Various conflicting views relating to the phylogenetic history of the interossei are reviewed. The primitive mammalian (marsupial) precursors of these muscles are shown to present a bilaminar arrangement: a dorsal layer of four bipennate abductor muscles (inserting into a proximal phalanx) is overlaid ventrally by a sheet of ten flexores breves, grouped in pairs, and inserting as wing tendons into either side of the extensor aponeurosis of the corresponding digit. The homologues of these muscles are identified in the hands of representative Primates including Homo. The dorsal abductors become the dorsal interossei proper; the flexores breves become the palmar interossei, which are therefore frequently more numerous than the four found in man. Certain of the flexores breves show a tendency to merge with those subjacent abductors with which they insert. Thus, the descriptive human dorsal interossei are composite muscles resulting from the amalgamation of a flexor brevis with a dorsal interosseous proper. Comparative morphology is shown to provide a logical basis for the understanding of the extensor apparatus of the human fingers.