Acanthocystis myriospina Penard, 1890, emend. Nicholls, 1983
Diagnosis: Periplast 6-26 μm, average 10 μm. Two types of siliceous scales. Radial scales numerous, very thin and delicate, 2.5-16 μm, straight or slightly bent at the proximal part and sometimes very numerous. Shaft 0.15-0.18 μm thick at the base, hollow and narrowing upwards (0.10 μm). The apex is abruptly cut off with a minutely serrated margin. Apex with three teeth, 0.08-0.25 μm long. Each tooth is usually sharply pointed, sometimes slightly branched or thickened and rounded. Basal disc 0.5-0.7 μm in diameter, usually with a thickened edge, slightly conical, shaft in the center. Tangential scales 2.4 x 1.3-3.6 μm, round, elliptical or ovoid with thickened margin, 0.015-0.030 μm (in width, with not always clearly developed axial rib or fold.) Up to 8 contractile vacuoles.
Remarks: Nicholls: Although the edges of several plate-scales were seen around the borders of some dried specimens, none ofthe preparations resulted in a clear view of intact plate-scales.
Distribution: lakes and streams. Canada (Nicholls, 1983), Chile (Dürrschmidt, 1985), Netherlands (Roijackers, unpubl.).