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Show 1876.] PROF. T. H. HUXLEY ON CERATODUS FORSTERI. 41 are all connected together by a strong ligament, which is continued to the pectoral arch. Moreover a small styliform cartilage passes from the last angle to the pectoral arch, and is connected with the dorsal end of the fifth branchial arch. It appears to represent the dorsal element of that arch. Johannes Miiller, fully appreciating the importance of the differences between the skull of the Chimaeroids and those of other " Elasmobranchii," and sagaciously remarking that " the skull of Chimeera is most like that of a tadpole "*, was thereby led to separate the Chimaeroids as a suborder of the Elasmobranchii under the name of Holocephali. It appears to me that he might have been justified in going still further; for, considering, in addition to the cranial characters, the structure of the vertebral column and of the branchiae, the presence of an opercular covering to the gills, the peculiar dentition, the almost undeveloped gastric division of the alimentary canal, the opening of the rectum quite separately from and in front of the urinogenital apertures, the relatively small and simple heart, the Chimaeroids are far more definitely marked off from the Plagio-stomes than the Teleostei are from the Ganoidei. In all other Fishes, except the Marsipobranchii, the mode of connexion of the mandibular arch with the skull is different from that which obtains in the Chimaeroids and the Dipnoi. The palato-quadrate cartilage is no longer continuous with the chondrocranium (though the' bony elements of that arch may unite suturally with those of the skull, as in the Plectognathi), but is, at most, united with it by ligament. Moreover the dorsal element of the hyoidean arch, or the hyomandibular, usually attains a large size and becomes the chief apparatus of suspension of the hinder end of the palato-quadrate cartilage with the skull. Skulls formed upon this type, which is exemplified in perfection in Ganoidei, Teleostei, and ordinary Plagiostomes, may therefore be termed hyostylic. But though the typical forms of autostylic and hyostylic skulls, as exemplified, e. g., by a Sturgeon, a Pike, and a Dogfish or Ray, on the one hand, and Chimeera, Ceratodus, and Menobranchus on the other, are thus widely different, certain Plagiostomes present a condition of the cranium which tends to connect the two by a middle form, which may be termed amphistylic. In the amphistylic skull the palato-quadrate cartilage is quite distinct from the rest of the skull; but it is wholly, or almost wholly, suspended by its own ligaments, the hyomandibular being small and contributing little to its support. The embryo amphibian is amphistylic before it becomes autostylic ; and, in view of certain palaeon-tological facts, it is very interesting that the link which connects the amphistylic with the ordinary Selachian skull is that of Cestracion (fig. 8). If the palato-quadrate cartilage of Chimeera were membranous in the centre, as it is in the tadpole, and if along three lines radiating from this centre the cartilage were converted partly into fibrous tissue and partly into a true joint, the result would be to produce a palato- * ' Vergleichende Anatomie der Myxinoiden,' erster Theil, p. 150. |