Surirella amphioxys

Valves have a length to width ratio between 2 and 3. The keel is low and no alar wings are visible. Valves are isopolar with cuneate apices and straight, parallel sides. The valve face is strongly undulate (structures termed "porcae"). These porcae appear as rectanglar structures in the LM. Each porca is 2-3 µm wide by 4-5 µm long and is traversed by 3-4 costae, with1-2 costae between each porca. Costae are 16-20 in 10 µm, extending from the margin to the apical axis, where they meet the axial line. Four to five rows of round areolae occur between the costae, visible only in the SEM. A distinct, flat central area occurs about the axial line. Around the valve margin are what appear to be large areolae in the LM, but in the SEM these “areolae” are actually indentations in the valve face corresponding to fibulae in the interior of the valve. Areolae on the surface of the indentations directly beneath the keel penetrate to the mantle. Indentations, or portulae occur on the mantle face, corresponding to those on the valve face.