Pinnularia macilenta

Valves are linear-lanceolate. Specimens at the larger end of the size range have a slightly gibbous central area. Apices are broadly rounded and in larger specimens, slightly subcapitate. The axial area is broad, about 1/4-1/2 the width of the valve. The central area is wide, irregular, and rhombic. A small, asymmetrical transverse fascia is usually present. The central area has four large markings, although they are not always visible in LM. The raphe is broadly lateral with terminal fissures having the shape of a question mark. Striae are radiate in the center, becoming convergent toward the ends.

The iconotype illustration (Ehrenberg 1837) shows a diatom with linear valves, straight margins, and a narrow axial area, a concept followed by Krammer (1992).

Later, Reichardt (1995) examined the type material from Cayenne and concluded that P. subgibba Krammer 1992 is a subjective synonym of P. macilenta. We note, however, that specimens from type material illustrated by Reichardt (1995) are more linear and narrow (7-13 μm).

In a slightly different interpretation, Krammer (2000) stated that only the variety P. subgibba var. hustedtii Krammer 1992 is conspecific with P. macilenta. Our specimens are most similar to those presented as P. macilenta in Krammer (2000), although they differ by being are wider and more lanceolate.