AXIAL SKELETON OF THE OSTRICH. 391 
AXIS VERTEBRA (natural size), 
Aspects. 
Fig. 8, lateral ; 9, preaxial ; 10, postaxial ; 11, dorsal ; 12, ventral: 13, preaxial; 14, lateral of immature bone. 
Letters as before, except a2, prezygapophysis; 7s, neural spine; 0, odontoid process; pl, rib-like or pleurapophysial lamella ; 
pz, postzygapophysis ; 7, running backwards and outwards to parapophysis; f, fossa (for ligament) on postaxial aspect of neural 
spine. 
The whole vertebra in the adult consists of one bone. 
The centrum may be considered as regards its four surfaces :— 
The preaxial surface, which joins the postaxial surface of the quasi-body of the atlas, 
presents a crescentic, transversely extended articular surface, more or less strongly 
concave dorso-ventrally, but nearly straight transversely. 
Its ventral margin is slightly convex, generally flattened, and never produced me- 
dianly. 
Its dorsal margin is not easily defined, as the preaxial articular surface of the centrum 
is prolonged dorsally on to each side of the ventral aspect of the root of the odontoid 
process. Its dorsal margin, however, if we count such prolongations as belonging to the 
preaxial surface of the centrum, must be described as strongly concave (figs. 8, 9, @¢). 
The postavial surface differs greatly in shape from the preaxial surface. Its ventral 
part is prolonged ventrad into a hypapophysial process ; but apart from this it exhibits 
an articular surface the outline of which approximates slightly to that of an hour-glass 
(fig. 10, pe). This articular surface is strongly concave dorso-ventrally and strongly 
convex transversely. Its dorsal and ventral margins are both very decidedly concave, 
