| OCR Text |
Show 24 PROF. G. B. HOWES ON THE SKELETON AND [Jan. 18, If the above suggestion should prove to have weight, the condition of the basal parts of the Polypterus fin, in which the mesopterygium is in no way in connexion with the shoulder-girdle, can only be a lowly one. It may not be inappropriate here to call attention to the conception lately put forward by Baur (2, p. 663) concerning the morphology of the cheiropterygium. He returns to Gegenbaur's first position, and maintains that the limb of the land-animal has been derived directly from the ichthyopterygium. In that case the Ceratodus fan, as it stands, can only represent the initial phase in a line of modification of the ichthyopterygium, culminating in Protopterus (to include Lepidosiren. Cf. Ayers, Jenaische Zeitschr. vol. xviii. p. 479, 1885, and Schneider, op. cit.). Davidoff claims that the Ceratodus pelvic fin (7, p. 127) " trotz der Einfachheit des Ganzen, sich bedeutend komplicirter gestaltet, als bei den friiher bearbeiteten Fischen." He uses the words in a physiological sense, it is true, but that in face of his concluding statement that (p. 160) " das Endergebnis aber besteht darin, dass von der Ceratodus-Extremitat sich diejenige der Haie ohne Schwier-igkeiten ableiten lasst." This is, in my opinion, far from proven. VII. Conclusions. 1. That the characters of the skeleton of the Ceratodus paired fins are inconstant, except for those of the preaxial parameres of the pectoral fin and the basal mesomere of both pectoral and pelvic fins. 2. That a metapterygium is always present in the fore limb, in a reduced condition and usually confluent with the second mesomere. 3. That traces of what appears to represent a metapterygium are occasionally to be met with in the hind limb, under conditions which point to atavism. 4. That the basal mesomere of the Ceratodus fin may conceivably have been derived from the metapterygium. 5. That the structural features of both paired fins of the Chimse-roids are identical, and characterized by the absence of a mesopterygium. 6. That the paired fins of the Plagiostomes and Dipnoi have, in all probability, arisen independently from a type of fin most nearly represented by that of the living Chimaeroids. 7- Proven incidentally.-That the basal cartilage of the Cestracion pectoral fin, usually regarded as the mesopterygium, is a compound of the pro- and mesopterygia of other Plagiostomes. VIII. List of Authorities referred to. 1. BALFOUR, F. M.-On the Development of the Skeleton of the Paired Fins of Elasrnobranchii. Proc. Zool. Soc. 1881, p. 656. 2. B A U R , G.-Ueber das Archipterygium und die Entwicklung des Cheiropterygiums aus dem Ichthyopterygium. Zool. Anzeiger, vol. viii. p. 663 (1885). |
