Interesting one this. Labelled as an Actinoptychus from Sendai, Japan. Single example on the slide. Prepared by Thomas Edward Doeg. 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. 51 images stacked in Zerene.
As for what this is. Superficially it looks like Actinoptychus senarius. However to me it seems half way between a normal valve and an ‘initial cell’ (a few examples the these on this site). So perhaps an aberrant form?