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Show 1876.] PROF. T. H. HUXLEY ON CERATODUS FORSTERI. 5.') cisely stated in his ' Grundriss der vergleichenden Anatomie,' 1874, p. 493, in the following words : - "The very various forms of the skeleton of the free limbs are deducible from a fundamental form of which only a few instances remain, and which, as representing the first and lowest condition of the fin-skeleton, I term the archipterygium. This is represented by a jointed cartilaginous stem, articulated with the pectoral arch and giving attachment, on each side, to a series of likewise jointed pieces, the radii. The whole structure, resembling a pinnate leaf, is singularly like the supporting apparatus of many Selachian gills, and thus throws a gleam of light upon the phylogeny of the limbs. " Ceratodus presents this form of fin-skeleton, which was perhaps usual among the Crossopterygidce, at present represented only by Polypterus. The biserial rays of the fin undergo different modifications. x-Vmong the Dipnoi the medial [postaxial] rays are retained in the form of thin rods of cartilage; while in the Selachians the lateral [praeaxial] rays attain a considerable development and constitute the greater part of the massive fin-skeleton. Of the medial [postaxial] rays but few remain, though they are sufficiently distinct to sanction the assumption of a former more extensive biserial arrangement of rays on the stem of the fin." The metapterygium Gegenbaur considers to answer to the axial skeleton of the archipterygium. The propterygium is formed by the union of the proximal praeaxial fin-rays. The mesopterygium is formed by a certain number of the succeeding praeaxial fin-rays. The only part of this interpretation with which I can agree is the determination of what Gegenbaur names the propterygium as the representative of the proximal praeaxial fin-ray or rays in most cases, but not in Chimeera, and probably not in Notidanus. In my judgment, the mesopterygium of Gegenbaur is the proximal piece of the axial skeleton, which constantly retains its primary articulation with the pectoral arch. His propterygium represents the proximal praeaxial fin-ray, and his metapterygium the proximal postaxial fin-ray in almost all cases ; and the ichthyopterygium, as the typical fish-fin may be termed, differs from the archipterygium not by the more or less complete suppression of the postaxial rays, but by the general abbreviation of the whole skeleton and the gradual connexion of more or fewer fin-rays (parameres) with the pectoral arch. In the effectual discharge of the function of the fish's fin, increase of breadth is needed ; and this increase of surface is obtained by the gradual approximation of more and more lateral elements of1 the archipterygium to the shoulder-girdle. Professor Gegenbaur has extended his theory of the limbs to the higher Vertebrata. He conceives that the axis of the archipterygium (which he considers to be the homologue of the metapterygium of the Selachian) is represented by the series of bones which is formed by the humerus, the radius, the radial segments of the carpus, and the radial digit or pollex ; while the ulna, the radial segments of the carpus and the ulnar digit, the other carpal bones, and the fourth, |
