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Show 1887.] PAIRED FINS OF CERATODUS. 17 receive refutation, proportionate to the support furnished for those more especially of Balfour, Haswell, and Dohrn already cited. As stated previously, the cartilage r of fig. 1 is ray-like, but stouter and more powerful than that of any ordinary paramere. In seeking light on this question, one naturally turns to Poly-pterus, the affinities between which and the Dipnoi, originally pointed out by Huxley (18), have nowhere been denied. The Polypterus pectoral fin is, as is well known, supported upon three basal elements. The mesopterygium (fig. 11, ms.) is held by all to represent that of the Plagiostomes, and no one has yet challenged Gegenbaur's determination (9, p. 148) of the homology between the elongated postaxial bar (mt.) of this fish and the metapterygium of the Plagiostomes and Chimceroids. Huxley says of this fin (19, p. 53) that "the Scyllium type is essentially preserved." Comparison of the Polypterus pectoral fin (fig. 11) with the pelvic fin of Ceratodus represented in fig. 1 would appear at first sight to suggest a homology between the basal postaxial bar (r) of the latter and the metapterygial bar (mt.) of the former. If this be justified, it would further appear, accepting the homology of the metapterygium of Polypterus with that of the Elasmo-branchii, that the two fins might have been derived along a line of modification characterized by the assumption on the part of the metapterygium of a ray-like character, and by the subsequent elongation of the mesopterygial plate (ms.). The probable truth of the latter assertion seems to me very great indeed. The mesopterygium is, in Polypterus (fig. 11, ms.), already elongated beyond the limits met with elsewhere, displacing in the process the marginal rays. Continue that elongation, and there could only result a Ceratodus-Wke product. As concerns the former supposition, however, comparison of the fin-skeletons represented in figs. 1, 3, & 4 is sufficient in itself to show that the proximal postaxial ray of fig. 1 most probably represents the distal one of those related to the proximal mesomere of fig. 4. Comparison of the latter (fig. 4) with the proximal end of the pectoral fin cf the same side of the same animal (fig. 8) shows unmistakably that in the plate-like structure resulting from the fusion of the basal ends of the two proximal parameres we have to deal with the last trace of the metapterygium, defining that, as must now be done, as a product of the confluence of the inner ends of the proximal postaxial rays, the distal ray being, from the nature of its relations therewith, one of the same series. Consideration of the above facts renders the homology of the supposed metapterygium of Polypterus somewhat doubtful. Gegenbaur, when pointing to the same, realized the similarity between both pro- and metapterygia so-called by him (fig. 11,pt. andmt.) and the marginal rays1. He at first suggested (10, p. 139) the possibility that the exclusion of the mesopterygium from connexion with the 1 The difficulty of interpretation of the supposed propterygium is greatly increased by the presence of the cartilage marked * in fig. 11,-by no means the least puzzling element in this fin. A s will be seen, it is grafted upon the anterior border of the propterygial rod; from it, however, it is perfectly distinct in PROC. ZOOL. Soc-1887, No. II. 2 |
