1886.] AND HYOID ARCHES IN A CRICTACEOUS SHARK. 223 



A third fossil in the National Collection (no. 49032) also belongs 

 to the same species, and exhibits the remains of the anterior portion 

 of what was evidently a complete fish at the time of its entombment. 

 It agrees in size with the specimen hei'e described, and exhibits 

 traces of the articular facette on the otic process, in addition to part 

 of the left cerato-hyal with its characteristic lower tnberosity. 

 Fragments of the shoulder-girdle are also preserved, and a portion 

 of (he base of the chondrocranium ; and the vertebral column is 

 shown for a length of 0"16 m., comprising about 33 well-calcified 

 asterospondylic centra. Unfortunately, all indications of dorsal 

 spines are wanting. 



As the typical Hybodonts range throughout the whole of the 

 Mesozoic strata, from the Muschelkalk to the Upper Cretaceous 

 inclusive ', and as it has been found impossible hitherto to recognize 

 more than one generic iy^&—Hybodus — on the evidence of spines 

 and teeth, it would be interesting to compare the modifications in 

 the skull of H. dubrisiensis with the corresponding structures in 

 earlier species. As yet, however, no information in regard to these 

 has been published, and the only deposits that have yielded 

 satisfactory specimens are the Lower Lias of Lyme Regis and the 

 Wealden of Pevensey Bay, Sussex. Of such fossils the British 

 Museum contains an unrivalled series, and I hope to attempt the 

 elucidation of the more important of these on a future occasion. It 

 must suffice at present to add that, though there are well-preserved 

 pterygo-quadrates from both the formations mentioned, there 

 appears to be none but the most uncertain evidence of an articular 

 facette on the otic process in any ; and if this observation can be 

 confirmed it will become of considerable interest when taken in 

 connection with the fact, pointed out by Prof. Huxley ", that the 

 postorbital articulation in the living Heptunchiis is only acquired 

 com])aratively late in the development of the foetus. It is also inter- 

 esting to note that one of the Liassic specimens (Brit. Mus. no. p340) 

 exhibits traces of a persistent notochord, with the arches alone 

 calcified, whereas in the Cretaceous form it has just been shown that 

 there are well-differentiated centra. The differences between the 

 anterior and posterior teeth are likewise more marked in H. dubrisiensis 

 than in any of the earlier species of which satisfactory remains are 

 known. It would appear, indeed, that there is distinct evidence of 

 specialization as the Hybodonts are traced through the Mesozoic 

 period, and it is almost certain that future research in regard to 

 structures other than teeth will lead to the subdivision of the multi- 

 tudinous forms hitherto grouped under one generic name. 



' Teeth indistinguishable frtiui Hi/bodu^, and originally described under this 

 nnnie, occur in the Carboniferous, but thej' have been proved to belong to a 

 distinct genus, Trisiychiun (T. Stock, " On the Structure and Affinities of the 

 genus TristychiKS, Agass.," Ann. & Mag. ?y"at. Hist. (5) xii. 188.3, pp. 177-190, 

 pi. vii.). There are also other Palreozoio Hybodontidfe, e. g. Ctcnacwnfhus, 



- Loc. cif. p. 44, fig. 0. 



