94 MAEINE EEPTILES OF THE OXFORD CLAY. 



forms the middle and upper part of the cup for the occipital condyle ; external to this 

 concave surface it is truncated by three oblique facets, one looking downwards and 

 forwards for union with the anterior wedge-bone, and two looking upwards, forwards, 

 and outwards for junction with the lateral (neural) pieces of the atlas. Behind these 

 the side of the odontoid is exposed to a considerable extent, its surface being gently 

 concave in all directions and much roughened anteriorly. Ventrally the bone is more 

 or less overlapped by the posterior angle of the atlantal wedge-bone, and on its postero- 

 lateral angle it bears the anterior portion of the surface of attachment for the head of 

 the second rib, the posterior part being borne on the anterior angle of the centrum 

 of the axis. No second ventral wedge-bone (between the odontoid and the centrum of 

 the axis) has been observed in any specimen. 



In the axis the anterior face of the centrum is usually closely fused with the 

 odontoid, but when fusion has not taken place it can be seen to be fiat or slightly 

 concave and much roughened, as if for a pad of cartilage : it is quadrilateral in 

 outline ; the nearly straight upper and lower borders are parallel with one another, but 

 the latter is the longer, so that the straight or very slightly concave lateral borders 

 diverge a little from above downwards. At their lower ends they are separated from 

 the slightly convex ventral border by the prominences bearing the two facets, which 

 form the posterior halves of the facets (r 2 .f.) for the heads of the second ribs. The 

 sides of the centrum are somewhat concave, and the ventral face somewhat flattened. 

 The posterior articular surface is gently concave and nearly circular in outline, the 

 vertical being a little longer than the transverse diameter. 



The bases of the pedicles of the neural arch {ax. a.) are very long, and, in fact, anteriorly 



they project beyond the centrum of the axis and have a short union with the odontoid ; 



posteriorly they extend to the edge of the axial centrum. The neural arch bears a 



long low neural spine, the form of which differs somewhat in the different species 



(see text-fig. 34). The upper border of the spine is somewhat thickened and roughened 



in front; it slopes gently upwards posteriorly where its edge is thin, and projects a 



little behind the centrum. The anterior zygapophyses are very small ; usually they 



are fused with the posterior zygapophyses of the atlas. The posterior zygapophyses {p.z.) 



are large and their articular surfaces look downwards and outwards ; between their 



bases there is a deep cleft, into which the lower end of the posterior border of the 



neural spine passes as it dies away. On the side of the arch there is a well-defined 



ridge running backwards and gently upwards from the anterior to the posterior 



zygapophysis. A little behind the middle of the base of the neural arch, close to the 



neuro-central suture, there is a small, short, diapophysial process [d.) which seems to 



have been directed downwards and forwards and articulated with the tubercle of the 



second rib, the head of which, as noted above, united with a facet borne partly on the 



odontoid and partly on the centrum of the axis. 



The length of the atlas-axis, the form of the neural spine, and some other points 



