STRUCTURE AND RELATIONSHIPS OF OPISTHOCELIAN DINOSAURS. 175 
are strong, but with the bifurcation of the spine they become reduced, 
and are replaced by a second short, stout pair which meet in the 
median line. In presacral 1 the articulating surfaces are concave and 
face downward to articulate with the prezygapophyses of the first 
sacral; in the succeeding vertebre they face outward and downward 
in the form of a wide V. At the point of convergence the articular 
surfaces are expanded into a prominent hyposphen. This is supported 
from below by a second pair of buttresses arising from the neural 
arch. In presacral vil both zygapophyses and hyposphen reach their 
strongest development. In1x the hyposphen disappears entirely; in 
x the zygapophyses are more elevated and widely separated. 
The prezygapophyses are supported chiefly by a single stout pair of 
buttresses arising from the anterior ‘margin of the pedicles. Their 
articulating surfaces are directed upward and slightly inward. In the 
anterior members of the series they are separated by a narrow notch, 
or hypantrim, into which the hyposphens of the preceding vertebre 
fit. In vil there is a pronounced widening of this notch correspond- 
ing with the disappearance of -the hyposphen from 1x. In 1x and x 
the prezvgapophyses are still more widely separated. | 
The capitular facet varies much, both in strength and in position, 
on the vertebra, as we pass forward in the dorsal series. (Plate L11.) 
In 1 the facet is very slight; it arises by three roots from the inferior 
surface of the prezygapophyses and from the lateral surface of the neural 
arch. It increases in strength in v and v1, and is slightly depressed by 
the shortening of the pedicles. In vir the decided shortening of the 
pedicles brings the inferior border of the facet almost to the margin 
of the centrum. ‘The facet itself is more than double the size of that 
in vi. In vil it is shifted downward so that half of its articular sur- 
face springs from the centrum. In 1x the facet is borne entirely on 
the centrum just anterior to the lateral fossa and partially replacing it. 
In x the transposition is complete, the facet having passed entirely 
below the anterior end of the lateral fossa. In the last two vertebre 
it is also much reduced in size. | 
