The present paper proposed to give for the first time a detailed account of the osteological structure of Stegosaurus, to be followed by systematic descriptions of all the type-specimens of that genus and other armored dinosaur remains contained in the United States National Museum collections. With one exception the present work is based entirely upon National Museum material, the exception being a specimen generously loaned by Mr. W. H. Reed, of the University of Wyoming, and described here as a new type species.