MICA, in natural history; a genus of in sects of the Ventres Testacea class and order. Animal ascidia ; shell bivalve, nerally gaping at one end; hinge with broad thick strong teeth, seldom more than one, and not inserted into the op posite valve. The animals of this genus perforate into the sand and clay at the bottom of the sea, burying themselves and their shells wholly or in part. There are about twenty-five species. M. de clivis has a brittle, semi-transparent shell, sloping downwards near the open end ; the hinge slightly prominent. It is found about the Hebrides, and the animal is in great esteem among the inhabitants. M. margaritifera inhabits most parts of the arctic circle, and is generally found in mountainous rivers, and about cataracts. It is about five inches long, and half as many broad. The shell is often corroded with worms ; it is noted for producing large quantities of mother of pearl and pearl; the latter is said to be a disease of the fish analogous to the stone in the hu man body. The river Conway, in Wales, was formerly famous for producing pearl of great size and value. M. dubia, shell with an oval and large hiatus, opposite the hinge, and the rudiment of a tooth within one valve. It is found near Wey
mouth; the shell is brittle, about the length of a horse-bean, and shaped like a pistachia-nut.
blYAGRUM, in botany, gold of plea sure, a genus of the Tetradynamia Silicu loss class and order. Natural order of Siliquosx, or Cruciformes. Crucifers, *Woolen. Essential character: silicle ter minated by a conical style, with a cell commonly one-seeded. There are ten species, of which M. perenne, perennial gold of pleasure. M r. Miller describes this as an annual plant, notwithstanding he gives it Linmens's epithet of perenne; the lower leaves are large, jagged, and hairy ; the stalks branching out from the bottom ; leaves about four inches long, and two broad; the stalks terminated by very long loose spikes of yellow flowers, succeeded by short pods with two joints, each including one roundish seed. Lin nails remarks that the lower joint of the silicle is strict and abortive ; the upper globular, striated, one-seeded. Native of Germany.