Triceratium sp. (like T. barbadense). Single example on the slide, mounted at a slight angle. No makers name but I understand this is by HRS Williams. 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. Monochrome converted Nikon d850 camera. 132 images stacked in Zerene (Pmax).
Original description (of T. barbadense): Greville, R. K. (1861). Descriptions of new and rare Diatoms. Series I. Transactions of the Microscopical Society, New Series, London. 9: 39-45. page 44; pl. 4, fig. 12. Scans of the relevant parts included above. The one on the slide does look like this, so for now I will agree with the suggested name T. barbadense.