CHAPTER VI. 



THE CAUDAL VERTEBRA OF HESPERORNIS. 

 (Plates XII and XX.) 



The tail of Hesperornis regalis presents some peculiarities of structure 

 not before seen in birds. It was composed, apparently, of twelve vertebrae, 

 and all of these are preserved, with the exception of that portion of the 

 last which formed the extreme end of the tail. The nmnber of caudal 

 vertebrae in Hesperornis regalis exceeds those in any known recent bird, 

 with the possible exception of the Great Auk (Alca impennis, Linn.), now 

 nearly or quite extinct. The number falls far short, however, of that in 

 the tail of Archceopteryx, which also differs essentially in its general 

 structure from the caudal extremity of Hesperornis. 



The anterior free caudals of Hesperornis are short, with high nemal 

 spines, and moderate transverse processes. They are opisthocoelian, as in 

 the corresponding vertebrae of Pavo and Geococcyx. The middle and 

 posterior caudals have very long, and horizontally expanded, transverse 

 processes, which restricted lateral motion, and clearly indicate that the tail 

 was mainly moved vertically, evidently as an aid in diving. The last three 

 or four caudal vertebrae are firmly coossified, forming a flat, horizontal, 

 terminal mass, analogous to, but quite unlike, the plough-share bone of 

 modern birds. 



The caudal vertebrae of Hesperornis are composed of compact osseous 

 tissue, without pneumatic openings. None of them possess zygapophyses. 



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