. Bulletin of the British Museum (Natural History), Geology. BRITISH WEALDEN SHARKS 315 the histological structure of the crown is described (Estes 1964 : 8, text-fig. 2d) as a fan-shaped radiation of dentine tubules from one central cavity which originated along a central longitudinal axis. The tooth crowns of L. breve breve (PI. 5, fig. 3 ; Text-fig. 29E) agree with this description. Below a rather thick layer of enamel the crown is made up of pallial dentine containing long, subparallel, much branched tubules which arise from the tips of vascular canals which end shortly after entering the
. Bulletin of the British Museum (Natural History), Geology. BRITISH WEALDEN SHARKS 315 the histological structure of the crown is described (Estes 1964 : 8, text-fig. 2d) as a fan-shaped radiation of dentine tubules from one central cavity which originated along a central longitudinal axis. The tooth crowns of L. breve breve (PI. 5, fig. 3 ; Text-fig. 29E) agree with this description. Below a rather thick layer of enamel the crown is made up of pallial dentine containing long, subparallel, much branched tubules which arise from the tips of vascular canals which end shortly after entering the centre of the base of the crown. Estes compares this structure with that seen in teeth of the Triassic hybodont Palaeobates (Jaekel 1889, pi. 10, fig. 2 ; Seilacher 1943, text-figs. 22, 29, 34), especially with P. nodosus Seilacher. But in Palaeobates (PI. 5, fig. 2) the dentine tubules are longer, parallel and more regularly branched than they are in Lonchidion, and they arise not from the tips of vascular canals but from a single open pulp cavity running the length of the tooth. The pallial dentine of Lonchidion is much more like that in the Triassic Polyacrodus, particularly in small species like P. minimus (Seilacher 1943, text-figs. 7-10 ; PL 5 fig. i), where there are just the same wavy, irregularly branched dentine tubules arising from short vascular canals near the base of the crown. In Polyacrodus the pallial dentine is a little thinner than it is in Lonchidion, perhaps because of the greater size of the teeth, but in other respects the correspondence is exact. The worn condition and absence of roots in most examples of this sub-species make it difficult to arrive at any clear idea of the variation in shape. The length of the crown ranges from 1-4-3-4 mm., the maximum breadth of the crown (at the labial process) from 0-4-I-2 mm. In five complete teeth the depth ranges from 1-0-I-35 mm., the ratio of depth to length from I-5-2-5. The crown is normally broader than deep,
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