408 MR. ST. GEORGE MIVART ON THE 
The centrum has its preaxial articular surface with its dorsal margin more concave 
and its ventral margin less concave than in the seventeenth vertebra. ‘The outline of 
the inferior margin of the whole centrum is modified by the hypapophysial process. 
The postavial surface of the centrum is relatively more extended transversely, with 
its ventral margin wider and more in excess of its dorsal margin than in the seventeenth 
vertebra, while at the same time the ventral margin is still less concave. 
EIGHTEENTH VERTEBRA (4 natural size). 
Fig, 40, Fig. 41. 
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Fig. 40, lateral aspect; 41, ventral aspect. Letters the same, except that d denotes the diapophysis. 
The ventral surface of the centrum is wide and but very slightly concave at either 
its preaxial or its postaxial part. ‘here is a very thick median hypapophysis extend- 
ing from the middle of the ventral surface, and more or less bifurcating ventrally into 
two short, rounded, diverging processes (fig. 41, hy). 
No diverging or other ridges connect the hypapophysis with the parapophyses; but a 
ridge may run postaxially from each parapophysis to the postaxial margin of the 
ventral surface of the centrum, the two ridges bounding that surface laterally, 
The postaxial margin of the ventral surface is less convex than in the seventeenth 
vertebra. 
If there is no free rib, the pleurapophysial band of bone representing it is extremely 
short antero-posteriorly. Each margin of it is concave, while from the ventral end of 
its postaxial margin a triangular blunt prominence (the last rudiment of the styliform 
rib of more preaxial vertebre) projects postaxially, but at the same time in a more 
ventral direction than that taken by any of the styliform ribs before described. Such 
is its condition in a highly ossified skeleton; but very often the pleurapophysial band 
appears as a separate rib, and in this free condition it is described below as the first 
rib (fig. 75, 1). 
This osseous band, when it is anchylosed to the vertebra, bounds externally a bony 
canal for the vertebral artery, which is so large that it exceeds in diameter the neural 
canal itself. 
This arterial canal is bounded internally by the neural lamina and inferiorly by the 
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