What does "hemihedral" mean?

Hemihedral: Having half of the similar parts of a crystals, instead of all; consisting of half the planes which full symmetry would require, as when a cube has planes only on half of its eight solid angles, or one plane out of a pair on each of its edges; or as in the case of a tetrahedron, which is hemihedral to an octahedron, it being contained under four of the planes of an octahedron. -- Hem`i*he"dral*ly, adv.