Pinnularia alabamae

The valves are large, linear, with an inflated middle part and subcapitate-cuneate apices. The length-to-width ratio is 5.5–7.7. The axial area is linear and about 1/4–1/3 of the valve width. Occasionally, there are irregular depressions on the external surface of the axial area, mostly along the external raphe fissure. The central area is round or elliptical, not always well separated from the axial area, usually wider on the primary valve side. The external raphe slit is only slightly curved so that the raphe appears lateral in LM. The central pores are small and round. The central nodule is prominent, with two depressions on its internal surface. The terminal raphe fissures are hook-shaped and extend onto the valve mantle. The striae (alveoli) are covered externally by silica plates perforated by pores arranged in 7–8 rows. The inner openings of the alveoli are approximately 1/2 the length of the stria in the narrowest part of the valve. Under LM their borders appear as two longitudinal lines: one transects the striae about 4–7 μm from their inner ends and is usually easily observed with LM, while another line runs 2–5 μm from the valve margin and is sometimes not detectable with LM. Striae continue to the valve mantle without any interruption at the valve face–mantle junction, where there are occasional spines on some virgae, which are widened at the spine base. The spines are not found at the valve apices, and their density is variable.