18 ODONTORNITHES. 



There are fourteen vertebra? in the coossified sacral series represented in 

 Plates X and XI, and there were twelve more in the tail, as shown in Plate 

 XII, making- the entire column consist of forty -nine vertebrae. This is a 

 very large number for this class, and is equaled in but very few recent birds. 



None of the vertebra? of Hesperornis were ankylosed together, except 

 in the sacral series, and at the extremity of the tail. None of the vertebra? 

 contained pneumatic openings, although some were lightened by medullary 

 cavities. 



The Atlas and Axis. (Plates III and XX.) 

 The atlas was evidently a short ring of bone, very similar to the first 

 vertebra in modem aquatic birds. Its articular cup for the reception of 

 the occipital condyle was not deep, and was cordate in outline. The atlas 

 articulated with the axis by well developed zygapophyses above, and by a 

 crescent-shaped tubercle below. The centrum of the atlas is coossified 

 with the axis, as the odontoid process. 



The axis (Plate III, figure 1) is well preserved, and resembles the 

 corresponding vertebra of the Loon (Colymbus torguatus, Brim.) ; but it is 

 one-third larger, and a much stouter bone. The posterior articular face is 

 much less oblique, being nearly vertical. The odontoid process is 

 relatively shorter and broader, and its articular face is more extended 

 backward, along the median line below. This process is slightly concave 

 above, in prolongation of the floor of the neural canal. The centrum of 

 the axis is expanded at each end, for its articular surfaces. The anterior 

 expansion is considerably greater, and somewhat more gradual, than the 

 posterior ; and, at the least expanded portion, its' thickness is less than two 

 millimeters. The anterior articular surface is reniforrn, the upper- outline 

 being emarginate, to make room for the base of the odontoid process. The 

 articulation is concave in both directions, but much more deeply concave 

 vertically than transversely. 



The anterior portion of the centrum of the axis is deeply excavated 

 at the sides, nearly on a level with the floor of the neural canal, for the 

 transmission of the vertebral artery. A very delicate bridge of bone 



