40	A PRACTICAL TREATISE ON GEMS.
	hemihedral, and tetartohedral,—the last forms with only a fourth part of their faces developed. The tetartohedral and many of the hemihedral forms are of rare occurrence, and only a few of the more common require to be here described. The hexagonal pyramids (figs. 36, 37) are bounded by twelve isosceles triangles, and are of three kinds, according
	as the lateral axes fall in the angles, in the middle of the lateral edges, or in another point of these edges, the latter being hemihedral forms. They are also classed as acute or obtuse, but without any very precise limits. The trigo­nal pyramid is bounded by six triangles, and may be viewed as the hemihedral form of the hexagonal. The dihexago-nal pyramid is bounded by twenty-four scalene triangles, but has never been observed alone, and rarely even in com­binations. The more common prisms are the hexagonal of six sides, and the dihexagonal of twelve sides. As the fundamental form of this system, a particular pyr­amid Ρ is chosen, and its dimensions determined either from the proportion of the lateral to the principal axis (1 : a), or from the measurement of its angles. From this form (niP). others are derived exactly as in the tetragonal