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Show 32 PROF. T. H. HUXLEY ON CERATODUS FORSTERI. [Jan. 4, This would seem to imply that the chondrocranium of vertebrated animals is formed by the coalescence of parts analogous to those which compose the bony skull of osseous fishes. As a matter of fact, however, the chondrocranium is never formed of such elements, but is built up, at a very early stage of embryonic development, by the union of primitively distinct parachordal, otic, and pleural elements*, which in no way correspond with the bones of the teleostean skull. And when Dr. Gunther subsequently (I. c. p. 522) speaks of " three groups of superficial labial cartilages," "the upper labial," the "supraorbital," and the "lower labial," the discrimination of such cartilages must, I think, be regarded merely as regional anatomy; and it must not be supposed that they have any thing to do with the cartilages to which the same names are applied in other fishes, several of which exist in Ceratodus, and will presently be described. When the osseous and merely fibrous structures are carefully removed, I find that the chondrocranium (figs. 4, 2, and 7) consists of a continuous cartilaginous mass, the interorbital region of which is much narrower than any other part, produced inferiorly and laterally into two stout suspensorial or palato-quadrate processes, with the pulley-shaped ventral ends of which the strong Meckelian cartilages are articulated. Anteriorly the orbits are bounded by the antorbital processes (A.O), which curve downwards in front of the eye. From these antorbital processes the cartilage is continued forwards to form the evenly curved roof of the ethmoidal region and its contained nasal chambers, and, bending down on all sides, ends in a free edge, which is slightly concave opposite each anterior nasal opening (an, fig. 4), and much more deeply excavated opposite the posterior nares (pn, fig. 4). The small process (a) which lies between the two excavations in question is connected by a strong fibrous band with the antorbital process (A.O), and this, by the ossified bar described by Dr. Gunther, with the postorbital process (P.O) ; but these structures have nothing to do with the chondrocranium. Behind the orbits, the skull suddenly widens out into two broad periotic masses, which lodge the auditory labyrinth. Anteriorly and ventrally these processes are continued into the suspensorial pillars (Qu) ; while behind they pass into thin but wide cartilaginous plates (O, C, fig. 4), which roof over the chambers in which the branchiae are lodged. I thought at first that Ceratodus had no labial cartilages; but at length I discovered two small upper labial cartilages in their right places, namely in the region of the nostrils. One of them (1, figs. 4 and 5) lies in the roof of the mouth, just in front, and on the inner side of, the posterior nasal aperture. It is fixed to the mesethmoidal cartilage (M.E) by fibrous bands, and is broader behind than in front. The inner edge of this cartilage is concave, the outer convex, and it has a nearly horizontal direction. The second cartilage (2, figs. 4, 5, 7) is stouter, and lies behind, and on the outer side of, the posterior nasal aperture. Its dorsal end is attached to the base of the skull and anterior part of the * "On the Structure of the Skull &c. of Menobranchus lateralisi' P. Z. S. 1874. |
