Actinoptychus trilingulatus from Samarang, Java (now Semarang, Indonesia). Single example on the slide. Mounted in Hyrax. No makers name, but this looks to one by JA Long based on the style and handwriting. I think this is an entire frustule rather than just a single valve, based on the appearance. 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. 98 images stacked in Zerene (Pmax).