Clypeolina marginata Penard, 1902
Diagnose: Test flexible, circular, ovoid or elongated elliptical, strongly compressed, lenticular in cross-section, formed of a double envelope; outer envelope composed of two opposed chitinoid valves covered with small scales and particles, usually with a keel; inner envelope a membranous sack closely investing the cell; aperture flexible, oval; long and thin filopodia, often branching.
Dimensions: Penard usually 80-140 µm, De Groot (1979): 110-115 µm; my measurements 30 -156 µm.
Ecology: Fresh water, in sediments of oliogtrophic and mesotrophic water. Feeding on green and blue algae.
I have found this rare and remarkable species in floating sapropel in a ditch of the nature reserve Naardermeer in the Netherlands (2011 and 2015). Other remarkable species in this sample were Paramphitrema lemanense and Hyalosphenia punctata. In 2012 I found this species in a shallow forest ditch in Gaasterland (GPS 52.88529, 5.56147) and in a shallow ditch in Den Treek, Amersfoort, both in the Netherlands and in a sample from China (Siemensma, unpublished, 2015). It has also been found by Julia Török in Hungary (pers. comm.).
This species was recorded earlier for the Netherlands by De Groot (1979).
Remarks: Clypeolina marginata is a rare species, but very easily overlooked by its resemblance with a piece of detritus. Small specimens are very hard to detect between detritus. They can be confused with Pseudodifflugia and Diaphoropodon mobile. Its taxonomic status is uncertain.