20 ODONTORNITHES. 



Vertical diameter of anterior articulation of centrum, 6.0™ 



f superior, 9.0 



Transverse diameters of posterior articulation of centrum, ) median, 7.0 



(inferior, 8.0 



Vertical diameter of posterior articulation of centrum, 8.0 



Transverse diameter of vertebra, across pre-zygapophyses, 13.0 



Transverse diameter of vertebra, across post-zygapophyses (approximate), 24.0 



Greatest diameter of pre-zygapophyses, _ 5.5 



Least diameter of pre-zygapophyses, _. 4.0 



f vertical (approximate), 7.0 



Diameters of post-zygapophyses, | horizontal) 7g 



Length of floor of neural canal, 30.0 



Length of roof of neural canal (approximate), 26.0 



Transverse diameter of anterior opening of the neural canal, 7.5 



Vertical diameter of anterior opening of the neural canal, 6.0 



Transverse diameter of posterior opening of the neural canal, 8.8 



Vertical diameter of posterior opening of the neural canal, 5.0 



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



The third vertebra of Hesperornis regalis resembles that of the Loon, 

 but has the articular faces of the centrum less oblique. The centrum is 

 compressed laterally, as in the axis, becoming quite narrow at and behind 

 the middle ; then expanding suddenly, for the posterior articulation. The 

 anterior articular face is supported by a more gradual expansion. The 

 face itself is somewhat oblique, looking downward at an angle of less than 

 30°. It is concave transversely, and convex vertically, as in ordinary 

 birds. Immediately behind this articulation, the under surface of the 

 centrum is excavated. Beyond the excavation, the hypapophysis gradually 

 becomes prominent, quite thin in front, but expanded and flattened into a 

 button-shaped process, below the posterior articulation. The latter face is 

 very similar in shape to that of the axis, but is somewhat larger, as shown 

 by the measurements, and slightly more oblique. 



The neural arch bore a less powerful spine than that of the axis. 

 The pre-zygapophyses are much longer, and larger than in the axis, and 

 they look inward and upward, instead of outward, as in that vertebra. 

 They are, however, turned only slightly inward. The post-zygapophyses 

 are less elongated than the pre-zygapophyses ; and, as in the axis, are 

 strengthened by a ridge beginning about the middle of the upper part of the 



