Abstract
A compound hypothecal pore, situated on the first antapical plate, is described from seven species of Protoperidinium by means of SEM. The pore, which is 0·85-1·3 µm long and 0·5-0·8 µm wide, is bounded by a raised rim and consists of 9–16 small perforations of approx. 0·1 µm diameter. All species possessing the hypothecal pore have a six-sided first apical plate (Group: Paraperidinium).