A slide labelled as ‘Navicula rhomboides‘ from Florida (more on that later). Prepared by Samuel Henry Meakin. Mounted in Sirax and dated January 1939. Olympus BHB microscope using 450nm LED light. Main stack using 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. 34 images stacked in Zerene (Pmax). No removal of the background and left as a full frame stack.
More on the name of the slide now. There is a lot going on here. In the main stack I’ve highlighted 3 diatoms, and here’s my thoughts on the ID’s (based on help from the Diatom Images Facebook group).
1. I think this could be Eunotia quadra. Striae distances are about right (625nm and 577nm as measured in ImageJ). Areolae spacing 317nm (again measured in ImageJ). Ref for comparison images for Eunotia quadra: Peter A. Siver, Paul B. Hamilton, and Jeffery Pelczar. 2009. New species of freshwater diatoms from acidic localities along the Atlantic Coastal Plain of the United States. Botany. 87(4): 409-427. https://doi.org/10.1139/B09-015
2. This is, I am fairly sure, Navicula rhomboides. Also known as Frustulia rhomboides. Striae spacing of two examples on the slide are about right (355nm and 359nm as measured in ImageJ).
3. This looks to be a Brachysira sp., likely Brachysira serians. Could be other species on there. Perhaps Brachysira neoacuta and/or Brachysira brebissonii, although that is very speculative. Striae spacing is about right (525nm and 359nm as measured in ImageJ).
The naming of Navicula rhomboides has quite a complex past, and it may well be that various diatoms species were considered together under the one name by Meakin. As an interesting aside, in the October 2005 edition of the Amateur Diatomist (Vol III, No III) by Little Imp Publications, there is an article by Ron Green called ‘The Amician Mystery (or the case of the missing Diatoms)’, in which he discusses whether the Amician Test slides were actually Navicula rhomboides or something else (I have an Amician Test slide here). In his article, Ron identified some unusual looking diatoms he called ‘Navicula[r] X’, which appeared at low frequency on his slides compared with Navicula rhomboides. The ones he identified look very much the Brachysira diatoms on this slide so could perhaps have been Brachysira serians.
This slide turned out to be more interesting than I originally thought it would be, and I may return to it again and see if I can fine a better (more complete) example of the Eunotia diatom, or indeed other species.