Description: Shell yellow or light brown, too variable in shape-circular, oval, triangular, quadrangular, more or less irregular in apertural and dorsal view (Fig. 13 A-F); with depressed aboral region and trapezoidal in lateral view (Fig. 13 G). Apertural surface smooth, with small pores around the basal border (Fig. 13 F); dorsally divided into segments by folding of the surface, with view of polygon or a star (Fig. 13 C-E, H). Shell composed of very small organic alveoli of about 0.5-0.6 μm (Fig. 13 H). Aperture s invaginated, circular or oval, bordered by a distinct lip and corone of about y to twenty large pores (Fig. 13 F, I). Cytoplasm abundant but not ll the whole shell; o en amoebae are encysted, with large spheroidal cyst and characteristic rejected apertural side; two typical vesicular nuclei of about 16-18 μm, with a single central nucleolus. Pseudopodia few, large, exceed the basal border; movement slow, with sliding on the substrate. Notes: A. catinus is similar to A. arenaria, but these two species may be distinguished by the size (A. catinus is bigger, with larger aperture), number of apertural pores (10-15 in A. arenaria, versus 18-35 in A. catinus), as well as by theirs ecology: A. arenaria inhabits mostly aerophilic and dry habitats, whereas A. catinus occurs mainly in Sphagnum mosses. Ecology: Frequent in wet Sphagnum mosses. Geographical distribution: Cosmopolitan.