Difflugia difficilis Thomas, 1954
Diagnosis: Test ovoid, circular in cross-section. Aboral end of the test commonly with a not prominent, but rounded and/or compressed protuberance, sometimes replaced by a foreign body (diatom) which acts as the absent horn. The sides of the test converge towards the pseudostome to end in a short collar; the pseudostome is circular with a diameter varying between one-half and two-thirds of the width of the test. Test hyaline, clear, coated with siliceous particles, arranged regularly. Nucleus vesicular, spherical, with one central nucleolus. Zoochlorellae present.
Dimensions: Thomas (1954): Length 83-93 µm; width 59-66 μm; aperture 20-27 µm; height of the collar, 6-9 μm.
Naardermeer: L 80-117 µm; W 54-78 µm; aperture 26-32 µm; collar 4.7-7.4 µm high (n=6); nucleus 18.5-22.9 µm (n=2).
Hartwerd: L 119-162 µm; W 85-99 µm; aperture 27-36 µm (n=12); nucleus 31.5-34.2 µm (n=2).
Den Treek: L 111-151 µm (n=20); nucleus 36 µm (n=1)
Ecology: Freshwater, sediment; France (Thomas, 1954), Belgium (Chardez, 1956), Africa (Gauthier-Lièvre et Thomas, 1958), Netherlands (in sediment of mesotrophic and eutrophic waters; 2019-2021).
Remarks: I found three populations which differed in size (see above). The Hartwerd population was found in a ditch near the village of Hartwerd in the nothern part of the Netherlands.