Two examples of Aulacodiscus (new species) in between two disc diatoms. Fossil marine material. No location given. Prepared by Eduard Thum. Olympus BHB microscope using 450nm LED light. As well as the wider field of view I imaged them using the 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. 100 images stacked in Zerene (Pmax). One is on its side, and looks to have moved from its original location.
As for what this, I am not sure. There is some pencil writing on the right hand label on the slide, but so far I have not been unable to read that.
It is similar to Aulacodiscus comberii, however I have only seen that with 3 processes, and both examples here have 4. The closest match so far looks to be Aulacodiscus szakalensis Pantocsek, 1892 from Pantocsek, J. (1892). Beiträge zur Kenntnis der Fossilen Bacillarien Ungarns. 3 Teile. Julius Platzko, Nagyy-Tapolcsany. Teil III, Susswasser Bacillarien. Anhang-analysen 15 neuer Depots von bulgarien, Japan, Mahern, Russland und Ungarn. Nagy-Tapolcsány, Buchdrucherei von Julius Platzko, 42 pls., available online at http://www.biodiversitylibrary.org/item/15525. Plate 28, Figure 415. This one is shown with 4 processes.
However with no location on the slide, it is very difficult to narrow it down. So for now it will remain as Thum wrote it – Aulacodiscus, new species.