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Show 1886.] AND HYOID ARCHES IN A CRETACEOUS 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 here 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 tuberosity. Fragments of the shoulder-girdle are also preserved, and a portion of the 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 1, and as it has been found impossible hitherto to recognize more than one generic type-Hybodus-on the evidence of spines and teeth, it would be interesting to compare tbe 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 2, that the postorbital articulation in the living Heptanchus is only acquired comparatively late in the development of the foetus. It is also interesting to note that one of the Liassic specimens (Brit. Mus. no. P 3 4 0 ) 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 multitudinous forms hitherto grouped under one generic name. 1 Teeth indistinguishable from Hybodus, and originally described under this name occur in the Carboniferous, but they have been proved to belong to a A t,-„„f „*„,,« TristvcMm (T Stock, " O n the Structure and Affinities of the ^ I ^ h i ^ t ^ A u n . 1 Mag. Nat Hist (5) xii. 1883, pp. 177-190, pi. vii.). There are also other Palaeozoic Hybodontidse, e. g. Ctenacanthus. 2 Loc. cit. p. 44, fig. 9. |
