Centropyxis orbicularis  Deflandre, 1929

Diagnosis: Shell in lateral view hemispherical, in dorsal view nearly circular;  shell usually build of numerous small quartz particles, sometimes mixed or replaced with small diatoms, embedded in a layer of organic cement; shell light gray or brown yellow, depending upon the density of the embedded xenosomes; xenosomes sometimes absent; aperture nearly semicircular, often with some larger quartz particles along the border.

Dimensions: Deflandre (1929) and Bonnet & Thomas (1960): Diameter 100-140 µm, height 70 and 50-95 µm respectively. Lueftenegger et al. (1988): Diameter 95-119 µm; height 61-80 µm. My measurements: Width 98-145 µm, height 62-96 µm.

Remarks: Very dark specimens are easily overlooked between debris.

References:
Deflandre, G. (1929): Le genre Centropyxis STEIN. – Arch. Protistenkd. 67: 322-375