This illustration of Diplodocus was prepared by the artist Mary Mason Mitchell in 1910 under the direction of Oliver Perry Hay (1846-1930). Hay was not an employee of the Smithsonian Institution, but he held the title Research Associate, and had office space at the National Museum of Natural History, Smithsonian Institution.
Hay, Oliver P. 1910. On the Manner of Locomotion of the Dinosaurs, Especially Diplodocus, with Remarks on the Origin of the Birds. Proceedings of the Washington Academy of Sciences, vol. 12, pp. 1-25.