660 MR. F. M. BALFOUR ON THE SKELETON [June 7, 



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, it' it has existed, must necessarily have consisted 

 of a series of parallel rays. 



Gegenbaur ' 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 investigations. 

 After Giiuther had worked out the structure of the fin of Cerutodus, 

 Gegenbaur suggested that it constituted the most primitive joems^2?i<)' 

 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 subjoined 

 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 cartilaginous rods 

 (rays), which diff'er in their size, segmentation, and relation to one 

 another. One of these rays is larger than the rest, and has a num- 

 ber of other rays attached to its sides. I have given the name of 

 archipterygium 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 archipterygium, the characters of which 

 enable us to follow out the lines of development of the skeleton of 

 the appendage. Cartilaginous arches beset with the rays form the 

 branchial skeleton. The form of skeleton of the appendages may be 

 compared with them ; and we are led to the conclusion that it is 

 possible that they may have been derived from such forms. In the 

 branchial skeleton of the Selachii the cartilaginous bars are beset with 

 simple rays. In many a median one is developed to a greater size. 

 As the surrounding rays become smaller, and approach the larger 

 one, we get an intermediate step towards that arrangement in which 

 the larger median ray carries a few smaller ones. This differentiation 

 of one ray, which is thereby raised to a higher grade, may be con- 

 nected with the primitive form of the appendicular skeleton ; and as 

 we compare the girdle with a branchial arch, so we may compare the 

 median ray and its secondary investment of rays with the skeleton of 

 the free appendage. 



"All the varied forms which the skeleton of the free appendages 



^ C. Gegenbaur, 'TJntersuchuiigen z. vergleich. Anat. d. Wirbelthiere' (Leipzig, 

 1864-6) : erstes Heft, Carpus u. Tarsus ; zweites Heft, Brustflosse d. Fische. 



"Ueb. d. Skelet d. Gliedniaassen d. Wirbelthiere im Allgemeinen u. d. Hin- 

 tergliedmaassen d. Selachier insbesondere," Jenaische Zeitechrift, vol. v. 1870. 



" Ueb. d. Archipterj'gium," Jenaische Zeitschrift, vol vii. 1873. 



"Zur Morphologie d. Grliedmaassen d. Wirbelthiere," Morphologisches 

 Jahrbuch, vol. ii. 1876. 



