Cyclostephanos dubius

Valves are centric, with distinct costae. The valves are strongly concentrically undulate and show complementarity, with the central area of the valve face being either concave or convex. Characteristic chambers, called alveoli, are present near the valve margin. In large specimens, the central areolae are often organized into radial striae. In small specimens, the central areolae appear to be scattered randomly. A single rimoportula is located on the mantle, on an extension of a costa. Several central fultoportulae are positioned on the valve face, forming an irregular ring. Fultoportulae are also present on the mantle, on an extension of every second to third costa. Costae number 9-12 in 10 µm.

Based on European (West Germany, Poland, and Sweden) specimens, Håkansson and Hickel (1987) distinguished two morphotypes of Cyclostephanos dubius. Morphotype 1 was distinguished by more heavily silicified specimens, with coarse structure. The diameter of the valves was reported as 4.9-22.5 µm, with 9-12 costae in 10 µm. Morphotype 2 was distinguished as more weakly silicified, with finer structure. Morphotype 2 diameter ranged between 11.2–35.0 µm, with 12-18 costae in 10 µm. A central rosette, or annulus, is present in most morphotype 2 specimens. The morphotypes also differ in the length of the costae. Morphotype 1 has costae 1.25 µm in length, while morphotype 2 has slightly shorter costae, 1.0-1.25 µm in length. This feature, however, is only visible in SEM. The fultoportulae of morphotype 2 are arranged in an irregular ring near the center of the valve face and visible in LM. This feature however is not visible for morphotype 1. The material examined from West Okoboji Lake consisted only of specimens conforming to morphotype 1. Hickel and Håkansson (1987) also describe the occasional presence of spines in both morphotypes, with no spines, to spines on every second to sixth costa, to spines on every costa. No spines were been observed in specimens from North America.

Note that Round's (1982) description of the genus Cyclostephanos was invalid. Consequently, all of the taxa transferred to Cyclostephanos at that time were also invalid. In Theriot et al. (1987), the genus was validated the name Cyclostephanos dubius was also published.