from visceral arches with their rays. 



His views on this subject are clearly explained in the sub- 

 joined passages quoted from the English translation of his 

 Elements of Comparative Anatomy, pp. 473 and 477. 



"The skeleton of the free appendage is attached to the 

 extremity of the girdle. When simplest, this is made up of car- 

 tilaginous rods (rays), which differ in their size, segmentation, 

 and relation to one another. One of these rays is larger than 

 the rest, and has a number of other rays attached to its sides. I 

 have given the name of arcJiipterygium to the ground-form of 

 the skeleton which extends from the limb-bearing girdle into 

 the free appendage. The primary ray is the stem of this archip- 

 terygium, the characters of which enable us to follow out the 

 lines of development of the skeleton of the appendage. Carti- 

 laginous arches beset with the rays form the branchial skeleton. 

 The form of skeleton of the appendages may be compared with 



1 C. Gegenbaur, Untersuchungen z. vergleich. Anat. d. Wirbelthiere (Leipzig 

 1864-5): erstes Heft, "Carpus u. Tarsus;" zvveites Heft, " Brustflosse d. Fische." 

 "Ueb. cl. Skelet d. Gliedmaassen d. Wirbelthiere im Allgemeinen u. d. Hinterglied- 

 maassen d. Selachier insbesondere," Jenaischc Zeitschrift, Vol. v. 1870. "Ueb. d. 

 Archipterygium," Jenaische Zeitschrift, Vol. vn. 1873. " Zur Morphologic d. Glied- 

 maassen d. Wirbelthiere," Morphologisches Jahrbuch, Vol. u. 1876. 



