PEESACRAL VERTEBRA OF ICHTHYORXIS. 129 



thickens backward, and nearly acquires the character of a hypapophysis 

 below the posterior articular surface. On each side of the centrum near 

 the middle, is a rather acute salient tubercle, just behind which the trans- 

 verse diameter of the centrum is at the minimum. Above and a little 

 behind these tubercles, are large pneumatic foramina. 



The posterior articular face of the centrum is much smaller than the 

 anterior, and is sub-quadrate in outline. It is distinctly concave vertically, 

 and less so laterally; and begins to assume the peculiar character seen 

 in the biconcave articulations of thejbllowing vertebrae. 



The pre-zygapophyses are sessile facets on the anterior margin of the 

 lateral walls of the neural canal. They project very slightly in front of 

 the g-eneral contour lines, much less however than in Ciconia, while in the 

 Tern these processes do not project at all. The post-zygapophyses are 

 larger than the pre-zygapophyses, and in form are sub-triangular, with 

 rounded angles. They are concave in a lateral direction, and strengthened 

 above by a stout ridge running out, somewhat beyond the zygapophysis 

 on each side, into a blunt point. This projection is much weaker than in 

 the Tem, where the post-zygapophyses are surmounted by prominent, 

 divergent processes. The neural spine was low, and stout, more elongate 

 antero-posteriorly, but less robust, than in the Tern. 



The following measurements show the principal dimensions of the 

 atlas and axis in one specimen of Ichthyornis victor: 



Measurements of Atlas and Axis. (No. 1733.) 



Length of atlas, ' 2.7 mm 



Length of centrum of axis, 7.0 



Length of centrum of axis, including odontoid process, 9.0 



Transverse diameter of cup in atlas, 3.5 



Transverse diameter of atlas, across zygapophyses, 7.0 



Transverse diameter of anterior articulation of axis, 3.5 



Vertical diameter of posterior articulation of axis, 2.7 



Transverse diameter of posterior articulation of axis, 1.8 



Transverse diameter of vertebra, across post-zygapophyses (approximate), 9.0 



17 



