Pyxidicula operculata (Agardh, 1827) Ehrenberg, 1838

Diagnosis. Shell discoid or hemispherical, composed of organic material. Colour varies from transparent or light yellow in young individuals to brown or red‑brown in older ones. Aperture almost as wide as the total shell diameter. Shell uniform in outline, with a slight thickening around the margin; this thickening forms a slightly depressed rim that is recurved to produce a small lip. Shell surface covered by regularly spaced pits or pores, but smooth along the apertural margin. A single vesicular nucleus with a centric nucleolus. Pseudopodia short, lobose or digitate.

Dimensions: Diameter 13.6—31 µm.

Ecology: On aquatic vegetation.

Geographical distribution: Very common, but easily overlooked.

Remarks:Portions of the broken rim show that it is a thin extension of the shell wall, which is constructed of a single layer of regular, interlocking alveoli. These alveoli are rectangular in vertical section, with a depth of about 0.3 µm, and have a solid content with thin walls. The contents of the alveoli did not permit embedding resin to penetrate the entire wall, resulting in poor sectioning quality. Nevertheless, sufficient information is available to reconstruct diagrammatically a cross‑section of the wall. Each alveolus is surrounded by a single membrane‑like structure, referred to as the shell matrix, and at the outer surface the shell wall is bilaminar. For further information see Ogden (1987).