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Show 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, if it has existed, must necessarily have consisted of a series of parallel rays. Gegenbaurl 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 Gunther 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 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 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 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 connected 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 1 C. Gegenbaur,' TJntersuchungen z. vergleich. Anat. d. Wirbelthiere' (Leipzig, 1804-5): erstes Heft, Carpus u. Tarsus; zweites Heft, Brustfiosse d. Fische. " TJeb. d. Skelet d. Gliedmaassen d. Wirbelthiere im Allgemeinen u. d. Hin-tergliedmaassen d. Selachier insbesondere," Jenaische Zeitschrift, vol. v. 1870. " Ueb. d. Archipterygium," Jenaische Zeitschrift, vol vii. 1873. "Zur Morphologie d. Gliedmaassen d. Wirbelthiere," Morpholosisches Jahrbuch, vol. ii. 1876. 6 |