STRUCTURE AND RELATIONSHIPS OF OPISTHOCG:LIAN DINOSAURS. 179 
the neural canal upward, leaving vertically elongate vacuities opening 
into the enlarged intervertebral chambers of the neural canal. In Iv 
and v the neural arches are shifted forward so as to overhang the 
intercentral sutures. The prezygapophysial articulation between 1 and 
11 is atrophied but still in evidence; that between 11, 111, and tv is lost 
entirely; between iv and v it has become ankylosed but otherwise 
little modified. The hyposphen-hypantrum articulation persists as far . 
as sacraly. (Plate L.) 3 
The neural spines of the sacral vertebre diminish regularly in length 
from first to last. The spine of sacral 1is similar to that of presacral 
I, except that the forward curvature is nearer the base. The spines 
of 11, 11, and iv are firmly united by their median plates and show 
from their strength and system of bracing that this was thé chief 
anchorage for dorsal and dorso-caudal muscles. The lateral plates 
in 11 are more strongly developed than in any other spine; their 
anterior buttresses which descend to the prezygapophyses are devel- 
oped into stout braces to resist antero-lateral strain. The posterior 
roots have disappeared entirely. The lateral plates of 111 are almost 
lost; those of Iv are well developed and braced in turn by the posterior 
root, while the anterior one has disappeared. 
The sacral ribs take the form of stout, blunt processes firmly 
ankylosed with the centra and expanded distally to articulate with 
the ilium. Those of the three primary sacrals are similar in general 
structure, save that the middle one is reduced in size; they coalesce 
distally to form the sacricostal yoke. The first and second ribs arise 
from the mid-lateral surface of the centra; the third has its origin at 
the anterior end of the centrum. The ribs of the dorso-sacral vertebra 
arise from the antero-superior margin of the centrum in conjunction 
with the base of the neural.arch. They articulate distally with the 
anterior surface of the great peduncle of the ilium. The ribs of the 
caudo-sacral vertebra are similar in origin to those of the true sacrals, 
but are expended distally into oblique plates, which are united with 
the diapophyses as in the anterior caudals. In this specimen the ribs 
are imperfectly united with the sacricostal yoke. They articulate 
distally with the posterior surface of the lesser peduncle and with the 
mesial surface of the iliac plate. (Plate xLvull.) 
The dapophyses show no essential modification from the thoracic 
type. They are more or less completely connected-with the sacral 
ribs by a thin vertical plate. On the right side of the museum speci- 
men, where the ilium is conjoined, this plate forms a continuous 
articular surface, connecting the ribs and diapophyses in sacrals 1 and 
ul, while in 11 and iv there remain only slight vacuities. In v, as 
