Pyxidicula operculata (Agardh, 1827) Ehrenberg, 1838
Diagnosis: shell discoid or hemispherical, composed of organic material, which varies in color, being transparent or light yellow in young individuals and becoming darker with age to be either brown or red-brown; aperture almost as wide as the total diameter of the shell; shell uniform in outline with a slight thickening around the margin; this thickening is a slightly depressed rim, which is recurved to give a small lip; shell covered by regularly spaced pits or pores, but the surface is smooth on 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. 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 and thin walls. The contents of the alveoli did not permit embedding resin to penetrate the whole wall, and as a consequence sectioned material gave poor results. Nevertheless, sufficient information is available to reconstruct diagrammatically a cross-section through the wall. Each alveolus is surrounded by a single membrane-like structure, which is referred to as the shell matrix, and at the outer surface the shell wall is bilaminar. See for further information Ogden (1987).