A slide labelled as Navicula silicula var. gibberula from Santa Fiora, Italy. Single example on the slide. No makers name but prepared by JA Long. Olympus BHB using 450nm LED light. 63x Leitz Pl Apo 1.4 objective, oil immersion. Olympus Aplanat Achromat condenser, oil immersion, slightly oblique lighting. 2.5x Nikon CF PL photoeyepiece. Monochrome converted Nikon d850 camera. 17 images stacked in Zerene (Pmax).
There is a description of this diatom in Elmore, C.J. (1921). The diatoms (Bacillarioideae) of Nebraska. University Studies, January-October, The University of Nebraska, Lincoln, 21(1-4): 22-214, 23 pls. Page(s): p. 89; pl. 11, fig. 424-425. Interestingly the one on this slide, looks less like the var. gibberula in figures 424 and 425, and more like the standard N. silicula in figure 423 on the same plate. As such I am not convinced that this one is var. gibberula (based on the images in the reference).