Acanthocystis spinifera Greeff, 1869 emend. Siemensma & Roijackers, 1988
Diagnosis: Cell diameter 27-30 µm; spine-scales 4.6-5.3 µm long, not abundantly present and rather fine, with a cylindrical shaft, which is irregularly bent and set on a circular basal plate, centrally or, mostly, slightly eccentrically; shaft tapers slightly to the apex; apex about 0.07 µm broad with two or three blunt teeth. Basal plate 0.7 µm in diameter. Plate-scales with distinctly concave sides, thin, flat and slightly ovoid, 3.4-4.4 X 1.7-2.6 µm, with a small marginal rim; surface smooth.
Remarks: The only species which resembles A. spinifera is A. clavata. A. clavata, however, has a centrally seated shaft and a closed blunt apex. lts spine- and plate-scales are nearly half the size of those of A. clavata, while the basal plate of A. spinifera is nearly twice the diameter of the basal plate of A. clavata.