130 ODONTORXITHES. 



The Third Vertebra. (Plate XXVII, figure 2.) 



The third cervical vertebra of Ichtlnjornis (number 1733) differs 

 much from the second, and in several respects resembles the corresponding- 

 vertebra in the Tern. The centrum is deeply excavated in front, and the 

 anterior articulation is much inclined to the axis of the vertebra. Behind 

 this excavation, the centrum abruptly becomes wedge-shaped, but the 

 lower edge thickens posteriorly, and, below the posterior articulation, is 

 about half as broad as the articular surface itself. 



The plane of the anterior articulation is nearer horizontal than vertical, 

 being inclined at an angle of nearly or quite 60°, with the axis of the 

 centrum. In a vertical section, this ai'ticulation is moderately convex, 

 while laterally it is strongly concave, thus presenting an approach to the 

 characters of the ordinary avian vertebral articulation. The posterior 

 articular surface of the axis, and of this vertebra also, fail to show this 

 peculiarity. This specialized feature, occurring at the first bend of the 

 neck, gives a direct hint as to the origin of the unique articulation in the 

 vertebrae of modern birds. The full explanation of this articular structure 

 will be given in a subsequent chapter. 



The posterior articulation is, like that of the axis, sub-quadrate in 

 outline, but is more elongate vertically than in that vertebra. This face 

 is concave vertically, and less so horizontally, except perhaps near the 

 margins, which are rounded. On the sides of the centrum near the lower 

 part of the anterior articular surface, rather strong parapophyses are 

 developed, which are connected with the diapophyses above by a strong 

 bridge of bone, or pleurapophysis. These together enclose on each side 

 a small, vertically oval, lateral foramen. 



Immediately above, and a little external to the lateral foramina, are 

 the strong and elongated pre-zygapophyses, which are directed somewhat 

 obliquely downward, outward, and forward, and are about equally convex 

 in both directions. They are proportionally longer than in the Tern, and 

 look more directly upward and less inward than in that species. Below, 

 they are strengthened by a ridge running up from the outer wall of the 

 lateral foramen. The post-zygapophyses are shaped much as in the 



