Few question marks with this one. The slides is labelled as Cyclotella minutula from Sweden. There are three examples in an arrangement above a sponge spicule. There is a Flatters and Garnett label on it, but the top label is the same style as a load of JA Long slides I got recently (although the handwriting is different). Bit of a mystery. Olympus BHB microscope using 450nm LED light. Main image with 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. 23 images stacked in Zerene (Pmax). I also made another version where I brought them closer together to reduced the dead space.
Back to the name. It says Cyclotella minutula. Cyclotella minutula Kützing 1844. Published in: Kützing, F.T. (1844). Die Kieselschaligen Bacillarien oder Diatomeen. pp. [i-vii], [1]-152, pls 1-30. Nordhausen: zu finden bei W. Köhne. Page 50, Plate 2, Figure 3. This name is currently regarded as a synonym of Stephanodiscus minutulus (Kützing) Cleve & Möller. Source of synonymy: Cleve, P.T. & Möller, J.D. (1882). Diatoms, Part VI, Nos 277-324. . Upsala: Esatas Edquists Boktryckeri.
However these don’t look like C. minutula (or S. minutulus) to me. To me they look more like a Lindavia sp. Any ideas, please let me know.