HOLOHEDRAL  CLASS (Holohedral;

Bipyramidal) . Here there are three dissimilar dyad axes of symmetry, each coinciding with a crystallographic axis; perpendicular to them are three dissimilar planes of symmetry; there is also a centre of symmetry . There ape seven kinds of simple forms: Bipyramid (figs . 54 and 55) . This is the general form and is bounded by eight scalene triangles ; the indices are { 111 } , 1211), {221} 11121, {3211, 11231, &c., or in general {hkl} . The crystallographic axes join opposite corners of these pyramids and in the fundamental bipyramid 111 1} the parametral plane has the intercepts a: b: c . This is the only closed form in this class; the others are open forms and can exist only in combination . Sulphur often crystallizes in simple bipyramids . Prism . This consists of four faces parallel to the vertical axis and intercepting the horizontal axes in the lengths a and b or in any multiples of these; the indices are therefore {110}, 12101, 11201 or {hko} . Macro-prism . This consists of four faces parallel to the macro- axis, and has the indices poll, {201} , .

Or {hon . Brachy-prism . This consists of four faces parallel to the brachyaxis, and has the indices loll), 10211 . {okl} . The macro- and brachy-prisms are often called " domes." Basal pinacoid, consisting of a

pail of parallel faces perpendicular to the vertical axis; the indices are loot) . The macro-pinacoid{loo} and the brachy-pinacoid {oio} each consist of a pair of parallel faces respectively parallel to the macro- and the brachy-axis . Figs . 56-58 show combinations of these six open forms, and fig . 59 a combination of the macro-pinacoid (a), brachy-pinacoid (b), a prism (m), a macro-prism (d), a brachy-prism (k),and a bipyramid (u) . Pinacoid . Hypersthene . Holohedral Orthorhombic Combinations .

Examples of substances crystallizing in this class are extremely numerous; amongst minerals are sulphur,

stibnite, cerussite, chrysoberyl, topaz, olivine, nitre, barytes, columbite and many others; and amongst artificial products iodine, potassium permanganate, potassium sulphate, benzene, barium formate, &c .