408 MR. ST. GBOEaE MIVAET ON THE 



The centrum has its _p>'eaxial 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 jjostaxial 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. 



EIGHTEE>^TH VERTEBRA (^ natural size). 

 Fig. 40. Fig. 41. 



hy. 



jiz 



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. There 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 jjarapophyses ; 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 vertebne) 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, i). 



This osseous band, when it is auchylosed 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 neui-al lamina and inferiorly by the 



