AYTAL SKELETON OP THE OSTRICH. 397 



It looks mainly ventrad, but slightly postaxiad also. 



On its doi-sal surface each postzygapophysis bears a prominent hyperapophysis, which, 

 however, is considerably smaller than in the axis vertebra (figs. 15 & 19 hp). 



The neural laminae develop a low neural spine, which, however, extends less both 

 dorsally and antero-posteriorly than does the neurapophysis of the axis. Instead of 

 i-ising 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,/). 



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,ras), 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). 



The Fourth Vektebka. 



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, pc). 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 thu'd vertebra, 

 except that, of course, its absolute length is greater. 



