Surirella ovalis

The valves are extremely variable throughout their vegetative cell cycle. Their length ranges from 16 to 61.5µm and their width from 11 to 35µm. The valves are all heteropolar and the poles vary according to valve length. The longest valves, between 41 and 75µm, had two cuneate apices, one being broader than the other. The middle length valves, between 31 and 34µm, had one cuneate apex and one broadly rounded apex. The smallest valves, between 14 and 27µm, had two broadly rounded apices, one being broader than the other. The intermediate sizes showed transitional states. Most of the valve faces were concentrically undulate with one or two undulations. The costae on the valve face are relatively coarse at 14-17 in 10 µm and every third or fourth costa in the margin is raised above the others, corresponding to the fibulae on the interior of the valve. The fibulae are marginal and are 5-6 in 10 µm.

Under SEM, the striae between the costae become visible. There are two rows of slit areolae irregularly orientated between each of the costae. Near the valve margin where the concentric undulation occurs and the raised costae begin, the striae split into as many as five rows. These striae lead into an areolated depression, or pseudoinfundibulum.

Krammer and Lange-Bertalot (1987) note on the type material that there is a single portula on the mantle between the fibulae. They also were not able to observe the areolae on the type material due to possible acidic degradation.