In some species of the fasciculate rugosan, Siphonodendron, the morphology of the fossula, columella and connecting processes is controlled by ecological conditions. A colony in a turbulent environment would have numerous connecting processes, a strong columella and a well-marked fossula, with a preferential orientation of the corallites if tbe water movement was in the form of a persistent current. In a quiet environment, the connecting processes are generally absent, the cardinal fossula is indistinct and the columella is weak or absent. In some species of Diphyphyllum, the morphology of the fossula and the disposition of the corallites are also controlled by ecological conditions.