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Show 1876.] PROF. T. H. HUXLEY ON CERATODUS FORSTERI. 45 spiracle, in correspondence with its homologue the tympano-eusta-chian passage. The dorsal and posterior edge of this process no less clearly corresponds with the spiracular cartilage in Cestracion, otherwise absent in Notidanus. The hyoidean arch is attached to the periotic region of the skull. It is very slender j and though closely bound by ligament to the mandibular arch, close to the articulation of the mandible, it can contribute little or nothing to the support of the latter. Remove the great otic process of the palato-quadrate arch of Notidanus, and bring its quadrate end further forward, and the result would be a typically amphistylic skull, such as exists in the larval Siredon and Triton*. The relations of tbe skulls of Notidanus, Cestracion, and Ceratodus m a y be thus expressed : - Notidanus most nearly approaches the amphistylic skull, such as exists in all the autostylic Vertebrata in the embryonic state; but it is considerably altered by the development of a great otic process from the mandibular arch. In Cestracion the palato-quadrate has become massive, and, in the region of the pedicle, is firmly united with the skull, while the otic process is a separate cartilage, connected only by ligament with the postorbital process above and with the palato-quadrate below. In Ceratodus the palato-quadrate has coalesced with the skull both by its pedicle and by its otic process; and the same change occurs in the autostylic skulls of the Amphibia. In the ordinary Plagiostomes, on the other hand, the palato-quadrate becomes smaller and more freely united with the skull. The otic process ( = spiracular cartilage) is smaller, and the hyomandibular takes a larger and larger share in suspending the mandibular arch to the skull, which is therefore eminently hyostylic. Turning now to osseous fishes, the skull which presents the nearest resemblance to Ceratodus is that of Polypterus. This will be obvious to any one w ho studies the excellent description of the chondrocranium of this fish given by Dr. Traquairf. But in the character of the palato-quadrate arch and the great relative size of the hyomandibular, the skull is as thoroughly and completely hyostylic as is that of any other existing Ganoid or Teleostean. Thus, having regard only to the structure of the cranium, the relations of Ceratodus may expressed as follows: Amphibia. -Ganoidei. Teleostei. CERATODUS.: -Cestracion. Baia. Ch imcera. Notidanus. -> • v sr- AUTOSTYLICA. AMPHISTYLICA. HYOSTYLICA. * P. Z. S. 1874, pl. xxxi. figs. 1 & 2. t " The Cranial Osteology of Polypterus," Journal of Anatomy and Ph™*- ology, 1870. J J |
