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Show 40 PROF. T. H. HUXLEY ON CERATODUS FORSTERI. [Jan. 4, I have already indicated the chief points of resemblance to the amphibian skull, and need not recapitulate them here. The most important feature is the manner in which the mandibular arch is connected with the skull. The part of the palato-quadrate cartilage which is united with the skull, between the exits of the fifth and second nerves, answers to the " pedicle of the suspensorium" of the amphibian, while its backward and upward continuation onto the periotic cartilage corresponds with the otic process. As in the Amphibia and in the higher Vertebrata, the mandibular arch is thus attached directly to the skull by that part of its own substance which constitutes the suspensorium. It may thus be said to be autostylic. Among fishes, the only groups which possess an autostylic skull, or in which the dorsal end of the mandibular arch is continuous with the cartilage of the brain-case, are the Chimaeroids and the Marsipobranchii. In Chimeera, the general form and connexions of the palato-quadrate cartilage are the same as in Ceratodus; but it differs from that of Ceratodus as that of the tadpole differs from that of a young Frog, or as that of Menobranchus differs from that of Menopoma; that is to say, the articular condyle is situated far more forward, and the gape is, in consequence, relatively shorter in the former than in the latter. There are the same large olfactory capsules in both cases. In Chimeera, however, these project beyond the termination of the ethmoidal cartilage, while in Ceratodus the latter projects beyond the olfactory capsules, which are more lateral in position, more elongated, and, in accordance with the general form of the head, much more depressed. Just as in Ceratodus, the palato-quadrate cartilage of Chimeera bears two teeth marked with radiating ridges, while two others, the vomerine teeth, are supported by the ethmoidal cartilage in front of these; and in both cases there is a tooth with radiating ridges on its surface in each ramus of the mandible. In the disappearance of the notochord and the articulation of the skull with the anterior coalesced vertebrae, the skull of Chimeera presents a higher degree of differentiation than that of Ceratodus ; while it is needless to speak of such aberrant peculiarities as its supracerebral interorbital septum, or the vast crest into which the skull is raised above the anterior part of the brain-cavity. In other respects, however, as in the inclination of the axis of the suspensorium already noted, the skull of Chimeera presents lower characters than that of Ceratodus. Among these may be reckoned the great size of the upper and lower labial cartilages and the condition of the hyoidean arch, which, except in size and some peculiarities of form, is altogether similar to the four branchial arches which follow it. Like them, it terminates, dorsally, in a flat, expanded, triangular piece, which is connected with the superjacent floor of the skull by muscles and ligaments, but by no direct articulation. The dorsal pieces of the succeeding branchial arches have the same form and attachments, and unite with the ventral segments at a sharp angle. These angles |
