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Acanthocystis bicornis
From Siemensma, 1991

Acanthocystis bicornis  Dürrschmidt, 1987

Diagnosis: Periplast 15—25 µm in diameter (measurement based on whole mount), with 3 types of scales. Radial spine-scales consisting of base-plate, shaft and furcate apex. Shaft narrow, cylindrical, 0.12 µm thick. Base-plate, 0.6-0.8 µm in diameter, circular, with a slightly upturned peripheral edge. Large spine-scales, 4-5 µm long, with 1 tooth on the inner edge of each branch of the apical furca (furca ca. 0.7 µm long). Distal ends canted. Small spine-scales, 2.0-3.2  µm long, a broader fork in the apex, usually with one or two teeth of varying length present on the inner edge of each branch. Tip of the branches biforked to a varying length. Tangential plate-scales 2.7 x 1.6  µm, elliptical, sometimes slightly curved, patternless, except for the axial thickening.

Ecology: Freshwater. New Zealand, Chile, Sri Lanka and Canada.

Remarks: A. bicornis is easily distinguished from all other species of Acanthocystis by the distinctly bifurcate tips of the small spine scales in addition to teeth on each apical branch. Nicholls (1983) reported Canadian specimens of A. turfucea with “tiny teeth on the bifurcate apices of the long spines”. However, the forks of the small spine scales are smooth and pointed. A. cornuta also has teeth along the inner edge of the branches of the apical furca but differs in that there are always three or more teeth present, and in that it has only one spine scale type.

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