Slide labelled Aulacodiscus crux (more on this in a minute). Single example on the slide mounted with the rim upwards towards the coverslip (underside view). 4 processes. Quite low contrast – mounted in Canada Balsam? No makers name but based on the handwriting it looks like Frederick Marshall. Olympus BHB microscope using 450nm LED light. 20x Nikon Plan Apo NA 0.65 objective. Olympus Aplanat Achromat condenser, oblique lighting. 2.5x Nikon CF PL photoeyepiece. Monochrome converted Nikon d850 camera. 33 images stacked in Zerene (Pmax). Couldn’t get close enough to use the 40x or 63x Leitz Pl Apo objectives.
Aulacodiscus crux Ehrenberg, 1844: 76 (as ‘Crux‘). Published in: Ehrenberg, C.G. (1844). Mittheilung über 2 neue Lager von Gebirgsmassen aus Infusorien als Meeres-Absatz in Nord-Amerika und eine Vergleichung derselben mit den organischen Kreide-Gebilden in Europa und Afrik. Bericht über die zur Bekanntmachung Geeigneten Verhandlungen der Königl. Preuss. Akademie Der Wissenschaften zu Berlin 1844: 57-97.
However I am not convinced this is right though. To me it looks more like Aulacodiscus mölleri (Schmidt’s Atlas, Plate 33, Figure 14). Interestingly Schmidt’s Atlas Plate 35, Figure 6, looks like a really good match. Translation “Without a location (Weissfl.), according to Grunow Aulacodiscus mölleri; but it has a different relief than Plate 33, Figure 14!! therefore probably sp. n.”. Also compare with Aulacodiscus mölleri on this site (here). This looks like a very good match, so is likely this or a variation. Hence, I will leave it is as more likely A. mölleri but with a question mark.