32	A PRACTICAL TREATISE ON GEMS
	as it were, cut off, and new ones produced in their place, which properly belong neither to the one form nor the other, but are edges or angles of combination. Usually, one form predominates more than the others, or has more influence on the general aspect of the crystal, and hence is distinguished as the predominant form, the others being named subordi­nate. The following terms used on this subject require ex­planation. A combination is developed when all the forms contributing to its formation are pointed out; and its sign consists of the signs of these forms, written in the order of their influence on the combination, with a point between. An angle or edge is said to be replaced when it is cut off by one or more secondary planes; it is truncated when cut by one plane, forming equal angles with the adjacent faces; and an edge is bevelled when replaced by two planes, which are equally inclined to the adjacent faces. It will be readily seen that such combinations may bo exceedingly numerous, or rather infinite; and only a few of the more common can bo noticed, simply as specimens of the class. Many others more complicated will occur in the descriptive part of this treatise. Among plenotesseral combinations, the cube, octahedron, and also the rhombic-dodecahedron, are the predoniinant forms. In fig. 16 the
	cube has its angles replaced by the faces of the octahedron, and the sign of this combination isO. In fig. 17 this process may be regarded as having proceeded still fur-