Single diatom on the slide. The slide states “This diatom is in the ctr [centre] of field when my scales read 20/15”. Additionally; “H Roser’s scales 38.5/13.5”. Mounted in Styrax and dated July 1941. Prepared by Samuel Henry Meakin. 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. 20 images stacked in Zerene (Pmax).
As for the diatom, I think it is perhaps Paralia sulcata var. biseriata f. coronata based on the appearance of it in the Oamaru Diatoms website (see here). However I acknowledge that a lot of these Paralia look very similar so it could be something else.