An Aulacodiscus sp. (labelled as A. crux, but I think this is more likely A. mölleri). Single example on the slide. Prepared by Arthur Charles Cole. Olympus BHB microscope using 450nm LED light. 40x Leitz Pl Apo 1.00 objective, oil immersion. Olympus Aplanat Achromat condenser, oil immersion, oblique lighting. 2.5x Nikon CF PL photoeyepiece. Monochrome converted Nikon d850 camera. 114 images stacked in Zerene (Pmax).
Aulacodiscus crux Ehrenberg 1844. 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. Page 76 (as ‘Crux‘).
However I do not think this is A. crux, and 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 A. mölleri but with a question mark.