Choanocystis rotundata (Nicholls, 1983) Siemensma & Roijackers, 1988
Diagnosis: Scales of two types. Spine-scales 3-12 µm long, usually slightly bent in the proximal one-quarter to one-half of their length. Spine shaft hollow, 0.12-0.2 µm in diameter, seated eccentrically at the base of a cleft in an ovate to circular base plate 1-1.3 µm in diameter. Base plate with a marginal rim. Apex of the spine shaft abruptly cut off at right angles without taper but with 12-20 minute marginal rounded teeth about 0.015 µm high. Plate-scales elliptical 3.5 x 1.5-2.5 µm, patternless except for an axial thickening positioned in the middle of the scale about two-thirds the length of the scale and a maximum width of about 0.15 µm in the midregion.
Remarks: The base plates of the spine-scales of A. rotundata are similar to those of A. perpusilla Petersen et Hansen; however, A. perpusilla spines are only 3-4 µm long and have a marked bifurcation at their apices.
Distribution: Canada. Freshwater.