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Show 1887.] PROF. T. J. PARKER ON CARCHARODON RONDELETII. 33 in so large a specimen a small cartilage imbedded in the immense jaw-muscles would be easily missed. 1 he foramina in the skull-wall have the same general disposition as m Lamna (cf. figs. 5 and 11, Plate V ) , the main differences between the two being the greater proportional size of the optic roramen (n.) in the latter genus, and the fact that the oculomotor foramen (m.) is on the same level as the optic in Carcharodon, while in Lamna it is in the same horizontal plane as the ophthalmic peduncle (op.ped.). In both the carotid foramen (corf.) is a short distance caudad of the optic. Between and below the foramina for the 3rd (Hi.) and the 5th (v.) nerves there is, in Lamna, a small aperture which does not seem to be represented in Carcharodon : possibly it transmits the 6th nerve. The glossopharyngeal and the vagus foramina (Plate IV. figs. I and 2, ix., x.) are both large, the latter in particular being of immense size. The jaws of Carcharodon (fig. 5) are chiefly remarkable for their great size, and especially for the extraordinary'depth of the mandible. In Lamna (fig. 11) their proportional size is considerably less. In another closely allied genus, Alopecias, the cranium has a more rounded form than in Lamna, and is similarly modified in accordance with the great size of the eyes. The rostrum is very thin and delicate, and is hardly at all calcified : its ventral or prenasal bar is perforated at its distal end by a vertical foramen. The jaws have about the same proportional size as in Lamna. The gill-bearing arches of Carcharodon closely resemble those of Lamna and of Scyllium \ The hyomandibular and ceratohyal (Plate VIII. fig. 27, c.hy) are large and stout, and the tongue is supported by a flat basihyal (b.hy) having a convex anterior and an excavated posterior border. The first branchial arch consists of aflat, subtriangular pharyngohyal, a stout epibranchial, and a similar but longer ceratobranchial (c.br. 1) which articulates with the basihyal, there being no first hypobranchial. The next three arches have, in addition, a short rod-like hypobranchial segment (h.br. 2-4). Between the ventral or inner ends of the second hypobranchials (h.br. 2) is a small nodular basibranchial (b.br. 2). The second and third hypobranchials are subequal, the fourth (h.br. 4) is barely half the length of its predecessors. The fourth and fifth pharyngobranchials have undergone concrescence; the fifth ceratobranchial (c.br. 5) is, as usual, much larger than the corresponding segment in the preceding arches. The last arch has no hypobranchial, its ceratobranchial segment (c.br. 5) abutting against an elongated flattened plate (b.br. 5), rounded in front and pointed behind, and probably to be regarded as a fifth basibranchial. To the inner face of the fifth ceratobranchial, near its dorsal end, a small irregular rod of cartilage is attached by fibrous tissue. Can this be the rudiment of a sixth branchial arch 1 The gill-arches are but slightly calcified, even the hyomandibular and ceratohyal having only a thin crust of bony matter which does not extend to their extremities. 1 W . K. Parker, op. cit.; Gegenbaur,' Kopfskelet der Selachier,' Leipzig, 1872. PROC. ZOOL. Soc-1887, No. III. 3 |
