Aulacodiscus sp. (not excavatus as written), from Singiliewsky. Single example on the slide. Abnormal form. Unknown maker. Olympus BHB microscope using white LED light. 20x Nikon Plan Apo 0.65 objective. Olympus Aplanat Achromat condenser, oblique lighting. 2.5x Nikon CF PL photoeyepiece. Monochrome converted Nikon d850 camera. 42 images stacked in Zerene (Pmax).
I went with white light for this as the diatom is very red in colour (and therefore very dark with 450nm light). I suspect that it has absorbed some of the red dye used to make the ring around the diatom.
Maybe Aulacodiscus eminens. In: 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
I recently acquired about 60 slides by this maker, so if you want to see others by them, search for AAAAA in the Search option at the top of the page, and I’ll include this in each of the pages for them.