MARSH.) 



APATOSAUKUS. 



167 



This enlargement of the neural cord in the sacral region exists to 

 some degree in reptiles and birds now living, but does not approach that 

 found in the Sauropoda, or especially that in the Stegosauria, where, 

 as will be shown later in the present article, this expansion reaches its 

 maximum, and its functional importance must make it a dominant fac- 

 tor in the movements of the reptiles in which it is so highly developed. 

 This great development has been found only in extinct reptiles in which 

 the brain was especially diminutive, and the relation of the two nervous 

 centers to each other offers a most interesting problem to physiologists. 



THE VKRTEBKj:. 



In PI. XVIII, fig. 1, is shown a posterior cervical vertebra of Apato- 

 saurus, and in fig. 2 of the same plate a dorsal vertebra is also repre- 

 sented, both being typical of the family Atlantosauridse. The cervical 



7 8 



Fig. 7. — Cervical rib of Apatosaurus ajax Marsh; outer view. 

 Fig. 8.— The same rib; inner view. 



Eoth figures are one-eighth natural size, a, anterior extremity; h, head; r. posterior process: 

 f, tubercle. 



vertebra, seen from behind, shows the deep, transverse cup of the poste- 

 rior articular end of the centrum, as well as the coossified cervical ribs, 

 both typical of the Sauropoda. A cervical rib of one species is shown 

 in figs. 7 and 8. ' 



The dorsal vertebra, seen from in front, presents the convex anterior 

 ball of the centrum, and also the massive neural arch of the vertebra, 

 with its elevated metapophyses, constituting a neural spine. The 

 expanded diapophyses, or transverse processes, are especially note- 

 worthy, as they aid in supporting the massive ribs, their extremities 

 articulating with the tubercle of the rib, while the head is supported 

 at the base of the arch by a sessile facet representing the parapoph- 

 ysis of the cervicals. The small neural canal in each vertebra is also 

 an interesting feature, especially when contrasted with the expanded 

 cavity in the sa«rum shown in fig. 3 of the same plate. 



