in natural history, the name of a large class of fossils, the characters of which are these: they are bodies composed of slender and scarce visible filaments, arranged into fine, even, and thin flakes; and those disposed into regular figures, in the several different genera, approaching to a rhomboide, or hexangular column, or a rectangled parallelogram; fissile, like the talcs, but they not only lie in a horizontal, but also in a perpendicular direction; they are flexible in a small degree, but not at all elastic; they do not ferment with acid menstrua, but readily calcine in the fire. Of this class there are seven orders of bodies, and under those ten genera. The selenites of the first order are those composed of horizontal plates, and approaching to a rhomboidal form; of the second are those composed of horizontal plates, arranged into a columnar and angular form: of the third are those whose filaments are scarcely visibly arranged into plates, but which, in the whole mass, appear rather of a flinted than of a tubulated structure: of the fourth are those which are flat, but of no determinately angular figure: of the fifth are those formed of plates, perpendicularly arranged: of the sixth are those formed of congeries of plates, arranged into the figure of a star; and of the seventh are those of a complex and indeterminate figure.

Of the first of these orders there are three genera. 1. The leptodecarbomites. 2. The pachydecarbomites. 3. The tetradecarbomites. Of the second order there are also three genera. 1. The ichthamblucers. 2. The lamblucers. 3. The oxucia. Of the third order there is only one known genus, the inambucia. Of the fourth order there is also only one known genus, the sanidia. Of the fifth order there is also only one known genus, the calotolpites. Of the fifth order, there are two genera. 1. The lepofra. 2. The trichofra. Of the seventh order there is only one genus, the symplexia.

The structure of the selenites of all the genera of the first order is exactly alike; they are all composed of a great number of broad flakes or plates, in a great measure externally resembling the flakes of the foliaceous talcs: these are of the length and breadth of the whole mass; the top and bottom being each only one such plate, and those between them, in like manner, each complete and single; and the body may always be easily and evenly split, according to the direction of these flakes. These differ, however, extremely from the talcs, for they are each composed of a number of parallel threads or filaments, which are usually disposed parallelly to the sides of the body, though sometimes parallelly to its ends. In many of the species they are also divided by parallel lines, placed at a considerable distance from each other, and the plates in splitting often break at these lines; add to this, that they are not elastic, and that they readily calcine. The structure of those of the second order is the same with that of the first; but that in many of the specimens of them the filaments of which the plates are composed run in two directions, and meet in an obtuse angle; and in the middle there is generally seen in this case a straight line running the whole length of the column and small parcels of clay infusing themselves into this crack, representing it the figure of an ear of grain so naturally, as to have deceived many into a belief that there was really an ear of grain there. The other orders consisting only of single genera, the structure of each is explained under the generic name.

in chemistry, called also gypsum spatulatum, a species of gypsum or plaster of Paris. See GYPSUM.