AXIAL SKELETON OF THE OSTRICH. 397 
It looks mainly ventrad, but slightly postaxiad also. 
On its dorsal surface each postzygapophysis bears a prominent hyperapophysis, which, 
however, is considerably smaller than in the axis vertebra (figs. 15 & 19 hp). 
The neural lamine develop a low neural spine, which, however, extends less both 
dorsally and antero-posteriorly than does the neurapophysis of the axis. Instead of 
rising gradually from the preaxial margin of the neural arch, it springs up suddenly, at 
some distance postaxiad from that preaxial margin (figs. 15 & 19, ns). It subsides less 
suddenly, at about the same distance from the postaxial margin of the neural arch as 
its origin is distant from the preaxial margin of that arch. It occupies rather less 
than the middle three fifths of the dorsal surface of the neural arch. 
The neural spine is excavated medianly on its postaxial surface (a fossa for a liga- 
ment thus being formed) in the same way that the same part of the axis is excavated 
(fig. 18, f). 
The preaxial surface of the neural spine of this third vertebra, however, is also exca- 
vated, and in such a way as to cause that spine to bifurcate preaxially (figs. 15, 17, & 
19,ns), the two preaxial margins of the depression projecting preaxiad at their dorsal ends, 
and making the short preaxial margin of the neural spine concave. 
The neural arch encloses a space of very little different width from that enclosed by 
the neural arch of the axis (fig. 17). 
When viewed above. the neural arch is subquadrate, the transverse diameter of the 
prezygapophyses about equalling that of the postzygapophyses. Its preaxial margin is 
much more strongly concave than is the corresponding margin of the axis ; its posterior 
margin may be slightly concave or nearly straight (fig. 15). 
Tue Fourth VERTEBRA. 
The fourth vertebra exceeds the third about as much as the third exceeds the 
second (counting the odontoid process) ; but in general appearance and arrangement and 
development of parts, the fourth vertebra very much more nearly agrees with the third 
than does the third with the second. 
As to the centrum, its preaxial surface is quite like that of the third vertebra, except 
that it is rather more extended transversely and looks slightly less ventrad (figs. 21 & 
24, ac). 
The postaxial surface only differs from the preaxial surface in the same antithetical 
way that the two corresponding surfaces of the third vertebra differ from each other 
(figs. 20 & 22, pe). It differs from the postaxial surface of the third in that its ventral 
margin is more strongly concave and nearly coincides with the concave ventral margin of 
the articular surface, and in that the transverse extent of the dorsal part of that surface 
is nearly equalled by the transverse extent of its ventral part. 
The neural surface of the centrum closely resembles that of the third vertebra, 
except that, of course, its absolute length is greater. 
