Centropyxis spinosa  (Cash, 1905)
Basionym: Centropyxis aculeata var. spinosa Cash, 1905
Synonym: Centropyxis bryophilus Dekhtyar, 1998

Diagnosis: Shell oval, with a neatly rounded anterior outline, purely chitinous, usually bearing relatively few adhering sand grains, and semitransparent; yellowish‑brown when young, gradually darkening with age, and sometimes partly or entirely covered with diatom frustules. Aperture usually oval, with the long axis parallel to the long axis of the shell, lobed or uneven in outline, variable in width, but always comparatively smaller than that of C. aculeata. The margin is reversed and usually has two bridges connecting the invaginated aperture with the opposite dorsal area. Spines are variable in number and length, composed of the same material as the test, and often curved, with a plug in the aperture at the upper end. A broad ventral region around the aperture has a characteristic smooth and uniform structure that is clearly visible under SEM and light microscopy.

Dimensions: Cash (1905): 120–140 µm.
Todorov and Bankov (2019): length 107–136 µm; height 42–46 µm; spines 14–28 µm; aperture diameter 28–37 µm.
My measurements: length 91–117 µm; L/W ratio 0.9–1.2; aperture 25–45 µm × 21–36 µm.

Ecology: In Sphagnum, or in water bodies in contact with Sphagnum.

Remarks: The lobed aperture is caused by the presence of two pillar‑like internal bridges and an anterior crescent‑shaped connection between the ventral and dorsal regions, resembling a headphone (see images below).