Arcella nordestina Vucetich, 1973

Diagnosis: In dorsal view the contour of the test is irregular, with a series of concavities; in lateral view the concavities are located in two or three rows. The test has a strong constriction, a kind of neck, just above the apertural region where it expands somewhat but does not form a marginal ridge. In apertural view two concentric circles are observed, corresponding to the edge of the test and the apertural tube which is very developed. The apertural side presents the smaller diameter of the test. The pseudostome is circular and has a full border. By transparency one can see a kind of star with five to seven digitiform arms that correspond to the deep fosetas of the dorsal face. The test is fragile, yellow in color and with small areoles.

Dimensions.. Diameter 52—60 µm; height 65—70 µm; diameter of the ventral (apertural ) side 35—40 µm; diameter of aperture 12—15 µm; height of apertural tube 10—15 µm; height/diameter: 1.16/1.25.

Habitat: Freshwater.

Geographical distribution: Lentic waters of the province of Corrientes, Argentina (substrate of Pistia stratiotes).

Remarks: According to Vucetich Arcella nodestina belongs to the group of A. mitrata Leidy – A. apicata Schaudinn. She considered A. nordestina as a species close to A. mitrata var. pyriformis Deflandre, which is distinguished by its smaller dimensions, the constriction in the lower third of the test and the deep concavities.

References: Vucetich M. C. (1973) Estudio de tecamebianos Argentinos, en especial los del Dominion Pampasico. Revta Mus. La Plata, Zoología 11: 287-332.