Slide labeled as “Coscinodiscus lewisianus var. ?” (I’m not convinced about this, more on that later). From Maryland, USA. Single example on the slide. Prepared by Frederick Marshall. Olympus BHB microscope using 450nm LED light. 63x Leitz Pl Apo 1.4 objective, oil immersion. Olympus Aplanat Achromat condenser, oil immersion, oblique lighting. 2.5x Nikon CF PL photoeyepiece. Monochrome converted Nikon d850 camera. 26 images stacked in Zerene (Pmax).
Back to the name. Coscinodiscus lewisianus Greville 1866. Published in: Greville, R.K. (1866). Descriptions of new and rare diatoms. Series XX [actually XIX]. Transactions of the Microscopical Society, New Series 14: 77-86, pls VIII, IX. Page 78, Plate VIII [8], Figures 8-10. This name is currently regarded as a synonym of Araniscus lewisianus (Greville) Komura. 1998. Published in: Komura, S. (1998). A perplexing morphotype of some centric diatoms inclusive of Coscinodiscus lewisianus Grev. and its allies. Diatom 14: 1-23, 147 figs [photographs], 1 text-fig.. Page 6. However this one lacks the parallel rows of areolae make this species (C. lewisianus) easy to recognize. As such I am not convinced this is a C. lewisianus variant.
Could be a Cestodiscus sp. (something like C. peplum maybe)? Overall appearance (but not shape) reminds me of this one – https://diatomimaging.com/cestodiscus-sp-hrs-williams/
Based on this, I’m leaving it with a question mark for now.