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 ‘ 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 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 Notidanide and Cestraciontidee 
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 Chlamydoselachidee 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. 
The remaius of Sharks, Rays, and Chimeroids 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 morpholog 
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 
‘1, H. Huxley, “On the Ceratodus forsteri, with observations on the 
Classification of Fishes,” Proc. Zool. Soc. 1876, pp. 40-45. 
2 B. D. Cope, “On the Structure of the Skull in the Hlasmobranch genus 
Didymodus,’ Proc. Amer. Phil. Soe. vol. xxi. (1884), pp. 572-590, with plate. 
See also further remarks by 8. Garman, “ Chlamydoselachus anguineus, Garm., 
a living species of Cladodont Shark,” Bull. Mus. Comp. Zool. Harvard Coll 
vol. xii. no. 1 (1885), pp. 28, 29. - 
15 
