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Show 1887.] PAIRED FINS OF CERATODUS. 21 me as conclusive against Huxley's view, as is the character of the embryonic fin against that of Gegenbaur." Examination of the pectoral fin of Ceratodus shows that the elements which I hold to represent the metapterygium and its rays (figs. 5, 6, & 8, mt. & r.) are related to the postaxial border of the second mesomere. There is at most a bare suggestion of a relationship to the proximal mesomere (m.p.). In the hind limb this is otherwise, for those parts of the skeleton which most nearly repeat the characters cf the presumed metapterygium of the fore limb are unmistakably connected (figs. 1 & 4) with the proximal mesomere. If they really represent the metapterygium and its rays as they occur among the lower fishes, it is, I think, not unlikely, p tting together these facts and those recorded by Davidoff, that while the metapterygium has for the most part disappeared, the proximal mesomere may, after all, turn out to represent the proximal end of that structure as defined by Huxley, early differentiated and segmented off. The above suggestion, should it be substantiated, would explain the fact that the proximal mesomere of Ceratodus is the only constituent of the fin-axis whose characters are constant. It would simplify our conceptions of the fins of the Ganoids and Dipnoi, and bring into harmony the supposed divergent modifications of the fins of opposite extremities ; while it would show the pelvic member to be, on the whole, less modified than is usually thought. I am disposed to think, moreover, that it receives support from the absence of preaxial rays in connexion with the basal mesomere of Ceratodus; from the complete exclusion of the mesopterygium from connexion with the shoulder-girdle in Polypterus ; and from the condition of the pelvic fin of that animal, already alluded to, no less than from the marked tendency towards an increased development of the proximal end of the pectoral metapterygium among the living Ganoids. Still more suggestive is the condition of the basal elements of a Protopterus pectoral fin represented in fig. 8 a, Wiedersheim has (as I have already mentioned, p. 5) shown that the proximal piece of the pectoral fin-skeleton of this animal bears ray-like elements. He describes a smaller distinct ventral (postaxial) one and a larger dorsal (preaxial) one, which is confluent with the main piece (proximal mesomere as compared with Ceratodus). I have examined two specimens ; in one of them the latter is much smaller than in his example, while in the other (fig. 8 a) there is no trace of it. I can onlv conclude therefore that it is a lobe of the basal mesomere, variable in character. Not so with the former ; that is in both perfectly distinct, being separated from the basal mesomere by a fibrous tract, such as subdivides any two segments from each other. In that specimen which was destitute of the preaxial process (fig. 8 a) its characters are still further noteworthy. It is elongated and shows traces of subdivision into two pieces, the basal one of which is swollen and enlarged in common with tbe proximal mesomere (m.p.), and from that it appears most clearly to have been derived. The second segment of the axis is in relation with both the proximal |