162 LIMESTONE FAMILY. 3. The apex flatly acuminated with six planes, which are set on the lateral planes. 4. The angles on the common base of the double py ramid truncated, thus forming a transition into the six-sided prism. 5. The acute lateral edges of the double pyramid sometimes truncated. (i. Twin-crystal. The double six-sided pyramids apparently pushed into each other, in the direction of their length, in which they are either (1) Unchanged in position, when the acute edges rest on the obtuse edges; or they are (2.) Turned around one-sixth of their periphery, so that obtuse edges are set on obtuse edges, and acute edges on acute edges; and the alternate angles on the common base have broken re-en tering angles. >7. Aggregated in single-rows, so that there is formed either an acute double, or a single four-sided py ramid. II. Six-sided Prism. It is equiangular, but generally with alternate broad and narrow lateral planes. It originates from the pyra mid N° 4.; and hence it presents the following varieties: 1. The six-sided prism, acutely acuminated witli six planes, of which two and two meet under obtuse angles, and each is set obliquely on the lateral edges. 2. The preceding figure, in which the six-pluned acu- mination is again flatly acuminated with three planes,
