OCR Text |
Show 52 PROF. T. H. HUXLEY ON CERATODUS FORSTERI. [Jan. 4, cavity. In like manner the proximal praeaxial ray (propterygium, has ascended along the praeaxial face of the axial cartilage, until it also is able to furnish a facet which completes the anterior part of the cup for the condyle of the pectoral arch. In Squatina, similar modifications have occurred ; but the axial cartilage remains large, and the large praeaxial and postaxial cartilages are directed respectively forwards and backwards, in accordance with the form of the vastly expanded fin. In Raia, yet further expansion is obtained by the separation of the axial and postaxial cartilages and the interpolation of postaxial rays between them. The proximal ends of these enter into the articulation, as the great postaxial cartilage had already done. The interpretation of the skeleton of the pectoral fin of Chimeera presents some difficulties. This skeleton consists (fig. 10) of:-(l)a proximal cartilage (I), which articulates by an excavated surface with the condyle of the pectoral arch ; (2) a flat, curved, elongated middle cartilage (Mt), which is inclined backwards; (3) a small semilunar distal cartilage (c), which fits onto the convex distal end of the last. At the end of the convex posterior edge of the distal cartilage is a small cartilaginous ray, consisting of a long basal and a short terminal segment. Similar rays, which gradually become longer, follow this along the free convex edge of the distal cartilage and that of the middle cartilage; but the proximal end of the latter bears a much stronger ray, with a wide base (R), which for the most part unites with the ventral edge of the proximal cartilage (I), but is connected with the middle cartilage (Mt) by its posterior produced angle. There can, I think, be no doubt that the proximal cartilage (I) in Chimeera answers to the proximal cartilage in Ceratodus. The small proximal postaxial cartilages also exactly correspond ; and the large proximal praeaxial rays no less closely answer to one another. But if this be so, it follows that the whole skeleton of the fin in Ceratodus is represented in that of Chimeera. The distal cartilage (c) in Chimeera is the result of the coalescence of the bases of a certain number of the postaxial rays, as is obvious on tracing the series round. Hence it would appear that all that can represent the series of median segments except the first is the middle cartilage (Mt). It further seems probable that this middle cartilage in great part, if not wholly, represents the second segment of the Ceratodus limb. The postaxial edges, a b and b c, correspond closely; but the edge ef, long in Ceratodus, is reduced to nothing in Chimeera ; while the edge c d, occupied exclusively by the third segment in Ceratodus, is greatly elongated and bears all the praeaxial rays in Chimeera. In order to change the skeleton of the pectoral fin of Ceratodus into that of Chimeera, all that will be necessary, if this comparison is correct, is that the third and following median segments of the former shall be gradually reduced, either by abortion or coalescence with the second, more and more postaxial fin-rays becoming attached |