DITRIGONAL

BIPYRAMIDAL  CLASS FIG . 76.—Crystal of a m a m 582 Hexagonal bipyramid of the second order, bounded by twelve faces (s in figs . 99 and 8o) ; indices {II2I} {IO2} . . Dihexagonal prism, consisting of twelve faces parallel to the hexad axis and inclined to the vertical planes of symmetry; indices {hiko} . Hexagonal prism of the first order limo), consisting of six faces parallel to the hexad axis and perpendicular to one set of three vertical planes of symmetry (m in figs . 71, 78-8o) . Hexagonal prism of the second order {10o}, consisting of six faces also parallel to the hexad axis, but perpendicular to the other set of three vertical planes of symmetry (a in fig . 78) . Basal pinacoid {000t}, consisting of a pair of parallel planes perpendicular to the hexad axis (c in figs . 71, 78-8o) . Beryl (emerald), connellite, zinc, magnesium and beryllium crystallize in this class .