Triceratium polycystinorum (also seen it spelt polycistinorum) from Kamischev, Russia (USSR). Single example on the slide. Sty.mix. and dated September 1969. C. 118µm bet[ween] angles. Prepared by ECP Bone. Olympus BHB microscope using 450nm LED light. 40x Leitz Pl Apo 1.00 objective, oil immersion. Olympus Aplanat Achromat condenser, oil immersion, slightly oblique lighting. 2.5x Nikon CF PL photoeyepiece. Monochrome converted Nikon d850 camera. 52 images stacked in Zerene (Pmax). Couldn’t get close enough to use the 63x Leitz objective.
Current name Entogoniopsis polycistinora. Good reference for this – Jakub Witkowski, Patricia A. Sims, Nina I. Strelnikova & David M. Williams, “Entogoniopsis gen. nov. and Trilamina gen. nov. (Bacillariophyta): a survey of multipolar pseudocellate diatoms with internal costae, including comments on the genus Sheshukovia Gleser”, (Phytotaxa 209), 89 pp, Figures 44-49, available here.