Frenopyxis stierlitzi Bobrov & Mazei, 2020
Diagnosis: Shell bilaterally symmetrical, broadly elliptic, sometimes almost circular, dorsoventrally flattened, yellowish to colorless. Aperture slightly eccentric on the ventral part of the shell, situated at the bottom of a wide funnel-like invagination. Aperture usually drop-shaped, with the narrow side oriented toward the nearest shell margin; occasionally circular. An internal thick organic lip surrounds the aperture and continues as a strut, which connects the aperture with the inner side of the shell wall and broadens at the point of connection between the strut and the wall. Shell surface smooth on the ventral side and densely covered by mineral particles on the dorsal side. Organic cement contains simple circular pores, regularly and densely distributed across the surface. This feature likely makes the shell wall fragile, necessitating reinforcement by the apertural lip and the strut attached to the shell wall.
Dimensions: According to Bobrov and Mazei, 2020: Shell length 37.80–45.90 μm; width 32.40–37.80 μm; depth 13.50–24.30 μm; distance between aperture and upper shell margin 8.10–15.30 μm; aperture length 8.10–15.12 μm; aperture width 6.21–13.50 μm; strut length 5.40–8.20 μm; strut width 3.24–6.75 μm.
Habitat: Type locality: Urban park “Bitsevsky Forest,” Moscow, in a mixed spruce–birch–linden forest. Found in a hollow of a spruce tree at a height of 0.5 m, in semi-decomposed litter with a small amount of fine mineral particles. Also reported from Berlin.
Remarks: F. stierlitzi is characterized by pronounced polymorphism in both shell size and shape. The general shell form is mostly elliptic, but in some specimens it approaches circular. The aperture is normally drop-shaped, but may be circular. A circular aperture occurs in both circular and elliptic shells. Variability of shell length and breadth within single populations is low, with a coefficient of variation between 4.64% and 8.85%. However, when comparing across populations, the overall size range increases considerably. The population from hollows 1.5 m high in spruce trunks, for example, is characterized by significantly larger shells than those from other sites. Aperture size shows much greater variability, with larger shells bearing proportionally larger apertures (after Bobrov & Mazei, 2020).