HOLOSYMMETRIC CLASS (Holohedra1; Dihexagonal-bipyramidal.) Intersecting in the hexad axis are six planes of symmetry of two kinds, and perpendicular to them is an equatorial plane of sym metry. Perpendicular to the hexad axis are six dyad axes of two kinds and each perpendicular to a vertical plane of symmetry. The seven simple forms are: Dihexagonal bipyramid, bounded by twenty-four scalene tri angles (fig. 77; v in fig. 8o) . The indices are { etc., or in general . This form may be considered as a combination of two scalenohedra, a direct and an inverse.
Hexagonal bipyramid of the first order, bounded by twelve isosceles triangles (fig. 70 ; p and a in fig. 8o) ; indices { 10 f1 } , { 20 2 i } • • • { hohl } . The hexagonal bipyramid so common in quartz is geometrically similar to this form, but it really is a combination of two rhombohedra, a direct and an inverse, the faces of which differ in surface characters and often also in size.
Hexagonal bipyramid of the second order, bounded by twelve faces (s in figs. 79 and 8o) ; indices { I I 2 I } , { I I 2 2 } . . . { h.h.2h.l1 .
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 { 'of o } , consisting of six faces parallel to the hexad axis and perpendicular to one set of three vertical planes of symmetry (m in figs. 71, 78-80).
Hexagonal prism of the second order { I I So}, 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 { 000i } , consisting of a pair of parallel planes perpendicular to the hexad axis (c in figs. 71, 78-80).
Beryl (emerald), connellite, zinc, magnesium and beryllium I crystallize in this class.