126 FLINT GENUS. Its fundamental cryftallization is an equiangular three-flded prifm, and this is either pevfeft or has cylindrical convex lateral planes, and is flatly acumi nated on both extremities by three planes, which, ^ the one extremity are fet on the lateral edges, but oU the other, on the lateral planes. The angles, edg^; and extremities of the acumination are often trun cated, and the edges fometimes bevilled. Whenfc' veral of thefe alterations occur at the fame time, cryflal has a very irregular afped, and is fomew!)^ difficult to determine. The lateral edges are f>' c ' quently bevilkd, and thus is formed a nipe-fided prifm, in which three and three planes meet uud er two cbtufe angles. When the edges of the be^' ments are truncated, a twelve-fided prifm is formed; but when the bevilling planes increafe fo much th at the original faces dilappear, an equiangular fix fid e ^ prifm is formed. Prifms are generally long, thin, and needle-fhaped’ fometimes they are alfo fiiort and thick; in other vat 1 ' eties, although very rarely, the acuminations me ct together, when a double three fided pyramid is for 111 ' ed, and the remainder of the prifm forms truncation on the edges of the common bafis L The lateral planes are generally fcrongly long‘ tu ' dinally fcreaked ; the acuminating planes are modi)' * Count de Boarnon defcribes a fpeclmcn in the colle&‘ on 0 Sir John St / ubyn, in which the prifm is completely wanti’ 0 ?' and is, what he confiders to be, the primitive %nre cf the t°° r niaiinc. Lontlor* Philofophiccl Tranfaflions for jSo:. f u fnioot-i