SELENITÆ, MOON STONE, in natural history, a class of fossils, naturally and essentially simple; not inflammable nor soluble in water; composed of slender filaments, ranged into fine and even thin flakes, and those disposed into regular figures, in the different genera, approaching to a rhomboide, a hexagonal column, or a rectangled inequilateral parallelogram; fissile like the talcs, but that not only horizontally, but perpendicularly also; flexible in a small degree, but not at all elastic; not fermenting with acid menstrua, and readily calcining in the fire.