Interesting slide this one. Five examples of Aulacodiscus sp. from Russia. Fossil material, mounted in Hyrax. Prepared by Northern Biological Supplies Ltd. Olympus BHB microscope using 450nm LED light. As well as the arrangement, I imaged two using the 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. Images stacked in Zerene (Pmax).
First is an example of what looks like Aulacodiscus pugnalus [pugnalis]. This diatom is referred to in Hanna, G.D. (1927). Cretaceous diatoms from California. Occasional Papers of the California Academy of Sciences 13: 5-[49]. Pages 14,15, Plate 1, Figures 6,7. In this it is spelt ‘pungalus‘ a couple of times as well as ‘pugnalis‘. In the WoRMS database it states that the accepted name is A. pugnalus (see here). For another example on this site, and more on the name, see here. Also referred to as Aulacodiscus archangelskianus var. pugnalis (pugnalus) published in: Long, J.A., Fuge, D.P. & Smith, J. (1946). Diatoms of the Moreno Shale. Journal of Paleontology 20(2): 89-118, 2 figures, plates 13-19. Page 97, Plate 13, Figure 12. I’ve not seen it from Russia before, it’s normally a Moreno Shale one from the USA. However the age matches.
Second is what I think is Aulacodiscus probabilis A.W.F.Schmidt, 1876. Published in: Schmidt, A.[W.F.] (1876). Atlas der Diatomaceen-kunde. Series I: Heft 9. pp. pls. 33-36. Aschersleben: Verlag von Ernst Schlegel. Plate 36, Figures 13,14. This name is currently regarded as a synonym of Moisseevia probabalis (A.W.F.Schmidt) Strelnikova, 2008. Published in: Strelnikova, N.I. & Tsoy, I.B. (Eds) (2008). Diatomovyye vodorosli Rossii i sopredel’nych stran. Iskopayemyye i sovremennyye [The diatoms of Russia and adjacent countries. Fossil and recent].. Vol. 2(5) pp. [1]-167 [1], index, incl. 155 pls. St. Petersburg University Press. [in Russian]. Page 90, Plate 116, Figures 1-10. An aberrant form example.
Also see Strelnikova, N. I., Fourtanier, E., & Kociolek, J. P. (2004). Morphological studies of Aulacodiscus species from the “Russian collection” of the California Academy of Sciences. Diatom Research, 19(2), 283–310. https://doi.org/10.1080/0269249X.2004.9705876.
Both of these I am leaving with questions marks as I am a little hesitant with the ID’s. As well as these two there are two nice examples of Aulacodiscus archangelskianus, and one I’ve not yet named.