Two examples of Podosira argus from Tamatave (also known as Toamasina, is a major port city located on the east coast of Madagascar). Mounted in Styrax. Prepared by JA Long. The one near the middle is broken, and the other has moved towards the red ring. Olympus BHB microscope using 450nm LED light. I imaged the (almost) intact one at higher resolution. 63x Leitz Pl Apo 1.4 objective, oil immersion. Olympus Aplanat Achromat condenser, oil immersion, oblique lighting. 2.5x Nikon CF PL photoeyepiece. Monochrome converted Nikon d850 camera. 55 images stacked in Zerene (Pmax).
Podosira argus Grunow 1878. Published in: Grunow, A. (1878). Algen und Diatomaceen aus dem Kaspischen Meere. In: Naturwissenschaftliche Beiträge zur Kenntnis der Kaukasusländer, auf Grund seiner Sammelbeute. (Schneider, O. Eds), pp. 98-132. Dresden: Dresden Burdach. Page 132.
Translation from the above;
“Another species in Weissflog’s collection, also from Monterey, is even more interesting. It has a diameter of 0.107 mm, a very thick cell membrane, and, like the previous one, is highly domed. On the inner, concave side, it is delicately radially dotted (16 stripes of 0.01 mm). On the convex side, however, a sharply defined (with a finely serrated edge), annular space of 0.077 mm outer and 0.04 mm inner diameter contains three to four almost concentric rows of large (0.004 mm) round dots (depressions?), which give this species a very distinctive appearance. For now, I’ll call it Podosira argus.”
I’ve italicized what I think is the important bit which talks about the very prominent ‘depressions’ in the surface.