Silicoflagellate with siliceous skeleton (basal ring) with four sides, but without the apical ring. Spines project from the corners of the outer hexagon, internal projections form irregular connections. Bars also arise inwards and may fuse to form a central arch. The cytoplasm extends over the skeleton, fine pseudopodia usually project from the tops of the spines. Many small plastids, single emergent flagellum. Mostly 30-80microns. Marine