Navicula sp. from Palos Verdes, California, USA. Single example on the slide. Mounted in Nirax (High RI?) and dated July 1993. Prepared by RH Taylor. Olympus BHB microscope using 450nm LED light. 50x Leitz Fluoreszenz NA 1.00 objective, water immersion. Olympus Aplanat Achromat condenser, oil immersion, oblique lighting. 2.5x Nikon CF PL photoeyepiece. Monochrome converted Nikon d850 camera. 22 images stacked in Zerene (Pmax). A slightly frustrating slide. I would normally have used the 40x Leitz or 63x Leitz Pl Apo objective, however I couldn’t get close enough with either, which makes me think it was mounted quite deeply. Some issue with the mountant, which are especially visible in the more hyaline areas.
Name options. Based on appearance I think this could be Navicula oswaldii. However it also looks like Navicula spectabilis var. excavata (image from “Miocene and Pliocene Marine Diatoms from California” by Walter W. Wornardt, Jr. A 1967 publication of the California Academy of Sciences, Figure 200). Navicula spectabilis var. excavata (Greville) Cleve 1895. Published in: Cleve, P.T. (1895). Synopsis of the naviculoid diatoms. Part II. Kongliga Svenska Vetenskapsak Akademiens Handlingar 27(3): 1-219, 4 pls. Page 61. This name is currently regarded as a synonym of Lyrella excavata (Greville) D.G.Mann. So I will leave it open for now.