Actinoptychus simbirskianus from Simbirsk, Russia. Single example on the slide. Prepared by WA Firth (although not the usual writing style for the labels). Olympus BHB microscope using 450nm LED light. 63x Leitz Pl Apo 1.4 objective, oil immersion. Olympus Aplanat Achromat condenser, oil immersion, slightly oblique lighting. 2.5x Nikon CF PL photoeyepiece. Monochrome converted Nikon d850 camera. 38 images stacked in Zerene (Pmax).
Actinoptychus simbirskianus A.W.F.Schmidt 1875. Published in: Schmidt, A.[W.F.] (1875). Atlas der Diatomaceen-kunde. Series I: Heft 8. pp. pls 29-32. Aschersleben: Verlag von Ernst Schlegel. Plate 29, Figure 11.
This one reminds me of the eye of the Martian in the 1953 film War of the Worlds, with its three fold symmetry.