An unusual monochorionic placenta with heterosexual twins is described. One half of the placenta presented features which are found only with monochorionic placentas. The findings in the other half were consistent with monochorionic placentation in part, but a membranous ridge (containing layers of amnion and chorion on histologic examination) at the base of the dividing septum was highly suggestive of dichorionic placentation. It is suggested that this anomaly arose as a result of incomplete disintegration of the chorionic layers of the dividing septum during development.