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Show 1876.] PROF. T. H. HUXLEY ON CERATODUS FORSTERI. 33 palato-quadrate cartilage just above the middle of the palatine tooth. It thence descends with an outward convexity and inward concavity, and terminates in the upper lip near the angle of the mouth. Dr. Gunther states (I. c. p. 524) that " the body of the mandible is persistent cartilage; but its entire outer and inner surfaces are covered by bone, forming an articular and a dentary piece The articular and dentary bones meet near the top of a low but strong coronoid process, and again at the symphysis, which is formed by fibrous tissue, and m a y easily be separated by the knife . . . . In front of the jaw the cartilage is expanded into a slightly concave lamella (lower labial cartilage)." Fig. 5. Ceratodus forsteri. TJnderview of the skull, showing the vomerine teeth (V.t), the palatine teeth, the mesethmoid cartilage (M.E), and the upper labial cartilages (1, 2) in place. The dotted lines An,p.n indicate the form and position of the anterior and the posterior nares. I find a persistent Meckelian cartilage, such as that here described; but as, after careful removal of the ensheathing bones, I have been unable to discover any separation between this lamellar expansion and the rest of the cartilage, I a m in doubt whether the lamella represents the lower labial cartilage or not. The analogy of the Frog, however, leads one to suspect that distinct lower labial cartilages m a y exist in the young Ceratodus. Dr. Gunther does not mention a third ensheathing bone (figs. 2 and 4, D) which is united by suture with the other two, and lies on each side of the symphysis on the ventral face of the mandible. It is a flat plate, of a triangular form, with a thick rugose inner edge for the attachment of the symphysial ligament. Its posterior edge is thin and concave; its external edge is also thin and overlaps the bone termed " articular " by Dr. Gunther, uniting with it by a squamous suture. The outer half of its dorsal aspect is smooth, and helps to support the ventral face of Meckel's cartilage ; the inner or symphysial half presents a broad rough triangular surface, which extends on the inner P R O C ZOOL. S O C - 18/6, No. III. 3 |
