Definition: An oceanic island, often having a characteristic ring-like shape surrounding a lagoon. Atolls are formed when coral reef grows around a volcanic island that later subsides into the ocean.
Definition: Marine sediment that accumulates within shallow regions of the oceanic basin close to continents, such as the continental shelf, or continental slope
Definition: A structural quality inhering in a bearer by virtue of the bearer's being thicker or more closely packed together; pressed tightly together.
Definition: An eye that consists of a cluster of three or four pit eyes that form a single structural unit but are separated from one another by pigment layers