Attention is called to rich coral banks and patches in the North Atlantic Ocean and along the coast of Norway at depths generally below 300 feet and in water temperatures down to 4°C. The framework of these banks is supplied by a few species of corals and hydrozoans. The associated fauna consists of nearly 200 species of invertebrates of which more than 100 are potential fossils. Considered as geological bodies these banks and patches form "organic reefs" (bioherms, biostromes) up to 200 feet thick and up to 1½ square miles large. It is suggested that environmental interpretation of ancient biohermal and biostromal deposits may be improved by paying close attention to the known facts of the distribution of calcareous algae and of hermatypic and ahermatypic coral and to the proportions in which these groups of organisms are associated. A few randomly selected examples of fossil "reefs" and bioherms are discussed in the light of the foregoing observations.