Tryblionella gracilis

Valves are linear-lanceolate and gradually taper to apices that are cuneate to sometimes slightly rostrate. The raphe is keeled and eccentric, fibulae 6-8 in 10 µm. The raphe is slightly constricted at the central nodule. A longitudinal undulation is located along the apical axis of the valve. Costae at the apices are easily distinguishable (6-8 in 10 µm), while costae near the valve center may appear thinner and less pronounced (7-11 in 10 µm). Costae may appear slightly curved and non-parallel across the valve undulation. Striae are difficult to observe with light microscopy.

The original description of Tryblionella gracilis by W. Smith (1853) is valid and legitimate. The taxon was renamed Nitzschia tryblionella Hantzsch in Rabenhorst when it was transferred to Nitzschia by Hantzsch in Rabenhorst 1848-1860. The change in the specific epithet was necessary because the name N. gracilis was already occupied. When the genus Tryblionella was resurrected by Round et al. (1990), the original name Tryblionella gracilis W. Smith 1853 was again recognized as the valid and legitimate name of this taxon.