OSTEOLOGY OF THE ARMOBED DINOSAUKIA. 57 



These He below the level of the diapophyses and between the lower portions of the 

 neural arches. 



In describing the nervous system of 8. ungulatus, Lull ' makes the following 

 interesting comments on the enlarged neural cavity in the sacrum: 



The neural canal in the sacrum is of startling dimensions, having a maximum enlargement of 1.14 

 cm. and a greatest width of 0.95 cm. and displaces nearly 1,200 cc. of water, thus giving it a mass more 

 than 20 times that of the brain. 



The brachial enlargement was the seat of innervation of the powerful fore limbs, while that, of the 

 sacrum was mainly the reflex and coordinating center for the control of the mighty muscles of the hind 

 limbs, but more especially of the powerful, active, and aggressive tail, which constituted the principal 

 means of defense. 



Marsh says : 



It is an interesting fact that in young individuals of Stegosaurus the sacral cavity is proportionately 

 larger than in adults, which corresponds to a well-known law of brain growth. 



Measurements of sacral vertebrae of Stegosaurus stenops. No. 4934. 



mm. 



Total length of three sacral centra 215 



Greatest expanse of transverse process of first sacral 145 



Greatest expanse of transverse process of last sacral .. 255 



Height of neural spine above zygapophyses, about '. 155 



Length of coossified spines (4 vertebrae) 312 



Total length of 5 coossified centra 272 



Caudo-sacral (cs.). — In specimen No. 4934 the vertebra designated as the first 

 or caudo-sacral is analogous to the vertebra in the 8. ungulatus sacrum which Marsh 

 considered the last" or fourth sacral (fig. 22). That it is a modified caudal appears 

 to be indicated by the low arch; the strong backward slant of the spinous process 

 which is separate from the coossified processes of the sacrals, the noncoossification 

 of the centrum with the sacrum, and the distinctness and manner of articulation of 

 the rib with the ilium. 



The rib of the caudo-sacral apparently has an origin similar to those of the 

 true sacrals. It is extended distally, forming a vertical plate of bone with expanded 

 ends. The inner or proximal end unites by suture with the side of the arch and the 

 body of the centrum, while the outer or distal end abuts against the inner, postero- 

 superior border of the iliuin, posterior to the inner end of the articular surface for 

 the ischium. The proximal articular end is confined entirely to the centrum of 

 the vertebra and does not meet the rib of the last sacral as in S. ungulatus. Like- 

 wise the distal end does not, and never could, in this specimen at least, enter into 

 the formation of the sacricostal yoke. This end is entirely distinct from those of the 

 true sacrals and its lower border is considerably above the acetabular border, whereas 

 in S. ungulatus (fig. 22), it is firmly coossified with the rib of sacral three and with 

 the other ribs of the sacrum enters subequally into the formation of the yoke. 

 (Compare figs. 22 and 23.) 



The centrum of this vertebra in S. stenops is distinguished by its broadly rounded 

 inferior surface, as compared with the keeled centra of the sacrals and the blunt- 

 wedge-shaped centra of the succeeding caudals when viewed from the side. 



i Amer. Journ. Sci., vol. 30, 1910, p. 372, fig. 9. 



