AXIAL SKELETON OP THE OSTRICH. 399 



The length of the free part of this process is less than half the antero-posterior 

 extent of the centrum, though its extremity may project beyond the antero-posterior 

 median point of the centrum. 



The postaxial margin of the pleurapophysial band is h-regular, often more or less 

 slightly concave, and proceeds obliquely dorsad and postaxiad. 



At the anterior part of this band a slight metapophysial prominence may appear 

 between the preaxial end of the parapophysis and the prezygapophysis. 



The zyga^ophyses closely resemble those of the thkd vertebra. 



The hyperajpophyses on the postzygapophyses are absolutely, and still more relatively, 

 smaller than in the third vertebra, though they are still noticeable prominences (figs. 20, 

 & 23, hp). 



The neui-al spine and the neural arch generally agree with those of the third 

 vertebra, except that the latter is less quadrate when viewed from above. This is 

 owing to the transverse diameter of the preaxial part of the vertebra (measured across 

 the prezygapophyses) considerably exceeding that of its postaxial part (measured across 

 the postzygapophyses), instead of these two dimensions being equal as in the third 

 vertebra. The postaxial margin of the neural arch is also always decidedly concave 

 (fig. 23), instead of being nearly straight as sometimes is the case in the third vertebra. 



The Fifth Vektebba. 



The fifth vertebra, though again a large bone antero-posteriorly, more closely 

 resembles in shape the fourth vertebra than does the fourth the third (fig. 1, C, v). 



The preaxial surface of its centrum quite agrees with that of the fourth vertebra, 

 while its postaxial surface differs only in having its articular surface relatively broader 

 and narrower dorso-ventrally in its middle (the two sides of the hour-glass coming 

 there nearer together), and in having the ventral margin of that surface very consider- 

 ably more extended than its dorsal margin, and, concomitantly, its lateral margins 

 rather more concave. 



The neural surface is similar to that of the centrum of the fourth vertebra, except 

 that it is perhaps rather more exposed at its preaxial end, through the slightly greater 

 cutting away, as it were, of the more preaxial part of the neural arch. 



The ventral surface of the centrum agrees with that of the fourth vertebra, except 

 that its postaxial part is more decidedly and extensively concave transversely, and that 

 the median hypapophysial ridge is stUl less developed. 



The neural arch is, of course, longer than that of the fourth vertebra, but is other- 

 wise very like it. The pleurapophysial band more decidedly fails to attain the post- 

 zygapophysis ; while at the dorsal end of its slightly concave, oblique, postaxial margin 

 a slight pointed process projects postaxiad, which process is the homotype of the slight 

 projection noticed as extending from the postaxial margin of the band in the axis 



