Amphidiniopsis hexagona cells are round, dorso-ventrally flattened, 28 - 42 microns long, 25 - 36 microns wide, approximately 17 microns broad. Thecal plates present: an apical pore, 4 apical plates, 2-3 anterior intercalary plates, 7 precingular plates, possibly 3 cingular plates, 6 sulcal plates, 5 postcingular plates, 2 posterior plates. Two forms were found, one with 3 anterior intercalary plates, the other with two. Two small hooks present at base of sulcus, one on the end of the right sulcal plate (Sd), the other on the end of the left sulcal plate (Ss). Thecal plates relatively thick, covered in small bumps and scattered pores, 0.1 - 0.2 microns diameter. Cingular plates covered by transverse ridges. Plate boundaries may have transverse striation. Possesses no chloroplast or eyespot. Nucleus large, (20 x 10 microns), oval and positioned centrally.
