Arcella arenaria Greeff, 1866
Diagnosis: shell always circular in apical view, more or less dome-shaped in lateral view; dorsal surface usually with slight folds, but very often flattened. Shell yellowish brown to dark brown in color. Aperture small, circular in shape, its diameter barely reaching 1/5-1/6 that of the shell. Many small pores, 8-28 in number, around the aperture. Height of the shell not less than 1/2 diameter of the shell. Neither remarkable border nor bosses around the lateral margin. Many nuclei present: the large ones usually 3 in number and arranged in an equilateral triangle, the small ones not constant in number, varying 3-5, arranged irregularly. The pseudopodia very short, 6 at most, many vacuoles developing along the ectoplasm.
Dimensions: in literature shell 75-130 µm in diameter, 21-50 µm in height, aperture 12-28 µm in diameter, pores 8-18.
Ecology: mostly in dryer mosses; on Hypnum and Polytrichum, also on lichens on trees and between Sphagnum.
Remarks: this species resembles Arcella catinus Penard; it can also be confused with Arcella artocrea. The folding of the dorsal surface is most distinct in lateral view. This species seems easy to identify.