Guide to Pinnularia

Frustules of Pinnularia may be large, up to 300 µm in length. The striae are alveolate. Internally, the striae are positioned within chambers. The openings of the chambers are evident as longitudinal lines that cross the striae. The raphe system may be straight or complex. Externally, the proximal raphe ends are expanded and bent slightly to the same side. Terminal raphe fissures are deflected and may form a distinct, curving bend (a question mark shape). The central area may be expanded to one or both sides. Living cells contain two plastids.

Pinnularia is closely related to Caloneis and some authors suggest they should be considered as the same genus.

Pinnularia contains a large number of species and is often abundant in low conductance, slightly acid waters.