The scleractinian coral Actinacis is an important component of reef communities from the Oligocene Lessini Shelf of Northern Italy. Actinacis occurs as massive-laminar colonies or thin to thickly branched thickets. In the present study we have used a combination od discriminant analysis and principal component analysis of corallite measurements to show that these two distinct growth forms cannot be distinguished on the basis of corallite characteristics. Thus, the two growth forms represent a single species, A.rollei, with a colony shape that varied as an autoecological response to the differeing environmental conditions that prevailed during deposition of the two facies.