Pleurosigma decorum from Colon (presumably Colón, Panama). Single example on the slide. Quite low contrast – mounted in Canada Balsam? No makers name but based on the handwriting it looks like Frederick Marshall. Olympus BHB microscope using 450nm LED light. 20x Nikon Plan Apo NA 0.65 objective. Olympus Aplanat Achromat condenser, oblique lighting. 2.5x Nikon CF PL photoeyepiece. Monochrome converted Nikon d850 camera. 20 images stacked in Zerene (Pmax).
Pleurosigma decorum W.Smith, 1853. Published in: Smith, W. (1853). A synopsis of the British Diatomaceae; with remarks on their structure, function and distribution; and instructions for collecting and preserving specimens. The plates by Tuffen West. In two volumes. Vol. I. pp. [i]-xxxiii, 1-89, pls I-XXXI. London: John van Voorst, Paternoster Row. Page 63, Plate XXI [21], Figure 196.