LTASSIC FORMATIONS. 
91 
The articular bases of the neurapophyses undergo corresponding modifications ; the 
joint-surface is subtriangular, somewhat protuberant in the anterior vertebrae ; but, after 
the diapophysis or rib-surface gets independent, that for the neurapophysis becomes 
longitudinal, narrow, and grooved. The neurapophysis rises, with a slight receding from 
the vertical position, for a height usually equalling its fore-and-aft breadth ; it develops a 
short prezygapophysis and inclines backward, with a postzygapophysial surface at 
its under and hinder part. The pair of neurapophyses having then coalesced send 
upward and slightly backward a subquadrate compressed neural spine, usually twice the 
height of the the subzygapophysial part or pedicle (ib., tig. 6, n) of the neurapophysis, and 
with gradually augmenting height and antero-posterior breadth as far as the midpart of 
the trunk. Towards the hind part the spines begin to lose height, but not breadth, until 
they enter the caudal region, when they gradually decrease in all dimensions, and disappear 
at or near the bend of the tail. Feeble emarginations at the fore and hind part of the 
pedicle form or bound the nerve-outlets. 
The contour of the centrum (PI. XXII) varies with the number and position of its 
lateral processes. At the fore part of the column it is more or less shield-shaped (fig. 3), 
with the angles of the upper border rounded off; at the hind part, where the rib-processes 
have descended (fig. 8) or have coalesced (fig. 11, dp), the base is below and the apex trun- 
cate for the neural arch ; further on, where those processes have disappeared, the contour 
becomes ellipsoid or elliptic, with the long axis vertical. 
The centrum is always short in proportion to its breadth and depth, but this varies 
in different species ; beyond the atlas and axis it is always biconcave (ib., fig. 6), but the 
contour of the concavity varies specifically. In some the sinking begins at the periphery ; 
in others in a feebler degree there ; in others a slight and narrow marginal convexity (fig. 
3) precedes or leads to the central concavity ; in others, again (fig. 8), a peripheral portion 
of the joint-surface is flat before it sinks into the central hollow ; in exceptional instances, 
the fore and hind concavities blend at a small central perforation, as they do in the 
Triassic ‘ Tretospondilia ’ of the Cape.’^ 
The pleurapophyses (dorsal ribs, PI. XXI, fig. 2, pi) are developed, as free movable 
elements, over a larger proportion of the vertebral column than in most other four-limbed 
Beptilia, extinct or existing (PI. XXVIII, fig. 1). They commence at the foremost seg- 
ment as shown in the description of the ‘ atlas ’ (PI. XXIII, fig. 5, p,i), gain slightly in 
length on the axis, in a greater degree on the third vertebra, and acquire their extreme 
length between the tenth and thirtieth (PI. XXI, fig. 2, pi, and PL XXVIII, fig. 1); 
beyond this they shorten, but continue as free elements, though short and straight 
(PI. XXI, fig. 4), along a great part of the caudal region ; their existence, as such, being- 
attested in detached centrums by the single sessile process (PI. XXII, figs. 9 — \l,dp) 
on each side : with the disappearance of the di-parapophysis the ribs cease. All the ribs 
^ ‘ Quarterly Journal of the Geological Society,’ vol. s.xxii, p. 43 (1875). 
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