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Show 50 PROF. T. H. HUXLEY ON CERATODUS FORSTERI. [Jail. 4, edges of the fin ; and they differ only in the relative extent of the central area, on which the fin-rays do not encroach. All the Chimaeroids and Plagiostomes are eminently crossoptery-gian so far as their fins are concerned; and therefore we might expect to find in the skeleton of the pectoral fins of these fishes a modification of the skeleton of that of Ceratodus. But in most of these fishes the skeleton of the fins has undergone such an amount of metamorphosis that it is difficult to reduce itto the type of Ceratodus. In Notidanus*, however, the skeleton of the pectoral fin affords the key to the nature of this metamorphosis. Here (fig. 10) there is an axial cartilage, the broad proximal end of which articulates with the pectoral arch. Distally it diminishes in diameter, and ends by a truncated face, with which another slender cylindrical cartilage, also axial in position, is articulated. I take these two cartilages to represent the shrunken axis of the fin of Ceratodus. The praeaxial basal angle of this axial mass is occupied by a distinct cartilage.. Whether this represents the proximal axial cartilage of Ceratodus, or whether it is the proximal praeaxial ray, is not clear. The praeaxial edge of the principal axial cartilage, at some little distance from this piece, presents a series of notches, with which are articulated a corresponding number of praeaxial rays, while, as has been already stated, a single ray is articulated to the base of the terminal axial cartilage. The uppermost or proximal praeaxial ray is two-jointed and broader than the others. O n the postaxial side there is a triangular cartilage (Mt), wide distally, very narrow proximally, where it is connected with the proximal end of the axial cartilages. Twelve postaxial rays are articulated with the wide distal edge of this cartilage. I conceive that this triangular postaxial cartilage is formed by the coalescence of the axial ends of the postaxial rays. The fin-skeleton of Notidanus thus results, in the simplest possible manner, from the shortening of the axis of such a fin-skeleton as that of Ceratodus and the coalescence of some of its elements. In Cestracion (fig. 11) the same process is carried a step further. Here there is a large cartilage (I) which articulates with the pectoral arch by a concave facet, and corresponds with the proximal axial cartilage of Ceratodus and Notidanus. United by ligament with its hinder concave margin is the triangular postaxial cartilage (Mt), which obviously answers to the similarly shaped postaxial cartilage of Notidanus, and which bears a series of postaxial rays, of which the first is directed almost at right angles to the axis of the fin, as in Ceratodus. The first praeaxial ray (R) is very large and bifurcates distally. The ray which is attached to the distal angle of the axial cartilage probably answers to the ray-like termination of the axial skeleton in Notidanus; but it is not distinguishable from the rest. In the Dogfish (Scyllium) (fig. 10) the further shortening of the axis gives rise to still greater changes. The axial cartilage (meso- * The figure represents the fin of the fcetal Notidanus to which I have already referred. The figure of the fin of an adult of the same species given by Ge°*en-baur (Untersuchungen, Heft ii.Taf. ix. fig. 2) shows essentially the same structure |
