Quadrulella symmetrica  (Wallich, 1863) Kosakyan et al., 2016

Diagnosis: Test ovoid or pyriform, with a rounded posterior end, laterally compressed towards the pseudostome. Test colourless, composed of square plates, regularly arranged in rows. The plates are smaller near the aperture (4–5 µm), then gradually larger (reaching up to 10–12 µm) towards the posterior end of the test. Pseudostome often curved and bordered by a thin organic lip.

Dimensions: Test length: 72–85 µm, breadth: 40–46 µm. Pseudostome: 20–23 µm wide.

Ecology: Q. symmetrica is found in wet mosses (Sphagnum or other), water streams, forest litter and soil, from all continents, except Antarctica.

Remarks: Morphologically very similar to Q. variabilis, from which it differs by the dimension of the test and the size of scale plates (L = 72–85 µm, maximum plate size 10–12 µm in Q. symmetrica versus L = 66–69 µm, maximum scale size 7–9 µm). It can be discriminated from Q. madibai, which has plates of similar size, based on its less slender and elongated test (L/ B ratio is 2.0–2.3 in Q. madibai versus 1.7–1.9 in Q. symmetrica). Moreover, the general outline of the test in Q. madibai is globally more tubular and does not present a distinct neck. Molecular data clearly separate these two species (sequence divergence up to 10%).

Detailed morphological and molecular observations are needed to clarify the true position of all described forms and their variability.

Following subspecies are distinguished:
Q. symmetrica var. irregularis f. minor (Laminger, 1971)
Q. symmetrica var. longicollis (Taránek, 1882)
Q. symmetrica var. longicollis f. lanceolata (Gauthier-Lièvre, 1953)
Q. symmetrica var. curvata (Wailes, 1912)
Q. symmetrica var. genuina (Taránek, 1882)
Q. symmetrica var. islandica (Decloitre, 1966)
Q. symmetrica var. kivuensis (Van Oye, 1958)
Q. symmetrica f. major (Chardez, 1967)
Q. symmetrica f. minor (Chardez, 1967)