Cyclostephanos dubius can be distinguished from Stephanodiscus alpinus and S. agassizensis by its more coarse structure and characteristic alveolae near the valve margin. In North America, the absence of spines in C. dubius also differentiates it from most Stephanodiscus species. Very small specimens of C. dubius might be confused with S. minutulus, but S. minutulus has distinct spines visible in the light microscope, even in small specimens.

LM scalebar = 10 µm
Cyclostephanos dubius LM5 Cyclostephanos dubius LM3 Cyclostephanos dubius LM4 Cyclostephanos dubius LM2 Cyclostephanos dubius LM1
8.6-18.5 µm

Stephanodiscus minutulus

(Kütz.) Round 1981

Stephanodiscus minutulus LM1 Stephanodiscus minutulus LM2 Stephanodiscus minutulus LM3 Stephanodiscus minutulus LM4 Stephanodiscus minutulus LM5
6-10 µm
Striae in 10 µm
8-13 based on circumferential density

Stephanodiscus alpinus

Hust. in Hub.-Pest. 1942

Stephanodiscus alpinus LM3 Stephanodiscus alpinus LM7 Stephanodiscus alpinus LM2 Stephanodiscus alpinus LM11 Stephanodiscus alpinus LM6 Stephanodiscus alpinus LM5 Stephanodiscus alpinus LM4
13.8-30.8 µm
Striae in 10 µm
12-14 fascicles

Also compare to

Stephanodiscus agassizensis

Håk. and Kling