* 49° ' CALC GENUS. mon bafis form a zigzag line, and the more obtufe lateral edges of the one are oppofed to the lefs obtufe lateral edges of the other pyra mid. Of this figure the following varieties occur : a. The extremity of the pyramid is fometimes more or lefs deeply and flatly acuminated by three fomewhat convex planes, which are fet on the more obtufe lateral edges ? b. T he angles of the common bafe are often more or lefs deeply truncated. When the truncating planes become fo large that they touch one another, the tranfition into the fix-fided prifm is formed. c. The lefs obtufe edges are fometimes bevilled, and the extremities fometimes more or lefs deeply truncated. d. If two double fix-fided pyramids penetrate one another in the direction of their axis, and one of them is turned around a fixth of its periphery, fo that the lefs obtufe la teral edges of the one cryftal come to be oppofed to the lefs obtufe lateral edges of the other, the refult is, a twin cryftal, re- prefenting a double fix-fided pyramid hav- ing three alternate re-entering angles at the common bafe, where the more obtufe la* t?ral edges are oppofed to one another. 2. Sis.-