TRIGONAL PYRAMIDAL CLASS (Hemimorphic-tetartohedral.) Here there is only the triad axis of symmetry, which is uni terminal. The general form { hkr} is a tri gonal pyramid consisting of three faces at one end of the crystal. All other forms, in which the faces are neither parallel nor per pendicular to the triad axis, are trigonal pyramids. All the prisms are trigonal prisms; and perpendicular to these are two pedions.
The only substance known to crystallize in this class is sodium p e r i o date , the crystals of which are circularly polarizing.