Cryptodifflugia oviformis Penard, 1902
Syn.: Difflugiella oviformis in Bonnet & Thomas, 1955
Syn.: Cryptodifflugia operculata Page, 1966

Diagnosis: SShell hyaline, rigid, ovoid, not noticeably laterally compressed, colorless or sometimes yellowish. Aperture with a thick collar. Diameter of the aperture usually about one-fourth and never more than one-third of the greatest diameter of the shell. Encystment with an operculum (usually biconvex) in the aperture. Pseudopodia few, slender, finely lobose, usually forming a very thin cone with a rounded tip, often with fused bases, free of granules, and emerging from a thin sheet of hyaline cytoplasm outside the aperture.

Dimensions: Shell length 14–35 µm.

Ecology: Freshwater, in various water types, also in soils; usually on aquatic plants and in Sphagnum. Very common, sometimes abundant.

Remarks: Shell wall consists of a thin outer organic layer and a much thicker inner, non‑crystalline calcareous layer (Hedley et al., 1977).
Page (1966) reported: “Most had a length between 16 µm and 20 µm. Apertures of the same 100 tests ranged from 3.4 to 6.9 µm in diameter, but only the largest test had an aperture of 6.9 µm; the majority had apertures with a diameter of 3.4 µm, and the mean was 3.7 µm, or approximately one-fourth the mean diameter of the broadest part of the test. The aperture in this species is circular and the inner rim of the test is thickened, forming an entirely internal collarette.
The test wall is about 1 µm thick and consists of a heavier inner layer and a thin outer layer, the latter very transparent and visible only by careful examination. Rigidity and fragility of the test are demonstrated by its shattering under pressure of the cover slip when a wet mount dries out or a cover slip is pressed down. No foreign matter was ever seen incorporated into the tests. Occasionally a little bacterial debris adheres to tests in culture, but no other foreign particles ever adhered even in the presence of soil and diatoms.”