Aulacodiscus heterostictus (labelled as Aulacodiscus septus, but I think this is wrong) from Singiliewsky, Russia. Single example on the slide, along with a sponge spicule. Prepared by JA Long. Olympus BHB microscope using 450nm LED light. Main image using a 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. 72 images stacked in Zerene (Pmax).
As mentioned above, I do not think this is A. septus (central region looks wrong – search above for an example as there is one on this site). Aulacodiscus heterostictus J.W.Barker & S.H.Meakin 1944. Published in: Barker, J.W. & Meakin, S.H. (1944). New genera and species of diatoms from Russia. Journal of the Quekett Microscopical Club, Series 4 1(6): 251-255, pls 38,39. Page 251, Plate 38, Figure 2.