STRUCTURE AND RELATIONSHIPS. OF OPISTHOC(ELIAN DINOSAURS. 233 
surface of the hyposphene. Similarly the articular surface of the 
postzygapophysis is continuous with that of the hypantrum. The 
two form a firmly interlocking joint capable of resisting lateral strain. 
The hypantrum is also slightly expanded inferiorly so as to prevent 
a downward displacement of the prezygapophyses. 
The prezygapophyses are supported by a single pair of buttresses, 
which also form the anterior margin of the neural arch, and by the 
anterior plate of the transverse process. The postzygapophyses are 
supported from above by a pair of stout buttresses which descend 
from the postero-lateral margin of the spine; laterally they are sup- 
ported by the posterior root of the transverse process and inferiorly 
by a pair of buttresses arising from the posterior margin of the neural. 
arch and attaching to the anterior surface of the hypantrum. In the 
sixth and seventh presacrals this support is strengthened by passing 
to the inferior surface of the hypantrum. 
The neural spines in this genus are single and median so far as 
known, and there is no reason to doubt that they continue so through- 
out the dorsal series. Unlike those of any other known members of 
the Opisthoccelia, the spines are short in the first presacrals, and 
become more and more elongate as far as the middle thoracic region. 
They are made up of the usual median plate expanded at the crest 
into a rugose knob and roughened on the anterior and posterior 
margins for the attachment of interspinous ligaments. They are 
flanked at the anterior and posterior margins respectively by a pair 
of greater and lesser lateral plates. The posterior plates arise from 
the superior root of the prezygapophyses, pass upward along the 
lateral border of the median plate near the posterior margin, and 
form a stout lateral support to the crest. The anterior lateral plates 
arise similarly from the base of the prezygapophyses and strengthen 
the anterior border of the median plate. In presacral 1 the spine is 
low, massive, and strongly reinforced by buttresses arising from the 
postzygapophyses. In 11 it is noticeably higher and less firmly 
braced. In Iv and v the base of the spine becomes broadened 
anterio-posteriorly by the diverging roots of the lateral plates. A 
lateral vacuity appears between the posterior and inferior roots: of 
the transverse process and the base of the postero-lateral spinous 
plate. In vi and vir the spine becomes more elongate and some- 
what more slender. 
