DORSAL VERTEBRAE OF HESPERORXIS. 55 



Length of floor of neural canal, 26.0 mm 



Length of roof of neural canal, 22.0 



Diameters of articulation for head of rih, 6.5- 7.5 



Length of hypapophysis, 5.0 



The Twenty-third Vertebra. (Plate V, figure 7.) 



The twenty-third, or last free dorsal, vertebra differs little from the 

 twenty-second, except in the diapophyses. The centrum is stouter, and 

 somewhat flattened below, with no hypapophysis. The articular faces are 

 larger than in the twenty-second vertebra. The diapophyses are short, 

 tubercular, and directed strongly forward. The capitular surface is not 

 quite as high on the side of the vertebra, hardly reaching above the middle 

 of the neural canal. The neural spine is slightly higher, but of less antero- 

 posterior extent than in the twenty-second vertebra. It is flattened and 

 roughened, before and behind, for ligamentary attachment. The roughness 

 extends much above the middle of the spine anteriorly, but not so far above 

 posteriorly, where it is surmounted by a 23rojecting angle in the outline. 

 In its natural position, this vertebra is almost entirely included between 

 the anterior projecting portions of the ilia. 



The following- measurements are taken from three vertebra? of 

 Sesperornis regal is, all in excellent preservation, from different skeletons, 

 found in the same geological horizon : 



Measurements of Twenty-third Vertebra. (No. 1206.) 



Length of centrum, 25.5 mm 



Transverse diameters of anterior articulation of centrum, 18.5-25.0 



Vertical diameters of anterior articulation of centrum, 1 1.0-15.0 



Transverse diameter of posterior articulation of centrum, 26.0 



Vertical diameters of posterior articulation of centrum, 12.0-1 7.0 



Transverse diameter of vertebra, across pre-zygapophyses, 18.0 



Diameters of pre-zygapophyses, 9.0-10.5 



Diameters of post-zygapophyses, 10.0-12.0 



Length of floor of neural canal, 26.0 



Transverse diameter of neural canal, at anterior opening, 6.5 



Vertical diameter of neural canal, at anterior opening, 6.5 



Diameters of articulation for head of rib, 6.0- 6.0 



Height of neural spine above floor of neural canal, 35.0 



Antero-posterior diameters of neural spine, 15.0-1 7.0 



Least transverse diameter of centrum, 14.0 



