a. 半面的, 完成一半的

a. 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.