A slide labelled as Coscinodiscus diophthalmus. Single example on the slide, tipping downwards, away from the coverslip, towards the bottom of the image. No location or makers name, but based on the style and handwriting this is by James Doke MacPhail. Olympus BHB microscope using 450nm LED light. 40x Leitz Pl Apo NA 1.00 objective, oil immersion. Olympus Aplanat Achromat condenser, oil immersion, oblique lighting. 2.5x Nikon CF PL photoeyepiece. 11mm tube to the camera removed to increase the field of view. Monochrome converted Nikon d850 camera. 117 images stacked in Zerene (Pmax).
Coscinodiscus diophthalmus Castracane 1886. Published in: Castracane, [C.A.] F. (1886). Report on the Diatomaceae collected by H.M.S. Challenger during the years 1873-76. In: Report on the Scientific Results of the Voyage of H.M.S. Challenger during the years 1873-76. Botany. Vol. II. (Murray, J. Eds), pp. [1]-178. Page 163, Plate XVI [16], Figure 4. A currently accepted name.
This looks very much like Coscinodiscus excavatus to me….. However, the C. excavatus I have do not have the open area right in the centre of the diatom, so maybe it is different?