Encyonema pergracile

Valves are elongate, linear-lanceolate and dorsiventral. The apices are narrow and acute, slightly deflected toward ventral side in larger specimens. The raphe is lateral with external proximal ends that are slightly dorsally deflected before the distal ventral deflection. Internally, the raphe appears to be continuous through the central area. The distal raphe has relatively long terminal fissures, that is, the helictoglossae are distantly spaced from each valve apex. The axial area is linear and slightly expanded at the central area. A single isolated punctum is present on the dorsal side of the central area, in line with a central stria. The external expression of the punctum is small and circular, while the internal expression is elongate and opposite a bifurcate cleft in the intermissio. Striae are slightly radiate becoming convergent near the apices. This is most apparent on the ventral side of the valve as the stria density increases. Striae are uniseriate and composed of transapically elongated areolae.

The specimens shown here appear to align most closely with E. pergracile, although they are more narrow (6.5-7.8) than the specimens described by Krammer (1997).

This taxon is part of a large, diverse species complex (Krammer 1997). It includes both Cymbella gracilis (Ehrenberg) Kützing and C. gracilis (Rabenhorst) Cleve. It appears that the type of Cymbella gracilis is ambiguous, and that the name is poorly circumscribed. Further investigation may show additional North American species.