Detailed examination of the remains of fossil mammals from the fluvio-marine beds (Upper Eocene) of the Fayûm has, in several cases, shown that the number of species of the various genera has been underestimated. This is especially the case with Palœmastodon, of which originally only one species, P. beadnelli, was recognised. Subsequently another form, P. minus, was named, and now it has become necessary to establish at least two other species, to one of which probably by far the greater number of the specimens collected must be referred.