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Show 1886.] AND HYOID ARCHES IN A CKETACEOUS SHARK. 221 gradual, but then very abrupt, while anteriorly the rise is much more uniform and produces a markedly tapering outline. Quite at the front, the cartilage has the appearance of being more robust than is the case further back ; but this is perhaps chiefly due to the infolding of the lower edge for the production of a trough for the membrane bearing the undeveloped teeth. On comparing this form of mandibular arch with the various modifications observed among living Selachians, it is at once evident that none agrees so closely as that of the two genera of Notidanidse. Heptanchus and Hexanchusl, indeed, exhibit an arrangement that differs in no essential particular from that just described in the Cretaceous Hybodont. In both cases there is not only a well-developed pterygo-trabecular process-homologous (as shown by Prof. Huxley ~) with the pedicle of the tadpole's suspen-sorium,- but also a distinct postorbital prominence and articulation, corresponding to the otic process in the tadpole3. The mode of articulation of the lower jaw is also nearly identical in each case ; and though the fossil is at present much crushed, it requires very little careful study to discover that the hollows for the muscles for raising the mandible were quite as deep in the Cretaceous Shark as they are in the living genera under comparison ; the upper border of the quadrate region, however, is much less thickened than in the Notidanidai and agrees more closely with that of ordinary Selachians. In the hyoid arch, the upper or hyomandibular element (figs. 1, 2, hm, and fig. 4) is comparatively small and slender. Its length is 0'037 m., and the cartilage is considerably arched and flattened in what appears to have been an antero-posterior direction. The proximal extremity is imperfect, but was evidently somewhat expanded at its articulation with the cranium; this end is also slightly twisted with respect to the axis of the rest of the element. Just below the bend, the cartilage appears contracted a little when viewed from behind, but soon expands again, forming a blunt tuberosity (t) on the side nearest the pterygo-quadrate ; and from this point it finally becomes gradually narrowed until its termination in the imperfectly-displayed articulation for the cerato-hyal. The cerato-hyal (figs. 1,2, ch) is 0*048 m. in length, and is completely shown on the left side of the fossil, though somewhat mutilated at the distal end ; the lower part, however, is well preserved on the right. The cartilage is considerably arched in the ordinary manner, arid is much less robust towards its upper end than in the rest of its length. Compared with the hyomandibular, it is remarkably stout. A little below the proximal end it becomes comparatively large and 1 See figures by C. Gegenbaur, " Untersuchungen zur vergleichenden Anatomie der Wirbelthiere.-III. Deis Kopfskelet cler Selachier," pi. x. I am also indebted to the kindness of Mr. Howes and Mr. Martin Woodward for every facility for studying the beautiful preparations of Heptanchus, Cestracion, &c. in the Biological Laboratory of the Normal School of Science. 2 T. H . Huxley, loc. cit. p. 40. 3 It is interesting to note that Prof. Cope's Permian Selachian skulls already referred to also exhibit this character. |
