CHAPTER X. 



THE PRESACRAL VERTEBRA OF ICHTHYORNIS. 

 (Plates XXII, XXVI, XXVII, and XXXIV.) 



The presacral vertebrae of Ichthyornis present characters more remark- 

 able than those of any other known birds, and hence those preserved are 

 here described in detail, and fully illustrated. The series is not complete 

 in any one skeleton yet discovered, but the large nxunber of vertebrae 

 secured in various specimens make clear the more important features of 

 this part of the structure. 



None of the vertebrae of Ichthyornis, in front of the sacrum, are 

 ankylosed together, and a number of them contain cavities in the sides, 

 which are apparently pneumatic. Among existing birds, the Terns appear 

 to bear the nearest general resemblance to Ichthyornis, and hence their 

 vertebrae are here used for comparison with those of that genus. 



The Atlas. (Plate XXVII, figure 1.) 

 The atlas of Ichthyornis, as in most living birds, is a ring of bone, 

 very short antero-posteriorly, compared with the succeeding vertebrae. In 

 Ichthyornis victor (number 1733), it is only about one-third as long as the 

 axis, exclusive of the odontoid process, the proportion being nearly the 

 same as in Sterna regia, Gambel. 



The centrum of the atlas in Ichthyornis is coossified with that of the 

 axis, forming a true odontoid process, which is robust, and somewhat 

 convex upon its upper surface. The hypapophysis presents a crescent 



