Arachnoidiscus decorus from Palos Verdes, California, USA. Single example on the slide. Fossil marine. W.D.F. #355. Mounted in Hyrax and dated April 1967. Also says N.E. Brown 6/3 and Wolle 91/6.7. 234µm diameter (I measured it top to bottom as well and got…. 234µm). Prepared by ECP Bone. Olympus BHB microscope using 450nm LED light. 40x Leitz Pl Apo NA 1.00 objective, oil immersion. Olympus Aplanat Achromat condenser, oil immersion, oblique lighting. 2.5x Nikon CF PL photoeyepiece. 11mm spacer to the camera removed. Monochrome converted Nikon d850 camera. 28 images stacked in Zerene (Pmax).
Arachnoidiscus decorus N.E.Brown 1933. 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 47 (= key), 71, Plate 6, Figure 3.
See also; Williams, D.A. (2023). Notes on the diatom collections of the Natural History Museum, London (BM) VIII: the types of Arachnoidiscus in Brown’s monograph (Brown 1933). Diatom Research 3(4): 181-208, 80 figures, two tables.