It is characteristic of the Macaca mulatta to have a bidiscoid placenta. Higher apes and human beings most often have a single discoid placenta. It is shown that placentation atypical for the species may occur in all primates, including human beings. Furthermore, it was demonstrated that atrophic chorionic villi remnants are present in the fetal membranes of placenta duplex of either human or macaque origin. These remnants occur only in decidua capsularis. Therefore, it is suspected that all primates, at least those with villous placentas, possess a decidua capsularis.