ATOLL (native name atollon in the Maldive Islands), a horse-shoe or ring-shaped reef of coral enclosing a lagoon. It is found in low latitudes. Its form may be likened to that of a partly submerged dish with pieces broken from its edge, the ring of islands standing upon a conical pedestal. The dish is formed of hard coral and the shells of various reef-dwelling mollusca, covered, especially at the seaward periphery, with a film of living coral polyps that continually extend the fringe, and enlarge the diameter of the atoll. The lagoon tends to deepen when the land is stationary on account of the death of the coral animals in the still water, and the disintegration of the "hard" coral of the inner ring, while waves and storms tear off blocks of rock and pile them up at the margin, increasing the height of the islands, which in course of time usually become covered by vegetation. The lagoon entrance in the open part of the horse-shoe is always to leeward of prevailing winds. See CORAL-REEFS.