136 ODONTORNTTHES. 



The Sixteenth Vertebra. (Plate XXII, figure 3.) 



A vertebra of the type sj>ecimen (number 1450) resembles both the 

 sixteenth and seventeenth of the Tern. It belongs in the dorsal series. 

 The centrum has its least transverse diameter at the middle, and sends 

 perpendicularly downward from the anterior half a sub-conical hypa- 

 pophysis, two-lobed at the apex. This process is wanting in the Tern. 

 The articular faces of the centrum are sub-circular in outline, somewhat 

 flattened above. Both are concave,, and the posterior is more deeply so 

 than the anterior. 



The centrum is very deeply excavated at the sides, above the 

 middle, leaving only a thin wall on the median line. The articulation for 

 the head of the rib is borne on the lateral wall of the neural canal, and is 

 obliquely oval in outline. The diapophyses are broken away in this 

 specimen, and the neural spine is very imperfect. The zygapophyses 

 of the opposite sides are near together. The pre-zygapophyses project 

 but little in front of the centrum, while the post-zygapophyses extend 

 nearly their whole length behind it. 



Another vertebra (number 1733), somewhat posterior to that regarded 

 as the fifteenth, has the centrum preserved, but is not figured. This cen- 

 trum is rounded below, slightly cai'inate on the median line, and sends 

 down three divergent tubercles near its anterior end. Two of these are 

 lateral, and somewhat divergent. The third is smaller, and more acute, 

 and is median in position, thus representing a hypapophysis. The anterior 

 face of the centrum is injured in this specimen, but appears to have been, 

 like the posterior face, sub-circular, and somewhat concave. On each side 

 above the middle, the body of the centrum is excavated, so as to leave only 

 a thin longitudinal median wall to support the floor of the neural canal. 

 The articular surface for the head of the rib is above this excavation, on 

 the lateral wall of the neural canal. 



