AXTAL SKELETON OF THE OSTRICH. 405 
The postaxial articular surface of the centrum has the transverse extent of its ventral 
margin very little in excess of that of its dorsal margin. The articular surfaces of the 
prezygapophyses look more directly dorsad and less inwardly; those of the postzyqya- 
pophyses are broader. 
The median unexcavated part of the neural spine is again more or less longer antero- 
posteriorly. The postaxial part of the neural arch still more nearly approaches in 
width the preaxial part of that arch than in the tenth vertebra; and its postaxial 
margin is less concave. 
Tue Twevrti VERTEBRA. 
The twelfth vertebra carries on the progressive modifications indicated in the 
eleventh vertebra, and it is slightly longer and more massive than that vertebra. 
The catapophyses may here first project ventrally as sharply marked processes 
(figs. 30 & 32, ¢). 
TWELFTH VERTEBRA (2 natural size). 
Fig. 29. Fig. 30. 
Aspects. 
Fig. 29, lateral; 30, postaxial; 31, dorsal; and 32, ventral. 
Letters as before; c, catapophysis. 
The transverse extent of the postaxial part of the neural arch nearly equals that of 
its preaxial part. 
The styliform rib here generally attains its maximum of length. 
312 
