PRESACRAL VERTEBRAE OF ICHTHYORNTS. 137 



The sixteenth vertebra of Ichtliyornis dispar measures as follows : 



Measurements of Sixteenth Vertebra. (Xo. 1450.) 



Length of centrum, 5.8 mm 



Least diameter of centrum, 1.2 



Vertical diameter of anterior articulation of centrum, 2.5 



Transverse diameter of anterior articulation of centrum, 2.5 



Vertical diameter of posterior articulation of centrum, 2.4 



Transverse diameter of posterior articulation of centrum, 2.4 



Length of hypapophysis, 1.8 



Transverse diameter of neural canal, 1.8 



Transverse diameter of vertebra, across pre-zygapophyses, 3.3 



Transverse diameter of vertebra, across post-zygapophyses, 2.5 



The Seventeenth Vertebra. (Plate XXVII, figure 5.) 



A dorsal vertebra (number 1733) may be best compared with, the 

 seventeenth vertebra of the Tern, to which it bears some resemblance 

 The centrum is but little contracted at the sides, and is almost evenly 

 rounded below. It presents near its anterior end two unequal tubercles, 

 the right one being the larger. The articular faces are slightly elongated 

 vertically, and nearly equally concave. 



The sides of the centrum in this vertebra are deeply excavated above 

 the middle for almost the entire length, so that only a thin layer of bone 

 is left to support the floor of the neural canal. This structure is much 

 like that in the corresponding vertebra of the Tern, and a little more 

 pronounced than in what may have been the preceding vertebra. The 

 articular surface for the head of the rib is situated above the anterior 

 part of this excavation, and is supported by a ridge running upward 

 and backward on the outer wall of the neural canal. 



The pre-zygapophyses are elongated, and approximate. The dia- 

 pophyses are both broken off. The post-zygapophyses are a little nearer 

 together than the j>re-zygapophyses. The neural spine was strong, but is 

 much fractured in the present specimen. 



This vertebra of Ichtliyornis victor has its principal dimensions as 

 follows : 



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