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; file-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.
Of this class Dr Hill makes seven orders, and under these orders ten genera. The first order is the selenitæ, with horizontal plates, approaching to a rhomboidal form; the second order is the selenitæ with horizontal plates, of a columnar and angular form; the third order comprehends those selenitæ whose filaments are visibly arranged into plates, but in the whole masses appear striated, not tabulated; of the fourth order, are the flat selenitæ, of no determinately angular figure; of the fifth order are the selenitæ formed of plates perpendicularly arranged; of the sixth order are those selenitæ formed of a congeries of plates, ranged in form
of a star; and of the seventh order are those selenitæ of a complex and indeterminate figure.
This fossil is found in strata of clay, usually of the blue tough kind; we have it in many parts in England, particularly about Shotover hills in Oxfordshire; in several places of Northamptonshire, Leicestershire, and about Epsom in Surrey. In medicine, it is a very powerful astringent; and is of effect in diarrhoeas, dysenteries, and hæmorrhagies. It stands also recommended as a cosmetic. The people of Northamptonshire call it stauneh, and use it in hæmorrhagies of all kinds with success.