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STRUCTURE AND RELATIONSHIPS OF OPISTHOCELIAN DINOSAURS. 173 
gradation, the vertebre will be described by taking up the several 
elements in turn and tracing them through the entire thoracic series. 
The centra pass from the less pronounced opisthoccelous type near 
the sacrum to the pronounced type of the cervical region. In the first 
presacral the vertical diameter of the centrum is equal to the lateral 
diameter; as we pass forward in the series the centra become vertically 
compressed while the lateral measurement remains approximately the 
same. The articular ends from 1 to vil are nearly perpendicular to 
the axes of the centra; with 1x the articular surface begins to incline 
forward, this feature becoming more and more pronounced until the 
centra take on the cervical type with presacral xt. The openings of 
the pleuro-central cavities in the first presacral are almost round and 
enter the centrum from above downward. With the strengthening of 
the pedicles and the depression of the centra, these assume a lateral 
direction. In the anterior members of the series they become elon- 
gate axially; in 1x and x they are partially replaced by the capitular 
facet. 
In cross section the typical centrum shows an external wall 
pierced by two lateral foramina leading to the pleuro-central cavities, 
which are separated by a thin median septum. This arrangement is, 
however, subject to regional modification. In the first presacrals 
there are a number of vacuities in the inferior portion of the centrum 
which do not connect with the lateral fosse. In1 these consist of 
small cavities within the posterior rim of ‘the centrum; in 11 the cavi- 
ties are large and occupy the lower half of the centrum, being separated 
from the lateral fossz by a wall two centimeters in thickness. 
The transverse processes arise from four roots:. (1) a horizontal plate 
which extends laterally from the prezygapophysis; (2) a second hori- 
zontal plate which arises from the anterior margin of the postzyga- 
pophyses and passing forward unites with the posterior margin of the 
first; (3) a vertical plate which arises from the base of the pedicle, 
passes upward and forward, and fusing with the inferior surface of the 
lateral plate, forms a bracket-like support below; (4) a similar but 
smaller vertical plate which arises from the lateral surface of the neural 
spine, passes outward and unites with the superior margin of the hori- 
zontal plate, supporting it from above. With the reduction and bifur- 
cation of’ the spine, the superior root becomes reduced, shifted 
backward, and in vi joins the posterior root. In 1x and x it is absent 
entirely. With the depression of the capitular facet, a second anterior 
root makes its appearance inv. This arises from the lateral surface 
of the capitular process and is noticeable as a slight ridge just below 
the anterior margin of the horizontal plate. In vi this remains very 
