50 



BULLETIN 89, UNITED STATES NATIONAL MUSEUM. 



There is not a sudden development of the spinous process as in Camptosaurus, 

 but rather a gradual growth, beginning with the sharp median ridge on the cervicals 

 and changing gradually into a distinct spine This change takes place at a point 

 anterior to the transition of the parapophyses from the centrum to the side of the 

 neural arch. 



Although the type of S. stenops has the complete presacral series preserved, 

 the exact point where the change from cervical to dorsal takes place can not be 

 conclusively demonstrated. This transition part of the backbone has been badly 

 damaged by crushing, and it is impossible to determine many of the principal 



vertebral characters. 

 2 The eleventh vertebra 



(counting from the skull) 

 shows plainly that the 

 parapophysis is on the 

 centrum. On the fif- 

 teenth it is found on the 

 side of the neural arch 

 at the base of the lamina 

 which rises to support the 

 under side of the trans- 

 verse process. The posi- 

 tion of the parapophyses 

 on the three intermediate 

 vertebrae can not be de- 

 tected, but one thing is 

 certain, the transition of 

 this facet occurs between 

 the eleventh and fifteenth 

 vertebrae and probably 

 takes place on either the 

 twelfth or thirteenth. 



The vertical elonga- 

 tion of the neural arch is a 

 somewhat rapid change, 

 beginning with the posterior cervicals and continuing into the anterior dorsal region 

 (figs. 18 and 19). These vertebrae were found articulated, there being one vertebra 

 between them. In 8. stenops (No. 4934) the maximum height of the dorsals is 

 reached on the twentieth vertebra from the skull. A selection from the vertebral 

 series of one skeleton is given in figures 18, 19, and 20. 



The neural canal in figure 18 shows the brachial enlargement, and in the more 

 posterior vertebrae, figures 19 and 20, its decreased size. Unlike S. ungulatus (fig. 

 21), where the neural canal is higher than wide, in this specimen it is subcircular 

 in outline. 



The spines gradually increase in height and also in the antero-posterior diameter 

 proceeding posteriorly to the mid-dorsal region. From this point back to the 



Fig. 19. — Anterior dorsal vertebra of Stegosaurus sp. Second vertebra 

 posterior to one shown in fig. 18. cat. no. 6531, u.s.n.m. j nat. size. 

 1, Side view. 2, Front view, d, Diapophysis; n, neural canal; p, parapo- 

 physis; 5, neural spine; z, anterior zygapophtsis; z' , posterior zygapophysis. 



