Genus Centropyxis Stein, 1857
Centropyxis is a genus of testate amoeba with a discoid, flattened test, somewhat beret-shaped. The dorsal surface is rounded, the ventral side is flat to concave, with a ventral aperture, which may be circular to uneven, but displaced towards one end. The shell may be flattened at the apertural end. At the posterior end or all around the periphery some spines may be present. Within a population individuals may have zero to thirteen spines. The shell surface is smooth, dorsally with more or less mineral particles or diatoms, cemented together by an organic material, ventrally polished-looking due to more cement and much smaller grains in the shell.
There are two shell types. The first type is bilaterally symmetrical, rounded, flattened more at the front than at the rear. The ventral part is flat. There are often spines at both sides and at the rear. The aperture is ventral, anterior, roundish, dorsal and has ventral lips recurved and invaginated. The ventral margin of the aperture is often connected to the dorsal face by some flat pillars or bridges. The shell can be organic or constructed with mineral particles or diatoms. Shape, number of spines and size are extremely variable. This type occurs mainly in freshwater habitats.
The second type has a circular shell or is elongated in ventral view. In lateral view the oral region is slightly flattened, the aperture is subterminal, only with the ventral lip incurved. No connections or bridges between the ventral and dorsal face. Some species of this type are common in freshwater habitats and sphagnum, but most species are found in drier mosses and humus.
More than 130 species and many varieties have been described, but many descriptions are not adequate. Identifying Centropyxis species is often extremely difficult or even impossible.