Aulacodiscus cretaceus (more on this later) from Panoche Hills, California. Single example on the slide. Some damage including a missing process. Prepared by JA Long. Mounted in Styrax. 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. 99 images stacked in Zerene (Pmax).
Ok, back to the name. This diatom is mentioned in Hanna, G. D. (1927). Cretaceous diatoms from California. Occasional Papers of the California Academy of Sciences. 13: 5-49 [including 5 plates]., available online at https://www.biodiversitylibrary.org/page/3143126. However it has two different spellings in this. On page 14 it is spelt A. cretaceous, while on the caption for the figure (Plate 1, Figure 5) it is spelt A. cretaceus, as it is on this slide. I suspect the correct spelling is A. cretaceous to match the Cretaceous geological period. Also, this example on the slide doesn’t seem to have the ‘large beads’ mentioned in the description in Hanna’s paper.