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Show 1876.] PROF. T. H. HUXLEY ON CERATODUS FORSTERI. 43 branchia, Cestracion exhibits a lower stage of organization than Chimeera, in which, as in Ceratodus, the mandibulo-hyoid cleft has disappeared. On the other hand, the hyoidean arch presents a form intermediate between that of the ordinary Selachians and that of Ceratodus and Chimeera. It is stout; and its dorsal element, still retaining a little of its original form, but much thicker and more cylindrical, is no longer united with the skull by ligament and muscle merely, but articulates with a process of the underside of the periotic capsule. Moreover its distal end is connected by strong ligamentous fibres with the posterior end of the palato-quadrate cartilage and with an inward process of the articular end of the mandible (the sustentaculum of Gegenbaur). In fact, the "epibranchial" of the hyoidean arch of Cestracion is just beginning to take on a new function, that of suspending the palato-quadrate cartilage and mandible to the skull. It is a true hyomandibular, though small and insignificant relatively to what it becomes in other Plagiostomes, in Ganoids, and in Teleostei. Had I been acquainted with the skull of Cestracion in 1858, I should have been spared the hesitation which I then felt* as to identifying the hyomandibular of Fishes with the summit of the hyoidean arch, and which has subsequently been removed by abundant evidence published by Mr. Parker and myself. In the general form of the skull, the position and proportions of the olfactory capsules, and the characters of the principal labial and alinasal cartilages, Cestracion has a stronger resemblance to Chimeera than is exhibited by any other Plagiostomes; and I take it to be one of the lowest of Selachian skulls. I am aware that in expressing this opinion I am diametrically opposed to Gegenbaurf, whose elaborate study of the Plagiostome'skull entitles his opinion to the greatest weight, and who regards Cestracion as possessed of one of the highest of skulls in its group, while Heptanchus and Hexanchus have the lowest. There is a certain ambiguity about the terms "highest" and " lowest;" but if by the former we understand the most extreme modification of the hyostylic type of skull characteristic of the group, then I should have no hesitation in regarding the skulls of the Rays as the highest of Plagiostome skulls, while Cestracion represents a low form of the autostylic type. Notidanus, on the other hand, appears to me to have an essentially low form of skull, so far as it is more completely amphistylic than any ordinary Plagiostome; but on this low form is superinduced a modification by which it approaches the higher autostylic skull. This is the union of the palato-quadrate arch with the postorbital * Croonian Lecture, 1858, and Lectures on the Theory of the Skull, 1864. See also " O n the Malleus and Incus," P. Z. S. 1869; ' Manual of Vertebrate Anatomy,' 1871, p. 85; and Mr. Parker's "Memoir on the Development of the Salmon," Phil. Trans. 1872. t ' Das Kopfskelet der Selachier,' p. 60. In controverting the opinion of Professor Owen that the Cestracion is less advanced in cranial development than Squatina, Gegenbaur observes, " So mochte ich gerade das Gegentheil behaupten und nicht etwan bloss beziiglich der Basalverhaltnisse des Craniums." |