PRESACRAL VERTEBRAE OF ICHTHYORNIS. 133 



of the lower surface, and extends downward, and a little forward. Its base 

 is about two-thirds the antero-posterior extent of the centrum, and, below, 

 it is broadly rounded. The anterior face of the centrum is slightly inclined 

 to the axis of the bone, so as to look somewhat downward. It is sub- 

 quadrate in outline, with rounded angles, and is slightly broader below 

 tlian above. The margin, except below, is prominent, and the face is 

 concave in both directions. The posterior face is more nearly circular, 

 in outline, but flattened above, and is more concave than the anterior. 



Strong diapophyses arise on the lower part of the anterior end of the 

 centrum, and are directed outward, backward, and downward. They are 

 united with slender pleurapophyses, which complete the lateral foramina 

 by union with the parapophyses above, just below and exterior to the pre- 

 zygapophyses. The lateral foramina are larger in proportion to the size 

 of the centrum than in the eleventh cervical of the Tern, as is also the 

 neural canal. 



The pre-zygapophyses look strongly inward, and somewhat upward 

 and backward, and are separated from the post-zygapophyses by a distance 

 only about equal to their length. Between the pre-zygapophyses, the 

 neural canal is broadly open above. The post-zygapophyses look almost 

 directly downward, and somewhat outward. They are well elevated above 

 the floor of the neural canal by its nearly perpendicular walls. The roof 

 of the neural canal is short on the median line, excavated by a longitudin- 

 ally oval pit below, and bears a rudimentary neru-al spine, less prominent 

 than in corresponding vertebra of the Tem. Below the neural spine, 

 behind, is a pit for ligamentary attachment. The neural canal is large. 



The following are the main measurements of this vertebra in the type 

 specimen of Ichthyornis dispar : 



Measurements of Twelfth Vertebra. (So. 1450.) 



Length of centrum, 5.5 mm 



Vertical diameter of anterior articulation of centrum, 2.5 



Transverse diameter of anterior articulation of centrum, 2.8 



Length of hypapophysis, 2.8 



Antero-posterior diameter of hypapophysis, at base, 3.0 



