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Show 1886.] AND HYOID ARCHES IN A CRETACEOUS SHARK. 219 to the cranium and the cartilages of the visceral arches. Employing the terminology of Prof. Huxley, published in this Society's ' Proceedings ' for 1876 l, it may be said that the skull in each of the three families just mentioned exhibits a nearer approach to the primitive amphistylic type than does that of any other adult living vertebrate, the hyomandibular taking very little share in the support of the mandibular arch, and the union of that arch by direct articulation with the cranium being only slight and sometimes almost wanting. The superinduced modifications in the Notidanidse and Cestraciontidse are very evidently in the direction of an autostylic arrangement- the former having a postorbital articulation of the pterygo-quadrate, and the latter a more extensive preorbital connection ; and in the Chlamydoselachidse there are somewhat similar tendencies, although the great extension of the pterygo-quadrate cartilage beyond the chondrocranium has apparently rendered the hyomandibular support of some importance. It would seem, in fact, that the oldest representatives of the Selachian order had skulls which were neither hyostylic nor autostylic, though their least altered descendants incline rather to the latter type; and that Notidanus and Cestracion especially, with Chlamydoselachus in a less degree, afford some slight glimpse into the early condition of the mandibular and hyoid arches from which the two later modifications have developed. Such being the conclusions based upon a study of living Selachians, it becomes of especial interest to determine to what extent they are confirmed or otherwise by the evidence of fossils. The remains of Sharks, Rays, and Chimseroids are abundantly scattered throughout most marine formations, from the Devonian to the latest Tertiary, and the biologist might thus be led to expect considerable information from this field of research. Unfortunately, however, " the imperfection of the geological record " presents its accustomed difficulties, and almost all the facts hitherto discovered relate merely to such hard structures as spines and teeth. There are also a few instances in which the entire fish has been described in a general way; but Prof. Cope's elaborate account" of some cranial fossils from the Permian of Texas appears to be the only contribution of importance that has yet been made to the morphology of the skull. Under such circumstances, I venture to offer to the Zoological Society a brief description of a Cretaceous fossil in the British Museum, which is particularly remarkable from the similarity of the archaic features it presents to those of the existing types already mentioned. . The fossil in question (no. 41675 of the B.M. register) was -1 Tw H Huxley "On the Ceratodus forsteri, with observations on the Classification of Fishes," Proc. Zool. Soc. 1876 pp. 40-45. 2 E D Cone " On the Structure of the Skull in the Elasmobranch genus Didymodus," Proc. Amer. Phil. Soc. vol. xxi. (1884), pp 572-590, with plate. See also further remarks by S. Garman, " Chlamydoselachus anguvneus, Garm a living species of Cladodont Shark," Bull. Mus. Comp. Zool. Harvard Coll vol. xh no. 1 (1885), pp. 28, 29. |
