Acanthocystis quadrifurca Nicholls, 1983
Diagnosis: Scales of two types; plate-scales elliptical to oblong 1.2-2.0 X 1.8-3.5 µm consisting of an oblong centrally positioned membrane with a thickened axial ridge and an outer rim 0.8 µm wide connected to the central membrane by 20-30 radial ribs which are sometimes forked or branched. Spine-scales, a straight or slightly bent shaft 5-10 µm long, 0.05-0.15 µm in diameter and seated on a circular baseplate 0.8-1.2 µm in diameter in the form of a shallow inverted cup. Ends of the spines separated into four flared arms about 0.1-0.2 µm long which taper to a sharp point.
Remarks: The spine-scales of this species might suggest some close relationship to A. trifurca at first; however, the main differences are in the spine shaft which is hollow with a three-pronged apex in A. trifurca but is solid with a four-pronged apex in A. quadrifurca. The plate-scales of A. quadrifurca with their striking “spoked wheel” pattern is very distinctive and cannot be confused with plate-scales of any other known Acanthocystis species.
Distribution: Freshwater. Canada.