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Show 18 PROF. G. B. HOWES ON THE SKELETON AND [Jan. 18, shoulder-girdle may have been due to a displacement of the same by two ravs. Should this be so, the metapterygium must there have disappeared, as from the Ceratodus pelvic fin, under the corresponding enlargement of the mesopterygial plate. The only alternative view possible is that the metapterygium does represent that of the Elasmobrancbs. If this be so, comparison of the pectoral fin of Polypterus (fig. 11) with that of the Plagiostomes, as represented in Scyllium, where the mesopterygium is relatively small, would seem to show that the loss of connexion between the metapterygium and its rays has been to no small extent due to a displacement of the latter by the elongation of the expanding mesopterygium, no less than by the simplification in structure of the metapterygium itself. The last step in the former process would appear, indeed, to be retained in the living Polypterus (** fig. 11). In the absence of embryological data further discussion of this difficulty would be fruitless. It is much more pertinent to observe that in both Ceratodus and Polypterus the initial step in the modification has been, in any case, one of elongation of the mesopterygium, and evidence has been adduced to show that in the Dipnoi (if not in Polypterus also) the metapterygium has been thereupon reduced and finally suppressed. The only traces of either it or its rays yet recorded in the Ceratodus pelvic member are forthcoming in fins whose postaxial parameres are more numerous and less specialized than is generally the case. If this simplification in structure of the most specialized portion of the pelvic fin-skeleton is, as I have attempted to show, reversionary to a condition which has been lost, the characters of those elements which reappear under the simplification, when compared with those to which they most nearly correspond in the pectoral fin, go far towards bearing out the presumed origin of the Ceratodus fin from a primarily expanded predecessor. V. On the Morphology of the Avis of the Ceratodus Fin. The entire axis of the Ceratodus fin is held by Huxley (19) Balfour1 (1), and v. Bautenfeld (22) to represent an elongated the two specimens examined by me. It has been figured by Wiedersheim (29, p. 199), but I have been unable to find a description of it. I think it not improbable that it m a y have been derived from the mesopterygium, the closely related lower anterior end of which may (as Wiedersheim has shown) insert itself between the supposed propterygium and the marginal rays. In the absence of embryological data further discussion of it would be useless. 1 Baur has recently called attention (3, p. 6) to the fact that Gervais has priority over Humphrey in the enunciation of the hypothesis that the paired fins are dismembered portions of a lateral fold. Gervais Writes (12):-" Si Ton considere que les rayons des nageoires impaires des poissons ont une analogie incontestable avec ceux dont la reunion formes les nageoires paires des memes animaux, e'est a dire leurs membres veritables, on est naturellement conduit a se demander s'ils ne seraient les homologues de ces derniers, et si lVtat d'isole-ment dans lequel ils restent les unes par rapport aux autres, ne resulterait pas de ce que chacun d'eux ne conserve pas completement ses rapports avec celui des segments osteodestniques dont il est tributuire. Alors on pourrait lea |