26 
TERTIAIJY VERTERRATA OF THE FATt)M. 
Elephas also the upper ends of the surfaces for the axis project inwards so as 
to form a constriction of tlic opening of tlie vertebra, separating the neural canal 
from the lower ])art occiphed by the odontoid process of the axis. 
In the axis vertebra (text-fig. 9) the odontoid process {od.jJ.) is blunt and rounded, 
and its base extends from the upper to the lower surface of the centrum. Its 
extreme end is rounded, with a slight ridge on its upper surface. The posterior 
half of its lower surface is occupied by a sharply-defined surface for articulation 
with the ventral bar of the atlas [v.fac .) ; this surface is gently convex from side 
to side. On either side of the odontoid there is a deep groove separating it from 
the main surfaces of articulation for the atlas {at.). These are roughly triangular 
in outline, the angles being rounded ; they are inclined to the long axis of the 
column at an angle of about 45°, and are gently convex except close to the inferior 
Text-fig. 9. 
Axis vertebra of Arsi/ioitJierium ziiteli : A, from front ; B, from right side. 
at., surface for atlas vertebra ; n.sp., neural spine ; odontoid process ; p.r., posterior zygapopliyses ; 
t.p., diapophysis and parapophysis forming transverse process ; v.c., vertebrarterial canal ; v.fac., ventral 
facet of odontoid process. 5 nat. size. 
angle, where there is a slight concavity. The upper edges of these atlantal surfaces 
are separated from the base of the neural arch by a deep groove. The pedicles of 
the arch are wide, and its upper portion is very massive ; the neural s})ine (u.sp.) 
is represented by a great tuberous mass of bone, which forms a single median 
prominence on the front of tlie arch, but posteriorly is divided into two by a large 
median depression. The posterior zygapopliyses (yi.2.) arc large ; their upper surface 
is roughened, and their articular facets consist of a larger, nearly circnlar, outer 
portion, looking nearly directly downwards, and a smaller inner face looking inwards 
and downwards. The posterior face of the centrum is very large, and its up})er border 
sometimes forms a prominent rim rising somewhat above the level of the rest of 
the upper surface. It is very slightly concave and is oval in outline, with its 
