170 



DINOSAURS OF NORTH AMERICA. 



articulation. This is shown in fig. 3. In the vertebra ngured, at the 

 base of the neural spine, there is a strong anterior projection, which 

 was inserted into the cavity between and above the posterior zyga- 

 pophyses of the vertebra in front. There appear to be no true lumbar 

 vertebrae, as those near the sacrum supported free ribs of moderate 

 size. The vertebrae in this region have both faces of the centrum nearly 

 flat or biconcave. An anterior dorsal rib is shown below. 



THE SACRUM. 



The sacrum in the present species consists of five well-coossified ver- 

 tebrae, and in the type specimen the centrum of the last lumbar is 

 firmly united with it, as shown in PL XXIII. The striking feature 



Fig. 9. — Proximal end of rib of Brontoiaurui ezcelsns Marsh; front view. 



Fig. 10. — The same bone; back view. 



Fig. 11. — The same; superior view. 



All the figures are one-eighth natural size. c. cavity ; h, head ; (. tubercle. 



about this sacrum is the large general cavity it contained. This was 

 divided in part by a median longitudinal partition, as shown in 

 PL XXIII, fig. 2. The septum, however, was not continuous the 

 whole length of the sacrum, so that the two lateral cavities were vir- 

 tually one. This extended even into the lateral processes. The trans- 

 verse partitions formed by the ends of the respective centra were also 

 perforate, so that the sacrum proper was essentially a hollow cylinder. 

 The cavernous character of the sacrum is one of the peculiar features 

 of the suborder Sauropoda, and was described by the writer when 

 the first species of this group was discovered in this country. The 



