STRUCTURE AND RELATIONSHIPS OF OPISTHOCCELIAN DINOSAURS. 235 
the composition of the sacricostal yoke and may be regarded as com- 
prising the primary sacrum. ‘The dorso-sacral is highly specialized, 
but its support of the acetabulum is a secondary function. The 
_ caudo-sacral is functional as an ilium-supporting vertebra, though 
its caudal affinities are still clearly marked. 
The centrum of sacral 1 is somewhat shorter than that of the last 
dorsal, and is irregularly convex on the anterior end. The pleuran- 
tral foramina are almost closed by the expanded sacral ribs. The 
centrum of sacral 11 is similar in length to that of 1, but is laterally 
expanded to meet the unusual development of its sacral rib. There 
is no evidence of a pleurantrum. The centra of 111 and Iv are less 
expanded laterally, and bear traces of the pleurantral foramen. 
That of v bears no trace of the lateral cavities. Its posterior end 
may have. been convex above and concave below, as is the typical 
structure, although this feature cannot be determined with cer- 
tainty. 
The sacral rib in 1 is developed into a broad, winglike appendage, 
whose primary elements can scarcely be traced. It arises from the 
upper half of the centrum anterior to the much-reduced pleurantrum 
and from the lateral surface of the neural arch. The capitular por- 
tion passes backward, joins the rib of sacral 11, and articulates with 
the mesial surface of the greater peduncle. The tubercular element 
is fused with the distal end of the diapophyses so that it is impossible 
to determine where the one ends and the other begins. It is probable, 
however, that the tubercular portion of the rib is represented by the 
rugose end only. It attaches to the crest of the ilium above and in 
front of the base of the greater peduncle. 
The second pair of sacral ribs are by far the. strongest of the 
series. The proximal ends are greatly expanded and attach to the 
whole lateral surface of centrum 11 as well as to the posterior half of 
centrum 1. The shaft is constricted at the middle, and the distal 
end is expanded to enter into the composition of the  sacricostal 
yoke. The third pair of ribs are much reduced in size. They arise 
from the anterior half of centrum 111 in common with the posterior 
margin of the second pair. The shaft is slight, but the distal end is 
expanded to form the middle section of the yoke. The fourth pair 
arise from the anterior two-thirds of the centrum. From the 
superior margins of ribs 11, 111, and Iv broad plates arise to connect 
with the corresponding diapophyses. The fifth pair of sacral ribs 
arise from the mid-lateral surface of the centrum, and passing diago- 
nally forward, unite with the distal ends of the fourth pair in sup- 
porting the lesser peduncle of the ilium. These bear traces of the 
