Pleurosigma angulatum, unknown mounter

Pleurosigma angulatum

I think this is Pleurosigma angulatum. The slide is labelled as Surirella gemma, but there are plenty of these on there as well. Mounted in piperine. No mounters name, but there is some engraving on the slide, and it looks to be dated [18]91. Olympus BHB microscope using 450nm LED light. Olympus 40x Dplan Apo UV NA 0.85 objective. Olympus Aplanat Achromat condenser, oblique lighting. Olympus 2.5x NFK photoeyepiece. Monochrome converted Nikon d800 camera. Single image (no stacking).