Envekadea metzeltinii

Valves are linear with almost parallel margins. The central part of the valve is gibbous in larger specimens and slightly convex in smaller valves. The axial area is narrow, less than 1/9 of the total valve width, linear and widens very slightly near the central area. The external central area is highly reduced. Internally, two distinct longitudinal ribs (also termed axial costae) border the axial area. The longitudinal ribs widen slightly at the central area of the valve. Near the apices, the longitudinal ribs diverge, forming a plate-like, rounded, hyaline zone, revealing the sigmoid course of the raphe.

The external raphe branches are positioned in a deep, narrow groove that is straight to very weakly undulate. The proximal raphe ends are straight and terminate in a depression, bordered by four slightly raised plates. Inside the depression, a raised central nodule is visible on which the proximal ends terminate. The external distal raphe ends terminate in opposite directions and the raphe is sigmoid. The raphe diverges from the sigmoid external groove, and follows a straight course. The internal raphe branches are situated on a sternum between longitudinal ribs that extend from apex to apex. Internally, the proximal raphe ends are hooked and the distal raphe ends terminate in short, simple helictoglossae.

Striae are clearly radiate, slightly geniculate and equidistant throughout the entire valve. In the central area, several irregularly shortened striae are inserted in the normal striation pattern. Near the apices, a sudden shift in stria orientation is present. Several very short striae are inserted near the axial area. Striae are uniseriate and composed of areolae that are variable in size and shape. Taxa within the genus are reported to have external hymenes covering the areolae (Gligora et al. 2009), but were never observed in E. metzeltinii due to rarity of valves in our collections and the severe oxidizing treatment used for LM and SEM. Most areolae are apically elongated and rectangular. Near the central area and the apices, areolae are very irregular in shape, with a higher stria density than the rest of the valve. Short hymenes (perhaps degraded by cleaning methods) are visible by SEM inside the areolae. A hyaline zone separating the striae is present at the apices, delimited by the external distal raphe ends. Striae continue uninterrupted onto the mantle. A narrow hyaline zone is present near the mantle edge. Internally, striae are located between clearly raised virgae and composed of rounded, unoccluded areolae. Near the central area, striae diminish in width toward the axial area.

Due to the rarity of this taxon in our samples, no frustules in girdle view have been observed. This has also been found in the Yucatan region of Mexico, where it was identified as a species of Scoliotropis (Metzeltin and Lange-Bertalot 2007).