Arcella marginata Daday, 1905
Diagnosis: Shell roughly hemispherical, rather overhanging, with a flattened, smooth edge. In dorsal view showing a circular outline with an inner concentric circle. Dorsal surface covered with bumps that appear circular. Pseudostome circular with full edge. In lateral view, dorsal region regularly curved in the shape of a semicircle with a wavy edge due to the presence of the bumps which are numerous and tight against each other.
Dimensions: Diameter 140 µm; pseudostomal diameter 34 µm.
Ecology: Freshwater. Paraguay, South America.