A slide of Tetracyclus emarginatus from the Sweden (more on the name in a moment). A range of them in between two Eupodiscus diatoms. No makers name. Olympus BHB microscope using 450nm LED light. As well as an image with a 10x Nikon Plan Apo NA 0.45 objective, I imaged one using the 63x Leitz Pl Apo NA 1.40 objective, oil immersion. Olympus Aplanat Achromat condenser, oil immersion, oblique lighting. 2.5x Nikon CF PL photoeyepiece. Monochrome converted Nikon d850 camera. 20 images stacked in Zerene (Pmax).
The slide is labelled Tetracyclus lacustris.
Tetracyclus lacustris Ralfs 1843. Published in: Ralfs, J. (1843). On the Diatomaceae. Annals and Magazine of Natural History 12: 104-111, 1 p. [pl. 2]. Page 105, Plate 2, Figure 2. This name is currently regarded as a synonym of Tetracyclus glans (Ehrenberg) F.W.Mills 1935. Published in: Mills, F.W. (1935). An index to the genera and species of the Diatomaceae and their synonyms. 1816-1932. Part 21 (Sy-Z, appendices, supplement). pp. 1571-1726. London: Wheldon & Wesley, Limited. Page 1602.
However after sharing the images on the Diatom Images Facebook page, someone commented that these are more like Tetracyclus emarginatus, and after looking I agree.
Tetracyclus emarginatus (Ehrenberg) W.Smith 1856. Published in: Smith, W. (1856). A synopsis of the British Diatomaceae; with remarks on their structure, functions and distribution; and instructions for collecting and preserving specimens. The plates by Tuffen West. In two volumes. Vol. II. pp. [i]-xxix, 1-107, pls 32-60, 61-62, A-E. London: John van Voorst, Paternoster Row. Page 38.
I have recently imaged a few slides by this maker, all of which are nice. If you want to see others search for CCCCC in the ‘Search’ function.