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The remaining simple forms are “hemihedral with parallel faces,” and from the corresponding holohedral forms two hemihedral forms, a positive and a negative, may be derived.

They crystallize in two complementary hemihedral forms, which are respectively right-handed and left-handed, i.e. enantiomorphous forms.

By some authors these two divisions are treated as separate systems; or again the rhombohedral forms may be considered as hemihedral developments 580 of the hexagonal.

These systems, however, only represented holohedral forms, leaving the hemihedral and tetartohedral classes to be explained.

Crystals belonging to many of the hemihedral and tetartohedral classes of the six systems of crystallization are devoid of a centre of symmetry.