Tetracyclus arceli  

Original Description: 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.