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Show 222 MR. A. S. WOODWARD ON THE MANDIBULAR [Apr. 20, is much compressed from side to side, the superior edge thus formed being well marked and sharp, but the lower appearing thicker (fig. 2 a). About two thirds of the distance from the upper extremity a gradual twist in the cartilage renders the remaining portion compressed almost from above downwards, and it ends distally in a triangular expansion, well shown on the right. The terminal edge was evidently articulated to a basihyal of considerable size, but of this no trace has been preserved. At one third of its length from the proximal end, the cerato-hyal exhibits a prominent tuberosity on the lower border (fig. 2, tu). This appears to be situated opposite a point corresponding to the attachment of the mandibulo-hyoid ligament of living Selachiaus '; but I have failed to discover indications of any similar prominence either in Heptanchus or the other specimens and figures to which I have been able to refer. It evidently represents a muscular insertion, and one of no small importance. In Teleostean fishes, the well-developed genio-hyoideus arises from an equivalent point in the hyoid arch, and by its insertion at the symphysis acts as the main depressor of the mandible 2. In the Selacbii, however, the function is undertaken chiefly by the coraco-mandibulares arising from the pectoral arch, and a differentiated genio-hyoid appears to be wanting. The elaborate researches of Vetter 3 in regard to the myology of the jaws and branchial arches in Heptanchus and Acanthias can leave no doubt that the tuberosity in question is that deep insertion of the great constrictor superficialis muscle which becomes the origin of the genio-hyoid in higher fishes ; and its marked character in the Cretaceous Hybodont may perhaps indicate that in this form the differentiation had already taken place to a certain extent. On the whole, the form of hyoid arch just described bears a greater resemblance to that of the Notidanidse than to that of any other living family. It agrees in the fact that the hyomandibular and cerato-hyal are most contracted at their point of union, but the elements are somewhat stouter than those both of Heptanchus and Hexanchus. The type specimen of Hybodus dubrisiensis (B.M. 36908) is of somewhat smaller size than the fossil here described, and as its cartilages are apparently less calcified, it probably represents a younger individual. It is comparatively imperfect, and except in the characters of the teeth and the upper border of the pterygo-quadrate, it does not admit of any detailed comparison with the foregoing descriptive account. There are, however, unmistakable indications of an articular otic process, besides a well-marked pterygo-trabecular eminence ; and a small fracture shows the slight thickening of the overturned edge of the quadrate region (fig. 6). 1 W. K. Parker, "On the Structure and Development of the Skull in Sharks and Skates," Trans. Zool. Soc. vol. x. p. 210, pi. xxxviii. fig. 2. 2 P. Owen, ' Anatomy of Vertebrates,' vol. i. p. 206, fig. 135. 3 B. Vetter, " Uutersuchungen zur vergleichenden Anatomie der Kiemen-und Kiefermusculatur der Fische.-I," Jenaische Zeitschrift, vol. vii. (1874), pp. 403-458, pis. xiv., xv. |
