492 CALC GENUS. 3. Three-fided Pyramid. 1. Simple three-fided pyramid, whofe fummit angle is of all degrees of magnitude, from obtufe to acute. 2. If the angles of the preceding figure are f6 deeply truncated that the angles of the trun cating planes meet each other, an oaaedron is formed. 3. The pyramid is often double, in which cafe the lateral planes of the one pyramid are fet on the lateral edges of the other. It prefents the following varieties. a. Flat double fix-fided pyramid, which has fometimes convex lateral planes. b■ If a number of the!e flat or obtufe pyramids are piled on one another, there is formed a fix-fided prifm acuminated by three planes, which are fet on the alternate and alter nating lateral planes. c. When this pyramid becomes very obtufe it gives rife to the lens. d. When the fummits of the pyramid become lefs obtufe, and approach to right angles, a figure differing but little from the cube is formed. s- When the fummits become ftill more acute, an acute double three-fided pyramid is formed. /. Jhc