Gomphonema semiapertum

Valves are lanceolate-clavate, with a rounded headpole and a narrower footpole. Striae are parallel to slightly radiate at the center of the valve, becoming more radiate near the apices. Striae are composed of a single row of distinct areolae that number 25-30 in 10 µm. The central axial area is wide and each branch is lanceolate. The central area is large, asymmetric and rectangular to elliptic. One side of the central area extends all the way to the valve margin; the other side is delimited by 3-5 shortened striae. One isolated stigma is located close to the proximal raphe ends on one side of the central nodule. The raphe is strongly lateral. Proximal raphe ends are small and inconspicuous. On deep focus, the internal proximal raphe ends are steeply deflected, into the shape of a barb, on the same side as the stigma. Terminal raphe ends are comma-shaped. The terminal raphe end at the footpole bisects a distinct apical pore field.

This is a large and distinctive but quite uncommon taxon, with extant populations apparently restricted to the Cascade Mountains of California and Oregon.