the fundamental pyramid, and then has its edges of combination replaced by a ditetragonal pyramid, and its sign is here 
In fig. 34 the polar edges of the pyramid are replaced by another pyramid, its sign being P.
In fig. 35 a
hemihedric form very characteristic of copper pyrites is represented, Ρ and P' being the two sphenoids, a the basal pinacoid, and b, c, two ditetragonl pyramids.
III. The Hexagonal System.—The
essential character of this system is, that it has four axes,—three
equal lateral axes intersecting each other in one plane at 60°, and one
principal axis at right angles to them. The extremities of the
principal axis are named poles, and sections through it and one
lateral axis, normal chief sections. The plane through the lateral axes
is the basis, and from its hexagonal form gives the name to the system.
As in the last system, its forms are either closed or open : and are divided into holohedral,
