(Parallel-faced hemihedral).
The elements of symmetry are a tetrad axis with a plane perpendicular
to it, and a centre of symmetry. The simple forms are the same here as
in the holosymmetric class, except the prism {hko}, which has only
four faces, and the bipyramid {hkl}, which has eight faces and is
distinguished as a "tetragonal pyramid of the third order."
[Illustration: FIG. 52.--Crystal of Chalcopyrite.]
[Illustration: FIG. 53.--Crystal of Fergusonite.]
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 (C4H10O4) also crystallize in this class.