Stephanodiscus oregonicus

Valves are cylindrical with strong concentric undulation. The central area is convex or concave, and sometimes slightly off-center. The areolae in the central area are disorganized or arranged in weakly radiate rows. Outside of the central area, fascicles are variable in width, becoming multiseriate near the valve margin. Areolae near the central area are larger than areolae near the margins. Fascicles at the valve margin may include as few as 3 or as many as 9 areolae. A spine is positioned at the distal end of each distinct costa, or interfascicle.

The circumferential density of fascicles is 3-4 in 10 µm.

Scanning electron micrographs and observations of S. oregonicus were published by Håkansson (2002).

Stephanodiscus excentricus Hust. was described from fossil material collected in Oregon and in my view, S. excentricus conforms to the type of S. oregonicus.

Håkansson and Kling (1989) report live specimens of S. oregonicus from Canadian lakes, along with live specimens of S. subtranssylvanicus, although this report deserves further verification. The taxa S. subtranssylvanicus Gasse and S. subtranssylvanicus var. minutula Gasse were described from Ethiopia (Gasse 1980) and such a disjunct distribution ought to be confirmed through examination of the type material. Nevertheless, Håkansson and Kling (1989) reported that smaller forms of S. subtranssylvanicus could not be distinguished from S. oregonicus. They also reported that both species lack valve face fultoportulae and have essentially the same ultrastructure. There is considerable overlap in the reported size ranges of these two taxa: 10-44 µm for S. subtranssylvanicus (Håkansson and Kling 1989) and 6-20.5 µm for S. oregonicus (Håkansson 2002).