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Show 1887.] PAIRED FINS OF CERATODUS. 23 condition. Its postaxial border is supported by a cartilage, admitted by all to represent the metapterygium (fig. 12, mt.). This appears to be produced out into a preaxial lobe, which is regarded by Davidoff (4. p. 470, pl. 28. fig. 3, and pl. 29. fig. 18), who last described it, as consisting of a single piece answering to the propterygium. It also recalls most closely that lobe from which Balfour held (I, p. 667) that both pro- and mesopterygia are derived. In a young Chimseroid pelvic fin examined by me (fig. 12), the lobe in question is seen to be formed by the fusion of three preaxial rays, and careful examination has shown that the last traces of an original separation between it and the metapterygium (indicated in the drawing by a dotted line) exist. Did that persist, the fin would correspond in all essential respects with the^pectoral member, as I have defined it; and I hold that this preaxial lobe is neither more nor less than the propterygium 1. Mivart comments (21, p. 465) upon the "close resemblance " between the pectoral and pelvic fins of the Chimaeroids. Comparing the pelvic fin of these animals (Callorhynchus) with the pectoral ones of Acanthias and Scymnus, he concludes (p. 456) that the basal cartilage represents, in the former, all three pterygia fused into one. The considerations put forward above, taken together with the fact that the mesopterygium never appears in the Plagio-stome's pelvic fin, beyond the insignificant degree observed by Haswell, appear to me to negative this view. The facts now under notice suggest, but do not prove, that the mesopterygium is never represented at all in the Chimseroids; and that with respect to that feature those fishes stand on a lower platform than do the living riagiostomes. Moreover, if the preaxial cartilages of their pectoral member represent the propterygium, as I believe, an absolute structural identity is proven between the pectoral and pelvic fins of the group. Both would appear to have been derived from the fins of an ancestor in which the mesopterygium was not differentiated ; and if so, that element must have been of comparatively late origin. Davidoff has pointed to the existence of structural similarities between the hip-girdles of Chimcera and Ceratodus (7, pp. 142-3); and if the magnificent array of structural affinities between the two, so successfully demonstrated by Huxley (19), have the weight which he assigns to them, I think it more than probable, if, as I have suggested, the basal mesomere of Ceratodus is a derivative of the metapterygium, that the paired fins of the Dipnoi may have arisen, side by side with those of the Plagiostomes, from some such form as is to-day represented by Chimcera-the fusion of the rays to form the mesopterygium having gone on independently, the intercalation of that structure between the applied bases of the pro- and meta-pterygia, so characteristic of the Plagiostomes, having been a comparatively late process. 1 The free ray represented at * in fig. 10 has been described by Davidoff (op. cit p. 471). The spur-like outgrowth of the same, which I think may not improbably represent the coalesced vestige of a second similar one, was not present in his specimen. |