I think this is a Triceratium of some type (perhaps Triceratium montereyi) from a strew slide from Haliotis shells, California. Unknown slide maker. 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).
EDIT – I had originally thought this might be my Henry Morland, although having looked more closely at the hand writing I am less sure of that now. My guess would be an English maker (or at least not American) as the date is written Day/Month/Year.