During five months of fieldwork, mainly carried out in Banyuls-sur-Mer (Pyrénées Orientales), but also in Marseille and Roscoff (Brittany), the systematics and ecology of the madreporarian corals were studied, especially of the species. Caryophyllia smithi Stokes & Broderip, 1828, Caryophyllia clavus Scacchi, 1835, and Coenocyathus dohrni Döderlein, 1913. Observation of these animals in their own environment gave an explanation of the different growth-forms within a species, which were formerly regarded as of specific importance. I did come to the conclusion that Caryophyllia clavus is a synonym of C. smithi. The species C. smithi has two growth-forms depending on some ecological data (depth, water movement, substrate): var. smithi and var. clavus. A slow transition from the one into the other was observed. Coenocyathus dohrni is not synonymous with C. smithi (cf. ROSSI, 1957); it is a species with specific characters in skeleton, shape of the polyp, and growth into a colony.