26 



TBETIAEY VEETEBEATA OF THE FAT'&M. 



Elephas also the upper ends of the surfaces for the axis project inwards so as 

 to form a constriction of the opening of the vertebra, separating the neural canal 

 from the lower part occupied by the odontoid process of the axis. 



In the axis vertebra (text-fig. 9) the odontoid process [od.p.) 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-%. 9. 



Ax4 



n^p. 



Bxi 



Axis vertebra of Arsinoitherium zitteli : A, from front ; B, from right side. 



at., surface for atlas vertebra ; n.sp., neural spine ; od.p., odontoid process ; p.z., posterior zygapophyses ; 

 t.p., diapophysis and parapopbysis forming transverse process; r.c, vertebrarterial canal; v./ac, ventral 



facet of odontoid process. 



I 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 spine (n.sp.) 

 is represented by a great tuberous mass of bone, which forms a single median 

 prominence on the front of the arch, but posteriorly is divided into two by a large 

 median depression. The posterior zygapophyses (p-z.) are large ; their upper surface 

 is roughened, and their articular facets consist of a larger, nearly circular, 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 upper 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 



