Arachnoidiscus hungaricus (but with a question mark after the name), from St Laurent, France. Single example on the slide. Mounted in Styrax and dated November 1936. Prepared by Samuel Henry Meakin. 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. 54 images stacked in Zerene (Pmax).
Arachnoidiscus hungaricus N.E.Brown 1933. Published in: Brown, N.E. (1933). Arachnoidiscus. An account of the genus, comprising its history, distribution, development and growth of the frustule, structure and its examination and purpose in life, and a key to and descriptions of all known species, illustrated. pp. 1-88, 7 pls. London: W. Watson & Sons, Ltd.. Pages 43 (= key), 61, Plate 5: 6.
No reason to doubt Meakin’s description of this as A. hungaricus at this stage. He left it with a question mark though, and I shall do the same.