Costae

Costae, the longitudinal ridges running down the outer side of a coral: they represent the continuations of the septa external to the wall or theca. They usually correspond in number to the septa, but are often less numerous. In compound corals the costae may either end off in the intermediate calcareous tissue (or coenenchyma), or the corallites (q.v.) may be united together by the costae.