An arrangement of four Triceratium arcticum (triangular form) prepared by Watson and Sons Ltd. Olympus BHB microscope using 450nm LED light. As well as the group, I imaged one using the 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. 38 images stacked in Zerene (Pmax).
Triceratium arcticum Brightwell 1853. Published in: Brightwell, T. (1853). On the genus Triceratium, with descriptions and figures of the species. Quarterly Journal of Microscopical Science, London 1: 245-252, pl. IV. Page 250, Plate IV [4], Figure 11.
These look mainly like Triceratium arcticum var. californica (Schmidt’s Atlas Plate 79, Figure 5) to me.