in natural history, the name of a genus of spars; the word is derived from the Greek ἀνομοιός, irregular, and πολύκειος, a rhomboidal figure. The bodies of this genus are pellucid crystalline spars of no determinate or regular external form, but always breaking into regularly rhomboidal masses; easily fissile, and composed of plates running both horizontally and perpendicularly thro' the masses, but cleaving more readily and evenly in an horizontal than in a perpendicular direction; the plates being ever composed of irregular arrangements of rhomboidal concretions. Of this genus there are five known species. 1. A white, bright, and shattery one; found in great quantities in the lead-mines of Derbyshire, Yorkshire, and Wales. 2. A milk-white, opake, and shattery one, found in some parts of France, and very plentifully in Germany, and sometimes in Wales and Scotland, and in the hills of Yorkshire. 3. A hard, dull, and snow-white one, found in some of the mines in Derbyshire, and in many of our northern countries. 4. A hard gray and pellucid one, found in the lead-mines of Yorkshire, and very common in Germany. And, 5. A pellucid and colourless one; this is found in the lead-mines of Derbyshire and Yorkshire. All these in some degree have their double refraction of the island crystal. See ISLAND-Crystal.