Amphicoelias

Amphicoelias was a genus of sauropod dinosaur,
The fossil was found in the Morrison Formation. It lived about 150 million years ago during the Upper Jurassic of what is now Colorado. 
A herbivore, "Amphicoelias" was moderately sized at about 25 m (82 ft) long–roughly the same length as "Diplodocus", to which it was related.