AXTAL SKELETON OF THE OSTRICH. 415 
The ventral surface is destitute of any hypapophysis; but its nearly straight median 
portion is in the form of a slightly marked antero-posterior ridge. On each side of this 
the centrum rounds off, so that its ventral surface is convex transversely throughout ; its 
lateral margins are, when viewed dorsally (fig. 50), strongly concave, owing to the ex- 
pansion of the pre- and postaxial ends of the centrum. The concavity of the preaxial 
margin is much as in the twenty-first vertebra; but the postaxial margin is less convex. 
The sides of the centrum and neural arch are continuous, without any trace even of 
a ridge; but there may be defects of ossification beneath the diapophysis. 
The parapophysis extends out less even than in the twenty-first vertebra ; and its arti- 
cular fossa is larger (especially dorso-ventrally), but shallower. 
The diapophysis is much longer and very much produced obliquely, postaxiad and 
slightly dorsad; its remoteness from the parapophysis is again augmented in passing to 
this vertebra from the twentieth. 
The convexity for the tubercle of this rib is, like that of the twenty-first vertebra, 
smaller and less marked than in the twentieth vertebra, and may be nearer to the post- 
axial margin of the diapophysis that it is to its preaxial margin. 
The distal part of the diapophysis is more antero-posteriorly extended than the more 
proximal part. 
Numerous irregular preaxial diapophysial excavations are placed beneath the ridge 
running from the prezygapophysis to the diapophysis ; and other large exavations appear 
behind the diapophysis and on each side of the postzygapophysis, extending close to the 
ventral side of the postaxial margin of the diapophysis, which margin runs inwards to 
the base of the neural spine. _ 
The neural spine is more extended dorsally, but not antero-posteriorly ; it has a 
slightly convex dorsal margin, and bifurcates again at each end (fig. 50, ns). 
That concavity which in the two preceding vertebre exists on each side of the preaxial 
root of the neural spine is here yet further deepened. 
The postaxial fossa of the neural spine is smaller; and the margins of the laminz 
which laterally bound it form an obtuse angle with the postaxial margin of that spine. 
The articular surfaces of the zygapophyses are again still smaller, and look almost 
entirely inwards and outwards respectively. Almost the whole surface of the postzyga- 
pophysis projects postaxially beyond the postaxial surface of the centrum. 
The twenty-second vertebra supports a long rib, which articulates by its distal end 
with the third sternal rib. 
THE TWENTY-THIRD VERTEBRA. 
This vertebra is so like its preaxial predecessor that little need be said in its descrip- 
tion. 
The postaxial surface of the centrum is relatively deeper dorso-ventrally, and its ven- 
tral margin is slightly concave. 
