ABSTRACTDyslocosaurus polyonychius, a new genus and species of diplodocid sauropods from eastern Wyoming, differs from other Diplodocidae in having at least four, and possibly five, unguals on the pes. The specimen is recorded as collected from the Lance Formation, which is consistent with the scant locality data, but the preservation and the known distribution of the family Diplodocidae suggest derivation from the Morrison Formation.