Surirella angusta

Frustules have length to width ratios between 2 and 5. The valve face is flat, isopolar, with cuneate or occasionally capitate apices. In valves beyond 25 µm in length, the sides are straight and parallel, while under 25 µm the frustule can be elliptical. The keel is not raised and no alar wings are visible. Fibulae are present on the interior of the valve surface, 7-8 in 10 µm. Every third or fourth costa on the valve surface is either raised or lowered from the plane of the other costae. These altered costae correspond to the fibulae below the valve surface. Not infrequently, raised costae can arise in double form, where no intermediate lower costae exist. The striae, visible in the SEM, consist of 2-5 rows of areolae situated between the costae, 23-28 in 10 µm.