A Considerable number of species of Tetrabelodont Mastodons are already known from various horizons and localities in the United States. Several of these from the Loup Fork deposits have been described by Cope, the best known being Tetrabelodon euhyphodon, T. campester, and T. productus. A fairly complete mandible recently acquired by the British Museum seems to indicate that yet another very distinct form existed contemporaneously with those just mentioned. This specimen, which forms the subject of the present note, was obtained from Mr. Sternberg, the well – known collector of fossil Vertebrates, who refers to it in his recently published work The Life of a Fossil Hunter (p. 123) under the name Trilophodon campester, Cope, from which, in spite of some similarity, I believe it to be very distinct.