(Holohedral; Ditrigonal scalenohedral).
In this class, which presents the commonest type of symmetry of the
hexagonal system, the triad axis is associated with three similar
planes of symmetry inclined to one another at 60 deg. and intersecting
in the triad axis; there are also three similar dyad axes, each
perpendicular to a plane of symmetry, and a centre of symmetry. The
seven simple forms are:--
[Illustration: FIG. 66.]
[Illustration: FIG. 67.