Arcella catinus Penard, 1890
Diagnose: Test in dorsal view oval, circular or quadrate; dorsal region smooth or divided into segments by folds of the shell surface, sometimes with pronounced ribs or folds. Aperture invaginated, circular or elliptical, with a small lip and surrounded by many large and small pores.
Dimensions: literature: test 73-140 µm in diameter and about 45 µm high, 15-18 pores; my measurements: 128-133 µm.
Ecology: fresh water among vegetation and in wet Sphagnum.
Remarks: The similarity between A. catinus and A. arenaria has led to some confusion in earlier descriptions. The two species may be distinguished by the size of the aperture and the number of pores surrounding it.