Three examples of what I think are Triceratium trisulcum var. cuneatum from Oamaru, New Zealand. This does not match the name on the slide though. Mounted in Styrax and dated May 1939. Very high contrast for Styrax. Prepared by Samuel Henry Meakin. Olympus BHB microscope using 450nm LED light. 20x Nikon Plan Apo 0.65 objective. Olympus Aplanat Achromat condenser, oblique lighting. 2.5x Nikon CF PL photoeyepiece. Monochrome converted Nikon d850 camera. 48 images stacked in Zerene (Pmax).
The slide label has these asTriceratium castelliferum Grunow in A.W.F.Schmidt 1888. Published in: Schmidt, A.[W.F.] (1888). Atlas der Diatomaceen-kunde Series III (Heft 31-32). pp. pls. 121-128. Leipzig: Fuess Verlag (R. Reisland). Plate 128, Figures 17,18. This name is currently regarded as a synonym of Sheshukovia castellifera (Grunow) J.Witkowski. Source of current name: Witkowski, J. (2022). Early Paleocene-Late Eocene diatoms from the Blake Nose Western North Atlantic Ocean. Nova Hedwigia, Beheift 152: [i]-v, [1]-381, incl. 2 figs, 7 tables, 156 pls.
However they look like Triceratium trisulcum var. cuneatum to me. Triceratium trisulcum var. cuneatum Grunow, 1890. Published in: Schmidt, A.[W.F.] (1890). Atlas der Diatomaceen-kunde Series IV: Heft [37/38]. pp. pls 145-152. Leipzig: Fues’s Verlag (R. Reisland). Plate 150, Figure 13 (as ‘cuneata‘).