THE CAMBRIDGE GREENSAND. 595 
is more valuable as an evidence of form of the centrum than might 
at first sight appear, obscured though its surface is with small 
adherent masses of phosphatite, marl, oysters, and the shelly base 
of a Gorgonia-like polypary. 
The striking features of the specimen are :—(1) the evidence that 
the neural arch was supported on pedicles of the centrum (fig. 1, p); 
(2) that the upper tubercle for the rib, usually called a diapophysis, 
was supported on this pedicle, and not on the neural arch itself; (3) 
the form of the odontoid process, which is compressed from below 
upward, and is almost a third of the length of the vertebra ; (4) the 
form of the centrum, which is wide and depressed in front, and 
®narrower, deeper, and subhexagonal behind ; (5) the obliquity of 
the posterior articulation, which extends further backward at its 
ventral than at its neural border ; and (6) the absence of all indica- 
tions of a separate wedge-bone element beneath the anterior articula- 
tion,—the last point being perhaps the most interesting of all, because 
the Upper-Greensand Ichthyosaurs and Plesiosaurs similarly cease 
to have this ossification marked either by form or suture in the axis, 
although it is a characteristic separate element of the skeleton in the 
species which represent those orders in the older Secondary rocks. 
The extreme length of the vertebra is 24 inches. The length 
along its neural border is about 2,3, inches ; “put, owing to the ex- 
cayation of a large vertical space for the atlas below the odontoid 
process, the length along the ventral surface is only 1,8, inch. The 
neural canal is smooth, relatively large, and excavated in the 
centrum as_a half-cylinder, which is ;8; inch wide in the hinder part 
of the vertebra. ; 
The anterior articulation for the atlas is vertical and subreniform, 
concavely impressed below the odontoid process, so that its veniral 
margin was prominent. This concave area is 58, inch wide, and 
is margined laterally by slight eminences, external ‘to which the bone 
is again concave. The odontoid process (fig. 1, 0) is slightly worn 
at its extremity and on its neural surface, but extends forwards in 
a broad wedge shape, also compressed from below upwards to some 
extent. The transverse measurement of the anterior face of the 
vertebra is 12 inch, vue its vertical measurement, from the 
neural canal downwards, i is =% inch. 
The posterior articulation, as already remarked, is inclined for- 
ward (fig. 1, pt), is well cupped concavely, is a regular hexagon in 
outline, and is wider than the part of the centrum anterior to it. 
Its articular margins are a little worn; but it measured fully an inch 
from the Ce to the ventral surface, and each side of the hexagon 
was about 5°, inch long. 
The basal surface appears to have been flat from front to back, 
though its anterior margin is slightly worn. The articulations for 
the rib were placed much as in the Wealden specimen already de- 
scribed (Q. J. G. S. vol. xxxi. p. 461), except that in the Green- 
sand fossil they are both relatively rather higher on the side of the 
centrum, and the parapophysis extends rather nearer to the anterior 
articular margin. 
