BIPYRAMIDAL  CLASS (Parallel-faced hemihedral) . The elements of symmetry are a tetrad

axis with a plane perpendicular 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 {hkll, which has eight faces and is distinguished as a " tetragonal pyramid of the third order." Combination of Tetragonal Prism and Basal Pinacoid . combination of a bipyramid of the first order with a ditetragonal bipyramid and the prism of the second order . Compare also figs . 87 and 88 . Examplesof substances which crystallize in thisclass are cassiterite, rutile, anatase, zircon, thorite, vesuvianite, apophyllite, phosgenite, also boron, tin, mercuric iodide .