(Holohedral; Dihexagonal bipyramidal).
Intersecting in the hexad axis are six planes of symmetry of two
kinds, and perpendicular to them is an equatorial plane of symmetry.
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 triangles (fig.
77; v in fig. 80). The indices are {213'1}, &c., or in general {hikl}.
This form may be considered as a combination of two scalenohedra, a
direct and an inverse.
[Illustration: FIG. 78. FIG. 79. FIG. 80.