CAUDAL VERTEBRAE OF HESPERORNIS. 85 



Measurements of Ninth Caudal Vertebra. (No. 1200.) 



Length of centrum, 16.0 mm 



Transverse diameter of anterior articulation, 10.0 



Vertical diameter of anterior articulation, 4. 



Transverse diameter of posterior articulation, 9.5 



Length of floor of neural canal, 15.5 



Transverse diameter of neural canal, at anterior opening, 2.6 



Diameter of vertehra, across transverse processes, 56.0 



Autero-posterior diameter of transverse process, at base, 10.5 



Anterior extent of hypapophysis in front of centrum, 8.0 



Transverse diameter of hypapophysis, 6.5 



Vertical diameter of hypapophysis, 2.0 



The Tenth, Eleventh, and Twelfth Caudal Vektebe^e. 

 (Plate XII, figure 10.) 

 The remaining caudal vertebrae of Hesperornis regalis are coossified 

 into a broad, horizontal, terminal bone, homologous with the pygostyle, or 

 plough-share bone, in existing birds. In this terminal mass, the tenth 

 vertebra retains the general form of the ninth, but its transverse processes 

 are less elongate, and less expanded. Its centrum is depressed, and there 

 is a small hypapophysis coossified with its anterior ventral surface. The 

 neural arch remains distinct, and there is a short neural spine. The 

 expanded transverse processes extend outward at right angles to the axis of 

 the centrum, and project but slightly below its under surface. Their 

 extremities are obliquely truncated by the removal of the outer posterior 

 angle. 



The eleventh vertebra is much smaller than the tenth, and firmly 

 coossified with it. Its transverse processes are slender, and are directed 

 forward as well as outward. Its hypapophysis is coossified with the 

 centrum, and in part with the posterior surface of the preceding' vertebra. 

 The neural spine is very low, but distinct. 



The twelfth vertebra, which apparently forms the termination of the 

 tail, is firmly ankylosed with the eleventh, but the greater portion of its 

 centrum has been broken off in the present specimen, and is lost. Its 

 general form is indicated in the restoration of Hesperornis in Plate XX. 



