Stephanodiscus niagarae

Cells discoid or barrel-shaped , with concentrically undulate valves that produce either a convex (scutate) or concave (lacunate) valve face. Valves appear undulate in girdle view in the LM. A ring of spines is present around the valve face, occuring in a single row at the end of every second or third costa at the junction of the valve face and mantle. Areolae are arranged in radiating striae separated by raised hyaline costae. In the valve center the striae are uniseriate, towards the margin striae become biseriate or triseriate. Areolae appear round or angular in the SEM, and round in high focus in LM. Convex domed cribra close the internal opening of the areolae. Fascicles closely spaced, separated by thin costae (also termed hyaline interfascicle). There are 7-9 fascicles in 10 µm, and 11-20 areolae in 10 µm. Every second or third interfascicle with a spine continues over the edge of the valve onto the valve mantle.

Spines are curved and tapered, with a sharply pointed tip. Marginal fultoportulae are present, arranged in a ring around the mantle, always occurring beneath each spine; central fultoportulae occur at the center of the valve face, singly or in a ring. The fultoportulae have 3 satellite pores and external buttresses. One or a few rimoportulae occur on the mantle-face junction. The rimoportulae have tubular external extensions that superficially resembles the spines. The rimortulae, however, are positioned slightly abvalvar the ring of spines. Rimoportulae are difficult to distinguish in LM, but they are shorter, straighter and less tapered than spines.