Slide labelled as Aulacodiscus margaritaceus var. from Forrester’s Hill, Oamaru, New Zealand. 7 processes, 185µm diameter. I’m not convinced by that name, more in a minute. Single example on the slide. Fossil marine material. Mounted in Cananda Balsam and dated October 1967. Prepared by ECP Bone. 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. Monochrome converted Nikon d850 camera. 87 images stacked in Zerene (Pmax).
Back to the name. It doesn’t look like A. margaritaceus (or variants) to me, but more like Aulacodiscus elegans. Aulacodiscus elegans Grove & Sturt 1887. Published in: Grove, E. & Sturt, G. (1887). On a fossil marine diatomaceous deposit from Oamaru, Otago, New Zealand. Appendix. Journal of the Quekett Microscopical Club, Series 2 3(19): 131-148, pls 10-14. Page 140, Plate XII, Figure 30. Although I will leave it with a question mark for now.