Slide of Cocconeis costata (more on the name in a minute). Single example on the slide. No mounters name, but prepared by Samuel Henry Meakin and mounted in Styrax. Some crystallization of the mountant. 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. 33 images stacked in Zerene (Pmax).
Back to the name. After a bit of research I came across the following paper – Romero, O. E., & Rivera, P. (1996). Morphology and taxonomy of three varieties of Cocconeis costata and C. pinnata (Bacillariophyceae) with considerations of Pleuroneis. Diatom Research, 11(2), 317–343. https://doi.org/10.1080/0269249X.1996.9705388. This shows an example called Cocconeis costata var. hexagona which looks very similar to this. So I think this is likely to be C. costata var. hexagona.