Hoplophoneus mentalis (Sinclair), mounted skeleton. Original description at the source site: Hoplophoneus Mentalis Sinclair, antero-lateral view of mounted skeleton; mounted by John Clark, Courtesy of Colorado University. Plate 28, in U.S.Geological Survey Professional paper 221. 1950.
(published in Jean Hough: The habits and adaptation of the oligocene saber tooth carnivore, Hoplophoneus, plate 28, U.S. Geological Survey, 1950, without copyright notice)
Note that it may still be copyrighted in jurisdictions that do not apply the rule of the shorter term for US works (depending on the date of the author's death), such as Canada (50 p.m.a.), Mainland China (50 p.m.a., not Hong Kong or Macao), Germany (70 p.m.a.), Mexico (100 p.m.a.), Switzerland (70 p.m.a.), and other countries with individual treaties.