Two examples of a Navicula sp. from Inverness, Scotland. More on the name in a minute. ‘Cover guag’ (presumably Coverslip gauge/thickness) 0.0035” (0.14mm). Dated 1895 (maybe October 1896). Prepared by William Allott Firth. Olympus BHB microscope using 450nm LED light. 63x Leitz Pl Apo NA 1.4 objective, oil immersion. Olympus Aplanat Achromat condenser, oil immersion, oblique lighting. 2.5x Nikon CF PL photoeyepiece. Monochrome converted Nikon d850 camera. 52 images stacked in Zerene (Pmax).
The note on the back says “2 valves (large) v. like “Smithii”. 1 has extremely fine double rows, other slightly convex [I think it says convex here].”. So, Navicula smithii? Certainly looks similar. Navicula smithii Brébisson 1854. Published in: Brébisson, A. de (1854). Note sur quelques diatomées marines rares ou peu connues, du littoral de Cherbourg. Mémoires de la Société Impériale des Sciences Naturelles de Cherbourg 2: 241-258, 1 pl. Page 253. This name is currently regarded as a synonym of Diploneis smithii (Brébisson) Cleve 1894. 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 96.
D. smithii comes in a variety of shapes, and this could be part of the ‘Diploneis smithii/fusca complex’. For example see: Mann, D. G. (1999). The species concept in diatoms. Phycologia, 38(6), 437–495. https://doi.org/10.2216/i0031-8884-38-6-437.1