. Bulletin of the British Museum (Natural History), Geology. 328 BRITISH WEALDEN SHARKS Clay. If we assume that L. heterodon, as the earliest known species of Lonchidion shows the most primitive condition, the genus must have originated as a heterodont, low-crowned, Hybodus-like form with very thick palhal dentine. The more homodont species, L. breve, would then have evolved by adoption in all the teeth of the slender, high-crowned, almost smooth form of the posterior teeth in L. heterodon, and by increasing the overlap between adjacent files of teeth. Fin Spines and Cephalic Spines of Lonchid
. Bulletin of the British Museum (Natural History), Geology. 328 BRITISH WEALDEN SHARKS Clay. If we assume that L. heterodon, as the earliest known species of Lonchidion shows the most primitive condition, the genus must have originated as a heterodont, low-crowned, Hybodus-like form with very thick palhal dentine. The more homodont species, L. breve, would then have evolved by adoption in all the teeth of the slender, high-crowned, almost smooth form of the posterior teeth in L. heterodon, and by increasing the overlap between adjacent files of teeth. Fin Spines and Cephalic Spines of Lonchidion In the late Cretaceous species Lonchidion selachos, Estes (1964 : 9) was able to include fragmentary fin spines and cephalic spines from locaUties at which teeth of the species occur. There is little doubt of the association between the teeth and the spines since L. selachos is the only hybodont known from the deposits. In the British Wealden and Purbeck the matter is complicated by the presence of small and im- mature hybodonts of other species. i,|/. B. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original British Museum (Natural History). London : BM(NH)
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