OSTEOLOGY OF THE ARMORED D1N0SAURIA. 



63 



Ribs. 



a 



Fig. 30. — Chevron of STEGOSAU- 

 RUS UNGULATUS HARSH. J NAT. 



size, a, Posterior view, b, 

 Front view. After Marsh. 



In Stegosaurus there are present cervical, dorsal, sacral, and caudal ribs. 

 Excepting the atlas, all of the other presacral vertebrae bear doublo-headed ribs. 



Cervical ribs. — With specimen No. 4934 there are eight cervical ribs present. 

 Four of these belonging to the left side were found articulated, as shown in figure 16. 

 They pertain to atlas, axis, fifth, and sixth cervicals, respectively. The remaining 

 ribs are from the right side, and, excepting the one for the atlas, the others are 

 disarticulated. There is one crushed against the proc- 

 esses of the twelfth vertebra, counting back from the 

 skull, but it can hardly belong to this vertebra. 



The ribs of the atlas and axis are much damaged, but 

 enough of these bones are preserved to show that their 

 arrangement was very similar to that of the living croco- 

 dile. They appear to verify the tentative arrangement 

 of these bones in Diplodocus by Holland. 1 



The ribs for the atlas are single headed and articu- 

 lated with the facets on the lower posterior side of the 

 intercentrum. This articular end has a width of 18 mm. 

 The rib for the axis articulates chiefly with the para- 

 pophysis and with the diapophysis by a cartilaginous 

 union, as in the crocodile. Both appear to have been long, tapering structures, 

 but of their exact shape little can be determined from the available specimens. 



The sixth rib (fig. 31) is a tri-radiate bone formed by the short and stout tuber- 

 cular and capitular processes with a pointed posterior branch, which is triangular 

 in cross-section. Anteriorly the indentation between the articular process is mod- 

 erately deep. On the external side, directly below the tubercular process, the 



ventral surface is pi-oduced outward, forming a 

 thin shelf of bone which curves slightly upward 

 and disappears toward the posterior end. This 

 shelf is not present on the posterior ribs of the 

 neck as shown by specimen No. 7616. The pos- 

 terior cervical ribs are longer and the articular 

 processes are lengthened, especially the tuber- 

 cular. The articular end of the capitular proc- 

 ess in these posterior ribs is much lengthened 

 dorso-ventrally, but compressed antero-poste- 

 riorly. The sixth rib has a length of 95 mm. 

 Dorsal ribs. — All of the dorsal vertebrae 

 bear double-headed ribs, and excepting those of the dorso-sacral all the others are 

 free. From a study of specimen No. 4934 it would appear that there were at least 

 15, and possibly one or two more,vertebrae carrying thoracic ribs, but of the exact 

 number I am now in doubt. 



With the above skeleton there are portions of 25 ribs, most of which are prac- 

 tically entire. Thirteen of these pertain to the left and the remainder to the right 



Fig. 31. — Sixth cervical rib, left side of 

 Stegosaurus stenops Marsh. Type. Cat. 

 no. 4934, U.S.N.M. I nat. size. Internal 



VIEW. C, CAPITULUM; I, TUBERCULUM. 



i Mem. Camegie Mus., vol. 2, No. 6, 1909, pp. 249-250, figs. 2-22. 



