SCALENOHEDRAL CLASS (

Bisphenoidal-hemihedral)  . Here there are only three dyad axes and two planes of symmetry, the former coinciding with the crystallographic axes and the latter bisecting the angles between the horizontal pair . The dyad axis of symmetry, which in this class coincides with the principal axis of the crystal, has certain of the characters of a tetrad axis, and is sometimes called a tetrad axis of " alternating symmetry "; a face on the upper half of the crystal if rotated through 90° about this axis and reflected across the equatorial plane falls into the position of a face on the lower half of the crystal . This kind of symmetry, with simultaneous rotation about an axis and reflection across a plane, is also called " composite symmetry." In this"class all except two of the simple forms are geometrically the same as in the holosymmetric class . Bisphenoid (orpirv, a wedge) (fig . 50) . This is a double wedge-shaped solid bounded by four equal isosceles triangles; it has the indices lilt), 12111, 11121, &c., or in general 1hhll . By suppressing either one or other set of alternate faces of the tetragonal bipyramid of the first order (fig . 42) two bisphenoids are derived, in the Fig . 53 shows a combination of a tetragonal 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 (C4Hio04) also crystallize in this class .