Argynnia patagonica Vucetich, 1975

Diagnosis: Shell with an almost circular outline, often as long as wide. In lateral view, it is very compressed; its thickness is equal to one third or less of the width of the shell. It has no neck and the edge of the pseudostome, formed by relatively large circular plates, does not have a chitinous lip. Shell wall embedded with circular and oval plates and abundant remodeled diatom frustules. The fundus has 1 to 3 small horns, sometimes barely outlined. Observed under high magnification, it can be seen that these elements are not hollow and are formed by an accumulation of covering plates.

Dimensions: Length 185-215 µm; width 180-209 µm; thickness 60-77 µm; horns 3-10 µm long; L/W 0.9-1.03.

Habitat: Freshwater. Surroundings of Monté Tronador, Río Negro, Brasil.

Remarks: The most similar species is A. columbiana Wailes, 1925, from which it can be distinguished by the greater compression of the shell, its almost circular outline and the different structure and smaller number and size of the horns.