Stephanodiscus hantzschii

Valves are centric with a flat valve face. Central fultoportulae are absent. Areolae 22 to 25 in 10 µm, easily resolvable in the light microscope. Areolae are arranged in radiate striae, grouped in fascicles. The fascicles are uniseriate at the valve center and become triseriate near the mantle. Several areolae may be grouped into a central annulus. Each interfascicle terminates in a prominent spine. Valve diameter and number of fascicles (37 to 45 per valve) is variable, although not directly related smaller valves do tend to have higher fascicle density. A single rimoportula is present near the valve face mantle junction at the same level of focus as the spine. The external opening of the rimoportula is a tube in the position of a spine, replacing a spine and appearing slightly shorter than a spine.

Stephanodiscus hantzschii is morphologically variable (Håkansson and Stoermer 1984, Kobayasi et al. 1985, Håkansson and Bailey-Watts 1993). The nominate form is described here, and has more heavily silicified valves, straighter fascicles and lower areola density compared to S. hantzschii fo. tenuis. Note, however, that S. hantzschii fo. tenuis was suggested to be an ecophenotype produced as a result of silica limitation (Stoermer et al. 1979).