Tetracyclus arceli

Valves linear at centre, shallower at the poles, tapering further towards the apices, length 20-25µm, breadth 10-15µm. Transapical ribs robust, mostly primary, few secondary and tertiary; 4-6 in 10µm. Striae parallel to each other and to the ribs, appear as simple holes through the basal siliceous layer; 10-20 in 10µm, 5-8 between (primary) ribs. Sternum central, linear, slightly raised; pore fields appear absent, although polar striae somewhat more compact than those composing valve surface.
Rimoportulae present, large slit (1—1.5µm), usually situated at a pole, often partly on mantle.
Cingulum partly known. Valvocopula, open band, attaching by crenulated lip overlaying ribs of valve; pars interior consisting of 2-6 distinct rows of poroids (which coalesce internally); ligula absent; septum extending from pars interior with small pore penetrating pole of band. Other bands not observed.
T. arceli is distinguished by its shape; in larger specimens the outline of the valve is more regular and less undulated than that of T. emarginatus (Ehrenb.) W. Smith (Williams 1987).