Auliscus mirabilis from California, USA. Single example on the slide, dipping down away from the coverslip slightly towards the left. Mounted in Sirax and dated May 1939. Prepared by Samuel Henry Meakin. Olympus BHB microscope using 450nm LED light. 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). Some issues with the mountant (filaments) but doesn’t look like mold. Crystallisation, maybe? Also, horizontal lines, and a granular texture. Seems to be at the base of the diatom rim or below. This one was difficult to stack, as there are details on different levels, and the end result is a little ‘messy’ to my eye.
Auliscus mirabilis Greville, 1863. Original description: Greville, R.K. (1863). A monograph of the genus Auliscus. Transactions of the Microscopical Society, New Series, London, 11(3): 36-53, pls II & III. Page 47, Plate 2, Figure 11.
From Greville, 1863: “Habitat, Monterey stone. An exquisitely beautiful diatom, and rare as beautiful.”. It certainly is a striking diatom. For another example of it, see here. The one in the link was imaged with a different objective and is in a lower refractive index mountant, which impacts the look of the final image. Same species, very different photos.