AXIAL SKELETON OP THE OSTRICH. 



413 



The dorsal margins of these laminse form nearly a right angle with the postaxial 

 margin of the neural spine. 



The articular surfaces of the zygapophyses are still smaller ; and the postaxial ones pro- 

 ject still more postaxially beyond the centrum than in the nineteenth vertebra (fig. 45, ps). 



The twentienth vertebra supports a long rib, which ends freely and does not come into 

 contact with any sternal rib, though it is related to the first of these (fig. 75, in). 



THE DOESAL VERTEBK^. 



The Twenty-first Veetebea. 



The twenty-first vertebra is so much like the twentieth that little need be said in its 

 description. 



The preaxial articular surface of the centrum is deeper dorso-ventrally. 



The hypapophysis is generally smaller, especially narrower from side to side, and its 

 preaxial margin is more concave ; it may show a rather more marked tendency to distal 

 bifurcation (fig. 48, hy). 



No oblique ridge traverses the side of the centrum ; but there may be some defects of 

 ossification dorsally, just ventrad of the diapophysis. 



The parapophysis extends less ; but its articular fossa (p) is larger. 



TWENTY-FIRST VERTEBRA (^ natural size.) 

 Fig. 47. Fig. 48. 



Ay 



Fig. 47, lateral aspect ; 48, ventral surface. Letters as before. 

 The diapophysial excavations are plainly to be seen ; the postaxial one has the form of a single foramen or fossa. 

 Besides these, there is a defect of ossification in the side of the centrum, just belovr the middle of the 

 diapophysis. 



The diapophysis projects obliquely postaxiad and slightly dorsad, as well as outwards, 

 and is much more remote from the parapophysis than in the twentieth vertebra. 



The tubercular convexity (<7) is less marked, and is situated almost as near the post- 

 axial border of the diapophysis as it is near its preaxial margin. 



