Frustules biraphid, symmetrical to apical and transapical axes. Valves are large (> 250 um) to small, striae (alveoli) with individual puncta not resolved appearing as thin lines or broad "costa-like" areas. Striae may be crossed by longitudinal lines on both sides of the axial area or such lines may be lacking. Raphe system straight to complex, the external proximal ends usually dilated and bent slightly in the same direction, distal ends deflected. Central area may be expanded to one or both sides, and often valves of the same frustules differ in the construction or degree of the central area.
The concept of Pinnularia followed here is that which includes Caloneis Cleve, a genus with species having finely alveolate striae and longitudinal lines near the margins.
Light Microscope Image(s)
Each image is accompanied by the genus and species, California Academy of Sciences slide number (ie. CAS 612010), location of the specimen on the slide, and dimension in microns.