ATOLL (native name atollon in the Maldive Islands)  , a horse-shoe or ring shaped coral reef enclosing a lagoon . The usual shape is that of a partly submerged dish with a broken edge, forming the ring of islands, standing upon a conical pedestal: The dish is formed of coral rock and the shells of various reef-dwelling mollusca, covered, especially at the seaward edges, 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 by the death of the coral animals in the still water, and the patchy disintegration of the " hard " coral, while waves and storms tear off blocks of rock and pile them up at the margin, increasing the height of the islands, which become covered by vegetation . The lagoon entrance in the open part of the horse-shoe is always to leeward of prevailing winds, since the coral growth is there slower than where the waves constantly renew the polyps' food supply . The conical pedestal rising from the depths is frequently a submarine volcanic cone or island, though any submerged peak may be crowned by art atoll .