Mastogloia calcarea

Valves are linear-lanceolate (post-initial and small valves are elliptical-lanceolate) with rostrate ends (post-initial valves have round ends, small valves have subrostrate ends). Striae are punctate, radiate, weakly curved at the central area, and become parallel at the apices. The striation pattern on the valve face continues unchanged onto the mantle. The central area is elliptical to polygonal and is often asymmetric. Areolae are round and internally occluded by a cribrum, 16–20 areolae per 10 µm. The raphe is straight to weakly undulate with a kink near the central area. The proximal raphe ends are slightly expanded and deflect in the same direction. The distal raphe ends are slightly expanded, extend centrally over the mantle and hook to the same side. There is a small hyaline area where the curve of the distal raphe ends occurs.

Internally, the raphe is straight and encased in a thickened sternum. The internal proximal raphe ends are straight and the sternum fuses into the thickened central nodule. The internal distal raphe ends are occluded by pseudosepta. The partectal ring is closed, forming a complete band, and lacks a flange separating the ring from the valve margins. A pseudopartectum is present between the apex and each terminal partectum. There is a furrow opening into a pore-like cavity by each pseudopartectum. The apices of the partectal ring have a narrow cleft that expands into a pyriform lacuna. One pore is present between the center most pair of partecta on each side of the partectal ring. The partecta are distributed along both sides of the partectal ring, except near the apices. Partecta are clearly distinguishable and evenly sized, but they are slightly larger closest to the apices, 5-6 partecta in 10 µm. Thickened virgae are present throughout the valve interior. Cribral pores become highly irregular near the sternum and central nodule. There are five partectal pores located near both sides of the apices. Live specimens produce mucilage strands from the partectal pores. Live cells contain 2 H-shaped plastids at each pole.