Aulacodiscus sp. from Ananino, Russia. Single example on the slide. Fossil marine material. Styrax mix and dated September 1966. Prepared by ECP Bone. Olympus BHB microscope using 450nm LED light. 40x Leitz Pl Apo 1.00 objective, oil immersion. Olympus Aplanat Achromat condenser, oil immersion, oblique lighting. 2.5x Nikon CF PL photoeyepiece. Monochrome converted Nikon d850 camera. 77 images stacked in Pmax.
As for what this is, I initially thought Aulacodiscus allorgei Chenevière 1934. Published in: Chenevière, E. (1934). Sur un dépôt fossile marin à diatomées situé à Kamischev (Russie centrale). Bulletin de la Société Française de Microscopie 3(3): 103-107, pls 5-8. Page 106, Plate 8, Figure 2. Visual ID based on Strelnikova, N. I., Fourtanier, E., & Kociolek, J. P. (2004). Morphological studies of Aulacodiscus species from the “Russian collection” of the California Academy of Sciences. Diatom Research, 19(2), 283–310. https://doi.org/10.1080/0269249X.2004.9705876. However the surface on this one is a ‘bumpier’ than expected and it didn’t look quite right. It also doesn’t match the example of A. allorgei on this site from a slide by JA Long (that one is much ‘smoother’ without the bumpy surface of this one).
I think it could be Aulacodiscus hirtus Barker & Meakin, 1949. Original description: Barker, J.W. & Meakin, S.H. (1949). New and rare diatoms. Journal of the Quekett Microscopical Club, Series 4, 2(6): 301-303, pls 37, 38. Page 301, Plate 37, Figure 1. The one on this slide lacks the ‘pear shaped’ processes of A. allorgei and has the more ‘shaggy’ appearance mentioned in the paper, which points me towards A. hirtus. It’s still a bit of a tentative ID though.