Surirella undulata

Valves are elliptic to lanceolate and symmetric about the apical and transapical axes with broadly rounded to slightly apiculate ends. Valves are roughly 1.5-2 times longer than they are wide. Along the margin of the valve, the alar canals are visible in LM. Fibula density is 3-4 in 10 μm. In SEM, striae appear radiate and convergent at the apices and transition to parallel toward the center of the valve. Striae are uniseriate and formed by fine areolae which become more dense toward the valve margin. Porcae are visible as transapical undulations across the valve face. Valves typically have nine porcae, up to eleven in larger valves. The raphe ends curve to the same side and are slightly enlarged.

This taxon was first described as Navicula undulata Ehrenb.1838. Later, Ehrenberg (1849) described S. undulata Ehrenb. 1845, without acknowledgment of the 1838 publication. Still later, Ehrenberg (1849) made the combination Surirella undulata (Ehrenb.) Ehrenb. 1849. Resolving if these taxa are of the same type or distinct entities needs to be investigated.

For many practitioners, this taxon is recognized and well-known as Cymatopleura elliptica (Bréb. ex Kütz.) W. Sm. 1851. However, C. elliptica is a later, subjective synonym (not of the same type).

Recent investigation into the phylogeny of the group (Ruck et al. 2016) supports a shared derived character, or synapomorphy, between Cymatopleura and Surirella. Simple portulae are found in both genera. These portulae form a cellular communication between the raphe canal and cell interior through fibulae. Because of this synapomorphy, Ruck et al. (2016) proposed a change in nomenclature to reflect evolutionary relationships. As a result, Cymatopleura taxa are now considered within Surirella.

This taxon also shares characteristics of the Surirella Pinnatae group by its isopolar valves and the marginal location of the raphe directly on the valve mantle (Jahn et al. 2017).