Arcella excavata Cunningham, 1919
Diagnosis: shell colorless or yellowish in young specimens and brown in older cells, oval or circular in apertural view and croissant-like in lateral view; large axis of the shell (length) usually in perpendicular direction to the shell’s protuberances, when the shape of the shell in apertural view is not circular; sometimes the large axis of the shell is parallel to the shell’s protuberances; apertural surface deeply invaginated forming two pronounced protuberances with circular or elliptical aperture at the bottom of invagination; aperture bordered by a small lip; aboral hemispherical region smooth or with regular depressions; shell wall composed of numerous alveoli, made of a proteinaceous material, and arranged in one layer; shell surface smooth or irregular with numerous small pores; usually 3-4 nuclei, sometimes 5-6, with central nucleolus.
Dimensions: Cunningham 55 µm, Stepanek 63-67 µm, Todorov and Golemansky 60-70 µm; my measurements 44 µm (average).
Todorov, M. and Golemansky, V.: Morphology, Biometry and Ecology of Arcella excavata Cunningham, 1919 (Rhizopoda: Arcellinida) - Acta Protozool. (2003) 42: 105 - 111.