Aulacodiscus lahusenii var. marginata (more on this later) from Singil (Singeliewsky and various other spellings). Single example on the slide, mounted with the underside facing the coverslip, i.e. rim upwards. Unknown maker. Olympus BHB microscope using 450nm LED light. 63x Leitz Pl Apo 1.4 objective, oil immersion. Olympus Aplanat Achromat condenser, oil immersion, oblique lighting. 2.5x Nikon CF PL photoeyepiece. Monochrome converted Nikon d850 camera. 39 images stacked in Zerene (Pmax).
So, the name. This does have similarities in appearance to A. lahusenii (some good images in Nina I. Strelnikova, Elisabeth Fourtanier & J. Patrick Kociolek (2004), Morphological studies of Aulacodiscus species from the “Russian Collection” of the California Academy of Sciences, Diatom Research, 19:2, 283-310, DOI: 10.1080/0269249X.2004.9705876). The overall shape, which I got a feel for when collecting the images, is a pretty good match for A. lahusenii. However this one has 5 processes, which would be unusual (normally 4, occasionally 3). Also the dot pattern on this one looks a bit too regular. There is an image of A. lahusenii var. marginata in Schmidt’s Atlas (plate 101, Fig 4), and with this having the processes inset from the edge, as Schmidt commented, it makes sense to have it as a marginata variant. But again, 4 processes not 5 and a different dot pattern. Where does this leave me? For now it is A. lahusenii var. marginata until I find anything better.
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.