BIPYRAMIDAL CLASS (Parallel-faced aemihedral)  . Here there is a plane of .symmetry perpendicular to the hexad axis; there is also a centre of symmetry . All the closed forms are hexagonal bipyramids; the open forms are hexagonal prisms or the basal pinacoid . The general form Ihikl} is hemihedral with parallel faces with respect to the general form of the holosymmetric class . Apatite (q.v.), pyromorphite, mimetite and vanadinite possess this degree of symmetry .