characterized by a narrow, slightly elevated ridge near the reef edge, the Lithothamnion ridge, which is lacking on the leeward side. The latter drops off vertically to depths of 100 to 200 feet on the seaward side, while the seaward slope of the windward reef falls away at an angle of about forty-five degrees. On the reef and in the lagoon,there is an abundance of colorful plant and animal life in which the keen competition between different species for space and food is very evident. On every hand there is evidence of rapid growth and simultaneous destruction. Masses of reef-building coral are competing with each other and with the coralline marine algae for space, one often overgrowing the other. fish gnaw wide scratches on the coral. Schools of green parrot- Fleshy patches of algae are pressed tightly against the surface of the coral to hold against the surges of the water pushed across the reef by the crashing breakers. urchins and clams grind niches into the hard coral, Sea some of them con- stantly feeding on the cover of bacterial and algal film which is as constantly being replaced. The clams, the corals, some small fish and other forms are ceaselessly removing from suspension in the water the small, often microscopic, plants, animals, and bits of debris which make up the plankton. In regions of quieter water, where sand has been deposited, sea cucumbers and spider snails, among the larger forms, turn the sand again and again in their gleaning for food.