The greatest volume of the very extensive literature on the myology of the mammalian forearm and hand appeared during the last thirty years of the last century, as a means of establishing phylogenetic sequences and relationships among the Mammalia. Although interest in this anatomical area has persisted to the present, the chief impetus for the more recent studies has been an interest in the structural and phylogenetic relationships between muscle groups and layers, and in the correlation of limb structure and function with ecology. Primates, insectivores, marsupials and monotremes are among the groups of mammals most thoroughly treated in the literature; the primates, because of their kinship to man; the insectivores, because of their primitiveness as the basal order of the eutherian stock, on the one hand, and their close relationship to the primates, on the other; and the marsupials and monotremes, because they, especially the opossums and monotremes, represent a primitive mammalian type and a possible link between the reptiles and mammals. The primary purpose of this investigation has been to study the structure and innervation of the flexor muscles of the forearm and hand of the opossum, Didelphis marsupialis virginiana Kerr (Hall and Kelson, 1959: 8), in sufficient detail to be able to establish the phylogenetic strctural and functional relationships between their various groups and layers. This species was selected because it is one of the most primitive and generalized living mammals and, therefore, structural and functional relationships found in it may be applicable to mammals, generally.