PLAGIHEDRALI CLASS  (Plagihedral-hemihedral; Pentagonal icositetrahedral; Gyroidal 2) . In this class there are the full number of axes of symmetry (three tetrad, four triad and six dyad), but no planes of symmetry and no centre of symmetry . Pentagonal icositetrahedron (fig . 40) . This is the only

simple form in this class which differs geometrically from those of the holosymmetric class . By suppressing either one or other set of alternate faces of the hexakis-octahedron two pentagonal icositetrahedra {hkl) and (khl) are derived . These are each bounded by twenty-four irregular Icositetrahedron . Dodecahedron . pentagons, and although similar to each other they are respectively right- and left-handed, one being the mirror image of the other; such similar but nonsuperposable forms are said to be enantiomorphous (evavrios, opposite, and µop¢$, form), and crystals showing such forms sometimes rotate the plane of polarization of plane-polarized light . Faces of a pentagonal icositetrahedron with high indices have been very rarely observed on crystals of cuprite, potassium chloride and ammonium chloride, but none of these are circular polarizing .