I think this is a Triceratium of some type from a strew slide from Haliotis shells, California. Unknown slide maker, although it could well be Henry Morland. Mounted in Styrax, dated 14.2.1920. Olympus BHB microscope using 450nm LED light. 63x Leitz Pl Apo NA 1.40 objective, oil immersion. Olympus Aplanat Achromat condenser, oil immersion, brightfield lighting. 2.5x Nikon CF PL photoeyepiece. Monochrome converted Nikon d850 camera. 31 images stacked in Zerene (Pmax).