O. C. Marsh — Skull and Armor of Stegosaurus. 417 



The large number of specimens of the Stegosauria now 

 known from the American Jurassic, and the fine preservation 

 of some of the remains, enable us to form a more accurate 

 estimate of the relations of the group to the other Dinosaurs, 

 than has hitherto been possible. The presence of a predentary 

 bone, and the well-developed post-pubis, are important charac- 

 ters that point to the Ornithopoda as near allies, with a com- 

 mon ancestry. These positive characters are supplemented by 

 some points in the structure of the skull, and the form of the 

 teeth. 



There are, however, a large number of characters in which 

 the Stegosauria differ from the Ornithopoda, and among these 

 are the following: 



(1) All the bones of the skeleton are solid. 



(2) The vertebrae are all biconcave. 



(3) All the known forms have a strong dermal armor. 



(4) The second row of carpals and tarsals are unossified. 



(5) The astragalus is coossified with the tibia. 



(6) The spinal cord was greatly enlarged in the sacral region. 



The relations of these two groups to each other and to the 

 rest of the known Dinosauria will be fully discussed by the 

 writer in his monograph on the Stegosauria. 



New Haven, Conn., October 24, 1887. 



