OLIGÆDRA, in natural history, the name of a genus of crystals composed of very few planes, as the name expresses. 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.