Figure 2. – Gyrodus circularis. (MNHN. JRE 540). Ground sections (MNHN-F-Histos 2743) (transmitted natural light). A: Axial section (see Fig. 1) of a molariform tooth inserted in a bony sock- et (sb). The dentine walls show numerous mineralized granules (arrowheads). The asterisks point to neighbouring teeth. The alveolar bony tissue runs lightly over the base of the crown. ac: acrodine; bo: bony tissue; oc: odontoblastic canalicles; od: osteodentine. B to D: Three successive transversal sections of five neighbouring teeth (see Fig. 1), crossing different levels of the pulp cavities. Section B (MNHN-F-Histos 2745) crosses the apical crown of the smaller teeth and the higher part of the pulp cavity of the larger tooth. Section C (MNHN-F-Histos 2746) crosses the mid part of the pulp cavities of smaller teeth and the base of pulp cavity of the large one. Section D (MNHN-F-Histos 2748) crosses a relatively low part of the smaller teeth and the base of the pulp cavity of the larger tooth as revealed by the bony arches that occupy the main part of the pulp lumen (arrowhead). The pulp cavities of the four left teeth are circular whereas the pulp cavity of the larger right tooth is lightly ovoid. The walls of the pulp cavities of the five teeth are perfectly circular and constituted of orthodentine. The circum pulpar dentine shows spheritic mineralization (see Figs 3, 4). Each tooth is surrounded by spongy bony tissue characterized by thin trabeculae (see Fig. 6). Scale bars: A = 500 μm; B-D: = 2 mm.