39                    A PRACTICAL TBEATISE ON GEMS. and in the third they lie in an intermediate position, or divide these edges unequally ; the latter being hemihedral forms. These pyramids are also distinguished as obtuse (fig. 25) or acute (fig. 26), according as the vertical angle is greater or less than in the octahedron, which, though intermediate, is never a tetragonal form. (2.) Ditetragonal pyramids (fig. 11) are bounded by sixteen scalene triangles,
	whose base lines are all in one plane. This form rarely occurs except in combinations. (3.) Tetragonal sphenoids (fig. 28), bounded by four isosceles triangles, are the hemi­hedral forms of the first variety of tetragonal pyramids. (4.) The tetragonal scalenohedron (fig. 29), bounded by eight scalene triangles, whose bases rise and fall in a zig-zag line, is the hemihedral form of the ditetragonal pyramid. The latter two forms are rare. Open forms.—Tetragonal prisms (fig. 30) bounded by four planes parallel to the principal axis; ditetragonal prisms by eight similar planes. In these prisms the prin­cipal axis is supposed to be prolonged infinitely, or to be unbounded. Where it is very short and the lateral axes