38 MAEINE EEPTILES OF THE OXFOED CLAY. 



downwards and forwards and marking the position of attachment of the anterior sub- 

 vertebral wedge-bone. The upper border of the centrum is divided into three slight 

 concavities, the median one forming the floor of the neural canal {n.c), the anterior 

 border of which is here usually rounded off, while the two lateral concavities {n.f.) are 

 the surfaces for union with the neural arch. These latter surfaces are continuous with 

 the diapophyses, the degree of development of which, as well as of the parapophyses, 

 varies greatly in different individuals and even on opposite sides of the same atlas. 

 Thus the diapophysis (d., d!) may be a clearly-defined, elongated, triangular surface, 

 and the parapophysis Qj., jo.') an equally clearly-marked round tubercle, situated on 

 the side of the centrum, nearer its posterior than its anterior edge : when this is so, 

 the arrangement of the surfaces on the axis is similar (see text-fig. 24, F). In other 

 specimens the parapophysis of the axis seem to have shifted forwards to the anterior 

 edge of the centrum (text-fig. 24, D), and may unite with that of the atlas to form a 

 single bony prominence. In some young examples the parapophysis may be confluent 

 with the diapophysis. 



As already mentioned, the axis is already fused with the atlas in the youngest 

 specimens found, and no trace of the line of junction remains in the region forming 

 the floor of the neural canal, though its position may be marked by a groove round the 

 remainder of tlie centrum. The surfaces for the neural arch are separated from the 

 corresponding facets of the atlas by transverse ridges, and are continuous externally 

 with the diapophyses. As above noticed, the parapophysis may be confluent with 

 that of the atlas, but more frequently is separable and situated near the posterior 

 border of the centrum. Tlie posterior face of this vertebra is roughly triangular and 

 deeply concave : in some specimens there is a slight oblique truncation of the lower 

 angle {w.h.f!), which probably indicates that a subvertebral wedge-bone between this 

 and the third cervical was present ; but neither it nor either of the anterior wedgo- 

 bones has been seen in situ, although that between the skull and the atlas was 

 certainly present and that between the atlas and axis probably so. In the atlas and 

 axis, as in the centra of the succeeding vertebras, very great variability in length is 

 observable in different individuals, and this is often so marked that it might easily 

 be regarded as a character of specific value. Further examination shows that these 

 differences are merely due to compression, the centra being frequently telescoped, as it 

 were, till they may be little more than half their original length (see text-fig. 31, A, B), 

 often at the same time showing little or no distortion in other directions. This seems 

 to be the result of the giving way of the inner spongy tissue. Apart, however, from 

 differences of form that can be thus explained, the variability in the shape of the 

 vertebrae is very considerable ; but since intermediate forms between the extremes are 

 usually to be found, it appears most probable that these differences may be regarded 

 merely as individual variations, such as might easily occur in an imperfectly ossified 

 skeleton in which much cartilage persisted throughout life. 



