128 ODOXTORNITHES. 



shaped articular surface in front, for articulation with the occipital condyle. 

 The angles of this crescent are prominent on each side above the odontoid 

 process, and during life doubtless gave attachment to a ligamentaiy bridge 

 above that process, completing a nearly circular cup, adapted to the 

 occipital condyle, as in existing birds. The hypapophysis is produced 

 below into a somewhat wedge-shaped process, pointing slightly backward, 

 and strengthened at the base behind by a low small tubercle on each side. 

 The posterior articular face of the hypapophysis is imperfectly exposed 

 in the specimen figured, but was evidently adapted to the anterior articular 

 surface of the axis, in such a manner as to allow only a rotary movement 

 about the odontoid process. This motion was, however, much limited 

 by the strong zygapophyses, which are much better developed than in 

 the Tern. They are strengthened by a ridge rising on the side of the 

 vertebra, and running upward, backward, and somewhat outward, and 

 ending, above the zygapophysis, in an obtusely pointed tubercle. A similar 

 form of zygapophyses is seen in the Egyptian Stork, Ciconia alba, but 

 in the Tern, these processes are rudimentary. The upper surface of the 

 atlas is not well preserved in the specimen here described, but appears 

 to have been destitute of a neural spine. 



The Axis. (Plate XXVII, figure 1-1 d.) 



The axis of Ichthyornis (number 1733) is proportionally a little longer 

 than in the Tern. The centrum is firmly coossified in front with the 

 centrum of the atlas, no trace of the suture remaining. Below the odon- 

 toid process, is a large articular surface, not exposed in the present specimen, 

 but evidently of a lunate form, somewhat concave, and pennitting little 

 motion except rotary. The lateral and inferior edges of this articular 

 surface are distinct, and rather acute ; and the articulation is bome upon 

 an anterior expansion of the centrum, which attains its greatest diameter 

 at the edges of the articular surface. These characters are well shown 

 in number 1775. 



The ventral surface of the centrum is evenly rounded for nearly one- 

 third of its length, then excavated on each side of a median carina, which 



