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Show 22 PROF. G. B. HOWES ON THE SKELETON AND [Jan. 18, mesomere and the swollen basal piece referred to ; and the whole condition of the parts is such as would have resulted did the proximal mesomere and its related lateral elements represent a shortened-up metapterygium. Beyond this, the pelvic member of Ceratodus differs most conspicuously from that of the Plagiostomes and Osteichthyes in the presence "of an elongated mesopterygium. Balfour first showed (1, pp. 666-7) that the development of the pelvic fin of the Shark is arrested at a comparatively early stage. He and subsequent writers regard the enlarged preaxial ray, which Huxley would hold to represent the mesopterygium, as the propterygium. He has also called attention to the fact that the mesopterygium is not there represented ; and the only anticipation of that structure forthcoming among the Plagiostomes, known to me, is the comparatively insignificant one described by Haswell (16. p. 23, pl. i. fig. 3) for Heptanchus indicus. VI. On the Homologies of the Chim&roid Fin-skeleton, as compared with that of Ceratodus. Huxley, discussing the morphology of the Chimseroid pectoral fin, insists (19, pp. 52-53) upon the close relationship which it bears to that of Ceratodus. The former fin is, as is well known, supported upon two basal elements, both of which are in intimate connexion with the pectoral girdle. The postaxial of these is held by all to represent the metapterygium. As to the preaxial cartilages:-Huxley (19, pp. 52-53), whose view demands that the mesopterygium " constantly retains its primary articulation with the pectoral arch," completely reverses Gegenbaur's determination (9, p. 145), and regards the smaller basal one as the mesopterygium, and the larger ray-like distal one which it bears as the propterygium. Mivart, on the other hand, insists (21, p. 478) on the absence of the mesopterygium, and regards both preaxial elements as homologous with the propterygium. Comparison of the fin of Chimcera with that of the Selachians, as represented in Hexanchus, appears to me to warrant his view. It is interesting, here, to recall Huxley's remarks upon the metapterygium, when dealing with Chimcera. Having asserted the belief that the metapterygium of Notidanus is " formed by the coalescence of the axial ends of the postaxial rays," he goes on to say (p. 53), " the metapterygial cartilage cannot, in Scyllium, at the same time represent coalesced postaxial rays, as the analogy of Notidanus would suggest, and the second joint of the axial skeleton as the analogy of Chim&ra .... indicates." Did the mesopterygium exist in Chimcera in the form so constant among the Plagiostomes -that of a fusion of the basal ends of the rays interposed between tbe pro- and metapterygia-he could, in comparing the Chimseroid and Ceratodus fins, only have come to the conclusion formed by me (p. 15), in describing the second pectoral mesomere of the latter. The pelvic fin of the Chimseroid is in an exceptionally interesting |