Oleiformis major (Penard, 1890) Soler-Zamora et al., 2023
Syn.: C. margaritacea var. major Penard, 1890

Diagnosis: Shell large, colourless or yellowish, transparent, retort-shaped laterally and elongated–elliptical in outline in ventral and in dorsal views, with maximum diameter in the mid-region of the shell. Cross section circular. In lateral view the shell is broadly rounded at the posterior end and curved at the anterior end towards the pseudostome, forming an elongated neck. Pseudostome circular and oblique, surrounded by about 35–40 apertural scales with clearly visible (SEM) denticles, rim with a thin covering of organic cement. Shell composed of numerous small (1.6–2.1 µm), circular or oval, plane, comparatively thick (1.5–2.0 µm), and non-imbricated scales, which are regularly arranged side by side in diagonal rows about 34–37 in the mid-region of the shell. Each scale is surrounded by a ring of six perforations in the organic cement, which give a characteristic appearance of the shell of C. major when it is observed under the light microscope.

Measurements. Shell length ranging in most specimens between 175 and 205 µm; shell diameter range between 75 and 81 µm. The axis of the pseudostome in all the specimens measured ranges within close limits, 23–28 µm.

Habitat: Freshwater sediment and mosses.