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Show 20 PROF. G. B. HOWES ON THE SKELETON AND [Jan. 18, the Ceratodus fin has arisen as an elongation of a primarily expanded predecessor, that such a fusion of the approximated ends of the rays could have resulted from that simple process. I am of opinion that the distal portion of the mesopterygium of Ceratodus has arisen on an elongation of a pre-existing plate, by a process such as is seen at its earliest phase in Polypterus. A difficulty, however, arises with respect to the basal mesomere of Ceratodus, which, if it represents the proximal end of the mesopterygium, differs from that of all other fishes in forming (in the pectoral fin, at any rate) the sole support for the base of the fin. Gegenbaur, who has paid considerable attention to this matter, at first acquiesced (10) in Huxley's belief in the close relationship between Polypterus and the Dipnoi. Commenting upon the pectoral fin of the former animal, he writes (p. 138) " ausser den Selachiern bei denen die zweieilige Form des Archipterygium in die einzeilige iibergeht, besitzt vielleicht nur noch Polypterus unter den lebenden Ganoi'den das primare Archipterygium im Flossenskelete." He goes on to advance the view that the fin of Polypterus represents a shortened-up derivative of the Ceratodus type ; but finally he reverts to his original position, holding, chiefly on account of the loss of connexion between the mesopterygium and shoulder-girdle, and of the great structural difference between the pectoral and pelvic fins, that (p. 140) "demnach kann ich das genannte Skelet von Polypterus nicht unmittelbar auf das primare Archipterygium beziehen, sonderu leite es, wie jenes der anderen lebenden Ganoi'den, von der secund-aren, nur eine Reihe von Radien besitzenden Form ab." The pelvic fin of Ceratodus appears, at first sight, to be exceptional in the possession of a wTell-developed mesopterygium. Davidoff has brought forward good evidence to show that the element hitherto regarded among Ganoids and Teleostei as the pelvic girdle is (5. p. 125, and 6. p. 433) homologous with the basal piece of the Ceratodus fin, and he terms it the basal segment of the metapterygium. He has shown good reason for believing that the true pelvis is seen for the last time among the Osteichthyes in Polypterus, where it is represented by two or three vestigial cartilages (cf. 6. p. 462, pl. 21, and Wiedersheim, 28) lying immediately in front of the applied ends of the basal pterygia of opposite sides. Upon careful consideration, I am disposed to accept his interpretation as it applies to the Ganoids and Teleostei, but I am more dubious about it as applying to the Dipnoi. Examination of either of the paired fins of Ceratodus in relation to the limb-girdle appears, at first sight, to favour Huxley's view that the whole fin-axis answers to the Selachian mesopterygium. If this be so, that element must, in elongating, have carried down with it the metapterygium, and the propterygium if present. Balfour, criticizing Huxley's view that the basal mesomere is the proximal piece of the axial skeleton of the limb of Ceratodus, says (1, p. 669), "The entirely secondary character of the mesopterygium and its total absence in the young embryo Scyllium appear to |