Surirella cruciata

Valves are obovoid with cuneate footpoles and broadly rounded headpoles. Valves are distinctly heteropolar in larger specimens, becoming less heteropolar in smaller specimens, with the valve length to width ratio ranging from 1.6 to 3.2. The valve face is distinctly and abruptly raised in the central third of the valve, depressed and shallowly concave in the apical thirds of the valve. The wings are well developed and raised prominently from the valve face. Raphe is situated in a raised raphe canal. The alar canals are narrower than the fenestrae, usually even in density throughout, but becoming occasionally variable in density in a single valve, 11–26 in 100 μm. The fenestrae are greater in hight than width in the central part of the wing, having 5–9 fenestral bars (8-9 in the middle part, 5 near apices), occluded or partially open between bars. The axial area is narrow, elevated in a shallow keel that is raised above the level of the porcae, without ornamentation or pores. The porcae are pronounced in the central part of valve, shallow and shortened in the outer thirds of the valve. The arrangement of porcae is opposite to alternate in the same valve. The porcae are unornamented, with a depression between porcae bearing minute granules, which are more or less visible in LM. The striae are bi- to triseriate, often becoming uniseriate towards the axial area and radiate at the apices. The areolae, 53–64 in 10 μm, are occluded externally, circular and opened with a small rim internally. Numerous silica plaques are present on the valve mantle near the edge with the valvocopula. The girdle bands are unornamented, thin and open. Chloroplasts were not observed.