Pinnularia sp. from Maria Madre Island, Mexico. Single example on the slide. Mounted in Naphrax and dated September [19]69. Prepared by Richard Gosden. 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).
As for what specifically this is, I’m not sure. Looks a bit like Caloneis lewisii, although the central lobe is smaller on this one.