STRUCTURE AND RELATIONSHIPS OF OPISTHOCELIAN DINOSAURS. 193 
PHB SAB VAS: 
The pelvic bones are well preserved, except the left ilium, which 
is waterworn. On account of its absence the pelvis is figured from 
the right side in Plate L, and the same parts reversed are used in 
Plate ti. The codéssification of the pubis and ischium on the 
right side establishes beyond question the correct relation between 
these bones. Their distal ends are more divergent than Marsh’s figure 
indicates. This, with the more forward inclination of the whole 
elvis, as evidenced by the downward curvature of the presacral ver- 
tebre, directs the ischium more backward and the pubis somewhat 
more downward than formerly figured in this genus. In fact, these 
relations approach very closely to those indicated by Marsh’s figure 
of Atlantosaurus.* 
The. zum is one-fourth larger than that figured in Marsh’s 
restoration, and shows the character of the crest which was want- 
ing in that specimen. It articulates mesially with the five coalesced 
sacral ribs and the corresponding diapophyses. The articulating 
surface for the ribs is a crescentic projection of the mesial surface 
just above the acetabulum. The greater and lesser peduncles articu- 
late below with the proximal ends of the pubis and ischium, 
respectively. 
The pudes are a pair of massive bones, broad in the proximal half 
and expanded into irregular, rugose knobs distally. They articulate 
proximally with the great peduncles of the ilium by broad cartilagi- 
nous connections, laterally with the ischia by their thin posterior 
margins, which extend downward from the acetabulum, and distally 
with one another by a cartilaginous union at the median line. The 
articulation with the ischia may in advanced age give place to coéssifi- 
cation, as has occurred in the right side of the pelvis under considera- 
tion. Anterior to the ischio-pubic articulation there is a rugose 
thickening of the pubic border evidently for union with its fellow at 
the median line. It is probable that the pubes were connected by 
cartilage from the ischio-pubic articulation to the distal end. The 
pubic foramen opens downward just below the anterior border of the 
acetabulum. (Plate L.) 7 
The zschza are much slenderer than the pubes. Their proximal 
ends are expanded into broad, articular surfaces, which connect superi- 
orly with the lesser peduncles of the ilia, and inferiorly with the pubes 
by the anterior margin of the blades. They form the postero-inferior 
boundary of the acetabulum. Theshaft curves outward and backward 
* Dinosaurs of North America, Pl. xvi. 
