Key Position: 2 Back to (1a)
2 a. Test circular in dorso-ventral view, plano-convex to hemispherical in lateral view, made of proteinaceous matter, surface smooth or punctate; aperture central, large and circular.

SEE Figure 2-1

 2 b. Test with or without spines, agglutinated with mineral grains, aperture sub-terminal or occasionally central, circular or oval, invaginated.

Proceed to 3

FIGURE 2-1

Arcella vulgaris Ehrenberg 1830
Arcella vulgaris EHRENBERG 1830, p. 40, pl. 1, fig. 6
Arcella vulgaris Ehrenberg REINHARDT et al. 1998, pl. 1, fig. 3
Remarks: Distinguished from Centropyxis aculeata "discoides" by having no agglutinated particles whatsoever, thus the test is hyaline and transparent.
Specimen from Crosswise Lake, near Cobalt, Ontario.