Placocista jurassica Penard, 1905

Diagnosis: Test small, distinctly more compact and less compressed than Placocista spinosa. Shell length 72–76 µm, thus much smaller than P. spinosa (typically 130–140 µm). In transverse section the test is strongly biconvex. Siliceous plates broadly oval; those bordering the pseudostome lack the denticulations characteristic of Euglypha. Lateral commissures may occasionally form a sharp angle, but this feature is inconsistent.

The shell bears numerous fine, straight, short spines without a basal swelling. These arise at the angles of the hexagonal plate junctions. Spines cover the entire shell surface, sparse on the broad faces but increasingly numerous toward the lateral margin; sometimes occurring in small groups of 2–4 from a single point.

Plasma filled with very small greyish granules; food particles rarely visible. Nucleus large, positioned posteriorly, containing 1–3 rounded nucleoli. Several small contractile vacuoles present but difficult to observe. Individuals are often retracted into the shell or encysting, sealing the aperture with a membranous operculum.

Dimensions: Penard (1905): Length: 72–76 µm; Width / thickness: not explicitly given, but described as relatively broad and less compressed than P. spinosa, with a strongly biconvex transverse profile.

Ecology: According tot Penard (1905) specimens were collected in Sphagnum from the peat bog “la Pile” above Saint‑Cergues, Switzerland. The species occurred together with Placocista spinosa but was much more abundant. Despite repeated sampling, no transitional forms between the two species were ever found, and P. jurassica was consistently present as a distinct, small, spine‑bearing species. Individuals were frequently observed encysting or withdrawn into the shell.

Distribution: Known from Switzerland.