OSTEOLOGY OF POLYGLYPHANODON—GILMORE 245 
Atlas.—The atlas is present only in specimen U.S.N.M. No. 15816, and 
as it remains in the original articulated position it is not available for 
detailed study. The neurapophyses are seen to arch over the neural 
canal, though separate on the median line, and a backward-projecting 
process carries the posterior zygapophysis for articulation with the 
axis. All other parts of this bone remain hidden in the matrix. 
Awxis.—The detailed description of the axis is to a great extent based 
on that of the type, only the incomplete spine being restored after the 
axis of U.S.N.M. No. 15816, as shown in figure 23. This element is also 
present in specimens U.S.N.M. Nos. 15566 and 15568, but both are less 
well preserved. 
type, natural size: A, Lateral view; B, ventral view. a, Axis; 0, odontoid; 3 and 4, third 
and fourth cervicals, respectively. 
In the type the axis was found articulated with the third and fourth 
cervicals, and a detached vertebra appears to represent the fifth. The 
anterior end of the centrum of the axis is broadly expanded, with the 
odontoid of the atlas completely fused with it. The articular surface 
of this end is broadly rounded from side to side and is much deeper 
dorsoventrally than the axis of an /guana of corresponding size. This 
face in the type has a greatest width of 11.25 mm. and a greatest depth 
of 8.75 mm., and the length of the centrum is 13.5 mm. Viewed from 
below, in front of the ball the centrum is regularly rounded from side 
to side, but the forward half is pinched together to form a sharp-edged 
process that projects downward on the median line (see B, fig. 238). 
This process may represent the fused axis intercentrum, although no 
trace of the line of coalescence is now discernible in any of the specimens. 
A small, rounded, spurlike, backward-directed diapophysial process is 
present on each side of the upper anterior end of the centrum. 
Prezygapophyses and postzygapophyses are well developed. The 
spinous process is incomplete in the type and has been restored in figure 
23 from U.S.N.M. No. 15816. It resembles the neural spine of Jguana. 
465455—42 
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