Arcella rukiensis Van Oye, 1926
Diagnosis: This species resembles an old clown hat, with three superimposed rows of facets. The upper row consists of four facets, the next six, while the third (aperture) consists of six, sometimes eight, and very rarely five. In general, 4-6-6 is the number of facets of the different rows. The row closest to the aperture is narrowed giving the test its typical shape. The aperture is in the middle, but whilst the rim of the aperture is retracted, the lower disk is provided at this height with an edge curved upwards so as to produce the shape of a hat. Number of nuclei always exceeds five.
Dimensions: diameter 100-120 µm; basal edge 120-125 µm.
Ecology: River Ruki near Eala, Congo (1923)
Remarks: This form is distinguished from all the Arcella's described by its large number of facets arranged in three rows and by the strangulation of the lower row which gives this species its special physiognomy.
Van Oye, P., 1926. Six rhizopodes nouveaux du Congo Belge. Arch. Zool. Exp. et Gen. 65:3,64-74.