An Aulacodiscus sp. from Totara, Oamaru, New Zealand. Single example on the slide. I’ll come back to the name in a minute. It is dipping downwards towards the left. Mounted in Nirax and dated July 1993. Surprisingly low contrast. Prepared by RH Taylor. Olympus BHB microscope using 450nm LED light. 20x Nikon Plan Apo NA 0.65 objective. Olympus Aplanat Achromat condenser, slightly oblique lighting. 2.5x Nikon CF PL photoeyepiece. Monochrome converted Nikon d850 camera. 81 images stacked in Zerene (Pmax). Large diatom, and the 20x was the largest magnification I could use.
This is not, as far as I can tell from the available information, Aulacodiscus rawsonii as per the label. I think it is probably Aulacodiscus margaritaceous var. debyi. Aulacodiscus margaritaceus var. debyi (Pantocsek) Rattray 1888. Published in: Rattray, J. (1888). A revision of the genus Aulacodiscus Ehrb. Journal of the Royal Microscopical Society 8(3): 337-382, 3 pls [V-VII]. Page 351.