AXIAL SKELETON OF THE OSTRICH. 411 
smaller excavation (either several small openings or one large foramen) is placed a little 
preaxially of the postaxial margin of the neural lamina, on the ventral side of the 
interzygapophysial ridge. 
The neural spine has its dorsal margin more antero-posteriorly extended than in the 
eighteenth vertebra; and consequently the margins of the ridges running from it to the 
postzygapophyses, instead of sloping gradually, run at first a little preaxially, thus 
forming a slightly acute angle. 
The postaxial excavation of the neural spine is much as in the eighteenth vertebra, 
though somewhat less deep. The preaxial surface of the neural spine is vertical, but 
cannot always be said to be medianly excavated; rather there is sometimes a slight 
depression on each side of it on the neural lamina. 
The prezygapophyses are broader than in the eighteenth vertebra, and look rather 
more inwards. The postzygapophyses are smaller and look even more outwardly. For 
the first time their transverse (here actually dorso-ventral) diameter may exceed their 
antero-posterior development ; and they are so produced that their articular surfaces may 
be altogether postaxial to the postaxial margin of the centrum. The preaxial margin 
of the neural arch begins to show a little median preaxially directed process (fig. 44). 
Tue TWENTIETH VERTEBRA. 
The twentieth vertebra closely resembles the nineteenth, with the centrum, diapo- 
physes, and neural spines slightly enlarged. 
The centrum has its preaxial articular surface relatively deeper dorso-ventrally ; and 
its lateral parapophysial terminations are not so much laterally produced. 
TWENTIETH VERTEBRA (3 natural size). 
Fig. 45, lateral aspect; 46, postaxial aspect. Letters as before. 
The pre- and postaxial diapophysial excavations of the neural lamina are the same as in the nineteenth 
vertebra; only the postaxial excavation is relatively the more extensive. 
The postawial surface has its ventral margin more conyex, but relatively shorter 
compared with its dorsal margin, than in the nineteenth vertebra; the whole articular 
surface begins also to be less convex transversely and less concave dorso-ventrally. 
3M2 
