A slide labelled as Aulacodiscus new species from Lewis Island, California, USA. Single example on the slide. Prepared by William Allot Firth. I have a few slides by Firth from ‘Lewis Is’, but so far have not been able to find out where this actually is. 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. 53 images stacked in Zerene (Pmax).
As for what the diatom might be, it looks quite like Aulacodiscus cretaceous (see here for more details on that).