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, pubhshed in this Society's ' Pro- 

 ceedings ' for 1876 \ 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 hving 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 hyoman- 

 dibular 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. 

 Tlie 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 ievi 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 simi- 

 larity 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 



^ T. H. Huxley, "On the Ceratodus forsteri, with observations on the 

 Classification of Fishes," Proc. Zool. See. 1 87(5, pp. 40-46. 



- B. D. Cope, " On the Structure of the Skull in the Elasmobranch genus 

 Bidymoclus," Proc. Amer. Phil. Soo. vol. xxi. (1884), pp. 572-590, with plate. 

 See also further remarks by S. Garmau, " Chlamydoselachus anguineus, Garm., 

 a living species of Oladodont Shark," Bull. Mus. Conip. Zool. Harvard Coll 

 vol. xii. no. 1 (1885), pp. 28, 29. 



15* 



