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Show 14 PROF. G. B. HOWES ON THE SKELETON AND [Jan. 18, were beginning to unite. The propterygium (pt.) is seen to compose fully the anterior third of the whole mass, its base being made up of two segments, the proximal one of which contributes nearly half the articular cup, entering at least as fully into the formation of the same as does its representative in Scyllium (cf. Huxley, 19, p. 48, fig. 10). In the adult fin (fig. 10) the original boundary line between the pro- and mesopterygia is represented by a groove indicated in Huxley's figure (here reproduced) by a dotted line. The pectoral fin of Cestracion is thus shown to conform to the Selachian type, being identical most nearly with that as represented by Acanthias (cf. Gegenbaur, 9, pl. 9. fig. 4, and Mivart, 21, pl. 77. fig. 2). Cestracion must, on the evidence now forthcoming, relinquish its position in the series established by Huxley ; it must, to say the least, change places with Notidanus. The main articulation of the fin of the last-named fish is established, as is well known, through the agency of the mesopterygium. Gegenbaur originally described a basal preaxial bar in Notidanus, and he homologizes it (9, p. 140) with his propterygium. This enters, if anything, more fully into the articulation with the shoulder-girdle than does its representative in Cestracion. There is connected with its distal end, in Hexachus, a smaller piece (cf. Gegenbaur, op. cit. pl. 9. fig. 1) which appears to represent the second segment of Cestracion, reduced, as an outcome of the great expansion of the front border of the mesopterygium. Mivart has suggested (21, p. 444) that the mesopterygium represents a coalescence of the pro- and mesopterygia; but I am inclined, upon careful examination of the specimen under my hand, to dissent from that view. The metapterygium of Notidanus enters into a feeble but definite connexion with the pectoral arch, such as is not the case in Cestracion. That the fins of these two genera differ from those of some of the Sharks is indisputable, but they do so to an insignificant degree, incapable in itself of supporting the " unibasal " type ; that, in face of the facts here adduced ', rests upon an insufficient basis. Returning now to Ceratodus; the lobe which, in the pectoral fin, carries the 3-5 proximal postaxial parameres (figs. 5 aud 6 mt.) is, as has been stated previously, marked off from the adjoining mesomere by a deep furrow. Gunther observes (14, p. 532) that the conjoined mass shows, in "horizontal section," lines of "the former divisions" into what he holds to correspond to the " three carpals " (pterygia) of most Plagiostomes. This has been denied by Huxley (19, p. 47). Setting aside this difference for the moment, I desire to call attention to the similarity of the furrow described above to that which 1 When, moreover, it is considered that in the pectoral fin of Polypterus, which Huxley relegates to the " tribasal " category, the mesopterygium is (as Gegenbaur pointed out, 10, p. 139) excluded from articulation with the limb-girdle, the statement that (p. 55) "the mesopterygium is the proximal piece of the axial skeleton, which constantly retains its primary articulation with the pectoral arch," must needs be modified. |