Aulacodiscus pellucidus from Sendai (written Sandai on the slide), Japan. Single example, dipping down away from the coverslip towards the bottom right. 7 processes. 63µm diameter (top to bottom), 65µm (left to right). Prepared by Laurence Miles. Olympus BHB microscope using 450nm LED light. 63x Leitz Pl Apo NA 1.4 objective, oil immersion.Olympus Aplanat Achromat condenser, oil immersion, oblique lighting. 2.5x Nikon CF PL photoeyepiece. Monochrome converted Nikon d850 camera. 65 images stacked in Zerene (Pmax).
Aulacodiscus pellucidus Greville 1864. Published in: Greville, R.K. (1864). Descriptions of new and rare Diatoms. Series XI. Transactions of the Microscopical Society, New Series, London 12: 8-14, pls I, II. Page 12, Plate I [1], Figure 5.
Must admit, I’m not 100% convinced by this ID, however it is similar. The one on the slide is about half the diameter of the one in the paper. Very small Aulacodiscus sp..