OLIGÆDRA, in natural history, the name of a genus of crystals composed of very few planes, as the name expresses. The word is compounded of oligo "a few,"
Oligarchy few," and oligo "a plane." The bodies of this class are crystals of the imperfect kind; being composed of columns affixed irregularly to some solid body at one end, and the other terminated by a pyramid; but the column and pyramid being both pentangular, the whole consists only of ten planes, and not, as the common kind, of 12.