Stephanodiscus alpinus has biseriate fascicles at the valve margin.

Stephanodiscus hantzschii has finer areolae and the valve face is flat, rather than concentrically undulate.

Stephanodiscus medius has a maximum of three marginal rows of areolae in each fascicle. That taxon is restricted to eutrophic waters.

Cyclostephanos novaezeelandiae has bifurcate interfascicles near the valve margin. Furthermore, fascicles are composed of 2 or 3 (at most 4) rows of areolae at the valve face/mantle junction.

LM scalebar = 10 µm
Stephanodiscus oregonicus LM7 Stephanodiscus oregonicus LM6 Stephanodiscus oregonicus LM5 Stephanodiscus oregonicus LM4 Stephanodiscus oregonicus LM3 Stephanodiscus oregonicus LM2 Stephanodiscus oregonicus LM1
13.0-25.6 µm
Striae in 10 µm
Fascicles 3-4

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

Stephanodiscus hantzschii

Grunow in Cleve and Grunow 1880

Stephanodiscus hantzschii LM7 Stephanodiscus hantzschii LM4 Stephanodiscus hantzschii LM3 Stephanodiscus hantzschii LM6 Stephanodiscus hantzschii LM1
11.8-20.3 µm
Striae in 10 µm
6-9 fascicles based on circumferential count

Stephanodiscus medius

Håk. 1986

Stephanodiscus medius LM10 Stephanodiscus medius LM8 Stephanodiscus medius LM3 Stephanodiscus medius LM5 Sargent 1 7 23 681 BW Stephanodiscus medius LM2 Stephanodiscus medius LM1
9.2-35.0 µm
Striae in 10 µm
6-10 based on circumferential density

Also compare to

Cyclostephanos novaezeelandiae

(Cleve) Round