56 



BULLETIN 89, UNITED STATES NATIONAL MUSEUM. 



The upper surfaces of the diapophyses of the sacrals have an unusual develop- 

 ment antero-posteriorly in Stegosaurus inasmuch as they coalesce into a continuous 

 sheet of hone which roofs over the entire area between the spinous processes and 

 the ilium with which their outer ends articulate and in adults are usually found 

 firmly coossified. This roof of bone is on a level with the superior flattened surface 

 of the ilium. The diapophyses of all the sacral vertebrae send downward thin 



vertical lamina that unite suturally 

 with the upper border of the sacral 

 ribs of their respective vertebrae, 

 thus forming bony partitions which 

 separate the large sacral foramina. 

 Internally these foramina are 

 bounded by the sacral centra and 

 externally by the expanded coa- 

 lesced distal extremities of the sacral 

 ribs, together with the internal 

 surfaces of the ilia. Unlike the 

 Sauropoda, there appear to be no 

 openings leading through these par- 

 titions from one foramen to the 

 other. 



The neural spines of all the ver- 

 tebrae giving support to the pelvic 

 arch are comparatively short, with 

 expanded summits. Their height 

 above the level of the ilium in S. 

 stenops is fully one-fourth less than 

 in S. ungulatus, as figured by Marsh. 

 The spines of the dorso-sacral and 

 three sacral vertebrae are coalesced 

 and form an elongated bony plate, 

 there being a single elongated aper- 

 ture between the spines of S 2 and S 3 

 (pi. 4). In S. ungulatus the spines 

 of the three sacrals are apparently 

 the only ones to thus become con- 

 joined. The direction of the spines 

 is almost straight upward, there 

 being a slight inclination backward in" the spine of the last sacral. 



The unusual development of the neural canal is well shown in plate 19, lower fig- 

 ure c. of specimen No. 7386. This large cavity appears to lie wholly within the neural 

 processes of the sacrals, and in this specimen has a greatest transverse diameter of 123 

 mm. The internal surface is comparatively smooth with the widest and deepest point 

 toward the anterior end. The large exits for the nerves leave the chamber through 

 vertically elongated openings leading into the sacral foramina (see fig. 24, e, e, e). 



Fig. 24. — Cast of neural cavitt in sacbum of Stegosaurus 

 ungulatus mabsh. all figs. } nat. size. 1, side view. 2, 

 Seen from abote. 3, Outlines representing teansverse sec- 

 tions THROUGH BRAIN AND SACRAL CAVITIES, CZ, ANTERIOR END; 

 b, BRAIN; e, e, e, FORAMINA BETWEEN SACRAL VERTEBRAE; p.EXIT 

 OF NEURAL CANAL IN LAST SACBAL VERTEBRA: S, SACRAL CAVITT. 

 ALL AFTER MABSH. 



