BIPYRAMIDAL CLASS (Parallel-faced hemihedral.) The elements of symmetry are a tetrad axis with a plane per pendicular to it, and a centre of symmetry. The simple forms are the same here as in the holosymmetric class, except the prism { hko}, which has only four faces, and the bipyramid { hkl } which has eight faces and is distinguished as a "tetragonal pyramid of the third order." Fig. 53 shows a combination of a tetrag onal prism of the first order with a tetragonal bipyramid of the third order and the basal pinacoid, and represents a crystal of fergusonite. Scheelite (q.v.), scapolite (q.v.), and erythrite also crystallize in this class.