Genus Frenzelina Penard, 1902

Diagnosis: Cell usually covered with a thin membranous test, which is hemispherical or nearly spherical, with a small or large aperture; very small mineral grains are loosely attached to the membrane; test in dorsal view usually elliptical; fine filopodia which pass the aperture in a stalk or bundle which can be relatively long; filopodia can be very numerous and branching; aperture flexible, circular to elliptical, sometimes rapidly changing; aperture somewhat tubular with a thickened rim; nucleus vesicular with central nucleolus.

Remarks: Two species have been described. The type-species is Frenzelina reniformis, described by Penard (1902) from some specimens collected from one single location. Some years later Hoogenraad (1910) described  Frenzelina minima. According to my observations, this species is identical with Frenzelina reniformis.
This species can be very variable. Within a sample one can observe naked specimens, specimens with a light covering and specimens with a heavy covered membrane. See Schönborn and Badewitz (2008).
Frenzelina can be very common in several types of fresh water.