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Show 1887.] PAIRED FINS OF CERATODUS. 19 mesopterygium, and by Gegenbaur (11) to represent the metapterygium. The points of difference between the two sets of observers are so well known that recapitulation of them would be superfluous here. I incline most nearly to Huxley's view, and hold that the axis represents mainly, if not wholly, the mesopterygium of the Sharks ; but I regard its condition in the latter as typical of its earlier and more primitive state. I moreover think it not unlikely that the short-lobed fin of the Crossopterygidm will prove to be of an earlier type of structure than that of the elongated one of the living Dipnoi. Extended obs'ervations along the lines already laid down by Traquair, in his Monograph on Tristichopterus (27), are greatly to be desired. That the elements described by him are probably, and that those described by Goldfuss (13) and Kner (20) in Xenacanthus are certainly, homologous with those of the axis of the Ceratodus fin, I fully believe, and we have here the foundation of a line of study which must soon yield fruitful resultsl. Wiedersheim is the only worker who, to my knowledge, has offered an opinion upon the last named. He says (29 p. 195), speaking of the Ceratodus fin, " dass dieser Organisationsplan der Brustflosse auch bei untergegangenen Fischgeschlechtern eine Rolle gespielt haben muss, steht unzweifelhaft fest und ich mochte dabei nur an den aus der Permformation stammenden Xenacanthus decheni erinnern." There can no longer be much doubt that the confluence so frequently seen between one or more rays and the mesopterygium of the Elasmobranchs represents the last trace of the process by which that structure is formed (cf. Dohrn, cited on p. 15). This granted, it becomes a question as to how far the axis of the Ceratodus fin, as here defined, represents a further extension of this fusion of primarily parallel rays or an elongation of the mesopterygial plate, as it exists in the Sharks. The fact that irregularity in distribution of the parameres is generally accompanied by that of the segmentation of the axis in Ceratodus (cf. especially figs. 1, 5, & 7), shows that there is an intimate connexion between the two ; and this is the more obvious on reflection that Davidoff has shown (7, p. 145) that the segmentation of the axis does not stand in constant relationship to the muscular attachments2. It is moreover inconceivable, if regarder comme autant de rayons membraux rested libres, et ils seraient les homologues de ceux qui, par leur asservation, donnent naissance aux membres proprement dits sur d'autres points du corps." 1 I look with great satisfaction upon the work now being done in this direction by Smith Woodward (cf. P. Z. S. 1886). It is time that some such check should be kept upon the deductions of the embryologist (cf. Baur, Morph. Jahrb. vol. viii. p. 453). 2 H e writes, " D a wir auch an der Muskulatur eine den einzelnen Segmenten der Stammreihe entsprechende Gliederung fanden, zo gewinnt diese Ansicht an Wahrsheinlichkeit, obwohl immerhin noch einzuwenden ist, dass die physiologische Bedeutung dieser Gliederung eine nur ausserst minimale sein kann, dass ferner auch die Zwischensehnen der Stammuskulatur in gar keiner naheren Beziehung zu den Segmenten der Stammenreihe stehen." I could find no difference between the muscles of the fin represented in fig. 6 and those described by him. |