An arrangement of three Navicula robusta var. perlonga from Borostelek, Hungary (now Borșa I think). Label says ‘Sx’ (Sirax maybe, would make sense as Sirax is a mountant where I have seen these ‘bubbles’ before), and X/27 (maybe October 1927). No makers name. Olympus BHB microscope using 450nm LED light. I selected one for imaging 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. 11mm extention to the camera removed to improve field of view. Monochrome converted Nikon d850 camera. 47 images stacked in Zerene (Pmax).
I’m guessing this is now known as Caloneis robusta var. perlonga (Pantocsek) Cleve 1894: 56. Published in: Cleve, P.T. (1894). Synopsis of the naviculoid diatoms. Part I. Kongliga Svenska Vetenskapsakademiens Handlingar Series 4 26(2): 1-194, 5 pls. Page 56. Homotypic synonym: Navicula perlonga Pantocsek 1889.