Arachnoidiscus sp., from the Hillaby (spelt Hilaby) deposit, Barbados. Single example mounted to the underside of the coverslip. Styrax was used as the mountant. No names name, but based on the style and handwriting this is by James Doke MacPhail. Also has the initials J.T.N.T. and the date 1911 written in red. Olympus BHB microscope using 450nm LED light. For the full image I used the 20x Nikon Plan Apo NA 0.65 objective, and oblique lighting. 35 images stacked in Zerene (Pmax). I also did an image of part of it using the 63x Leitz Pl Apo NA 1.4 objective, oil immersion. Olympus Aplanat Achromat condenser, oil immersion, oblique lighting. 2.5x Nikon CF PL photoeyepiece. Monochrome converted Nikon d850 camera. 48 images stacked in Zerene (Pmax). Also a crop of a single frame showing some of the faint features which were lost in stacking.
As for what this is, I am thinking Arachnoidiscus cognatus N.E.Brown 1933. See: 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. W. Watson & Sons, Ltd., London., 88 pp., 7 pls. Pages 43 ( = key), 57, Plate 5, Figure 7. But I am not certain on this, so am leaving it as Arachnoidiscus sp. for now.