Complete Installation of the Diplodocus Longus in National Museum, 1931, Smithsonian Institution Archives, Dinosaur Diplodocus longus as originally installed in the "Hall of Extinct Monsters" of the United States National Museum (now the National Museum of Natural History). The bones comprising the dinosaur were collected mostly in the Dinosaur National Monument in Utah in 1923, and over the next seven years were prepared and mounted primarily by Norman Boss and Thomas Horne, under the scientific direction of Charles Gilmore. This image was taken shortly after the dinosaur was installed in the exhibit hall in 1931..