PEOFESSOE OWEN ON THE DICYNODONT EEPTILIA. 
463 
passes anterior to, the iliac bone {ib. 62 ). It resembles in shape the human scapula, but 
is much thicker. The hinder and inner border of this sacral rib is thick, and smoothly 
rounded ; the front border is thinner, and is slightly concave ; the outer border appears 
to have been straight, but is somewhat mutilated. This expanded termination of the 
rib rests on the ventral side of the ilium, concealing much of that bone in a front view 
of the pelvis. It has not been anchylosed therewith ; decomposition of the ligamentous 
uniting matter, and subsequent partial dislocation, have allowed the matrix to insinuate 
itself between the sacral rib and the dium, as seen at a, fig. 2, Plate XXIII. 
Of the short, thick, transversely extended ribs of the five succeeding sacral vertebrae, 
the left of the penultimate (pZ) is the best preserved towards the pelvic cavity ; it pre- 
sents a smooth convex or rounded surface about an inch in breadth, is slightly bent 
with the concavity towards the outlet of the pelvis, and abuts against the ischium. 
The bodies of the sacral vertebrae (S 1 .... 5 ) are contracted at the middle, and slender 
there in proportion to the pelvis, but are rapidly and much expanded at their articular 
ends ; consequently they are very concave lengthwise, both below and at the sides ; but 
are smooth and convex transversely, yielding a semicircular transverse section. The 
parapophyses go off near the base of the neural arch. The first centrum (s 1 ) is nearly 
3 inches in length, and as much in breadth at the articular end, but is only I inch 3 lines 
across the middle; the rest ( 2 .... 5 ) slightly dimmish in length as they approach the tail. 
The ilium (Plates XXIII. and XXIV. 62 ) is a strong straight triangular bone, at least 
10 inches in length from the upper border of the acetabulum (Plate XXIII. fig. 2, <?) ; 
above which it is contracted to a breadth of 3^ inches and a thickness of 2 inches, and 
then expands to a breadth of 8 ^ inches, measured along its oblique anterior border or 
labrum, Plate XXIV. fig. 4, 1 1!. The front part of the anterior two-thirds of the ilium 
expands into a rough fiattened surface, 6 inches in length, and 3 inches in breadth ante- 
riorly {ib. fig. 4, r), to which the back surface of the expanded first sacral rib is liga- 
mentously attached. The inner surface of the ilium extends 6 inches behind this arti- 
culation, and is almost flat, but rather sinuous : coarse bony ridges or rays of ossification 
appear on this surface, near the labrum {i\ diverging thereto. The outer surface of 
the ilium is moderately concave transversely. This anterior expanded part of the ilium 
passes behind and in advance of the last pair of free ribs ; the relations of which, and 
of the first sacral ribs, remind one of those of the answerable vertebrse in the pelvis of 
birds. The ribs of the second and third sacral vertebrae also abut against the ilium. 
The ischium (Plates XXIII. and XXIV. 63 ), behind or beyond the acetabulum, forms 
a short and very thick prominence {ib. fig. 2 , i(), which receives the abutment of the 
ribs of the fifth sacral (Plate XXIV. pi 5 ) : below or behind this it receives a similar 
abutment from the ribs of the last sacral vertebra {pi e). The space, corresponding to 
the ‘ great ischiadic foramen ’ in Edentate mammals, is thus divided into two vacuities. 
The ischium becomes thinner, but is of great breadth where it forms the lower wall of 
the pelvis and converges towards its fellow to form, with the pubis, the long symphysis 
(Plate XXIII. fig. 2,3/); here it again increases in thickness. A fracture, with a slight 
3 E 2 
