I think this is Stictodiscus johnsonianus f. jeremianus (despite the label, and I’ll come back to that in a minute). Single example on the slide. No location or mountant information. No makers name but I understand this is by John Barnett. 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). Viewed from the underside with the rim facing the coverslip.
Ok, back to the name. The slide says “Stictodiscus buryanus var., rare”. S. buryanus are normally round or triangular, although I have seen a ‘square-ish’ one in Schmidt’s Atlas (Plate 442, Fig 5). However with S. buryanus, the dot pattern as it approaches the edges goes into 2’s and 3’s, whereas on Stictodiscus johnsonianus they remain predominantly singles as they do with this one. Stictodiscus johnsonianus f. jeremianus is a ‘square’ variant, and is shown in Schmidt’s Atlas (Plate 75, Figure 2 where it is described as Triceratium Jeremianum, and Plate 443, Figures 2-6, where it is described as Stictodiscus johnsonianus f. jeremianus). Accepted name is Triceratium jeremianum Schmidt in Schmidt et al., 1882. Based on the overall appearance I think this is Stictodiscus johnsonianus f. jeremianus rather thank an S. buryanus variant. Very pretty diatom with the radial pattern which is a feature of them nicely visible in the hyaline areas.