LIASSIC 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., fig. 6, n) of the neurapophysis, and 

 with gradually augmenting height and antcro-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 r 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. 1 



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 

 Reptilia, 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, pi), 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 (PL XXI, fig. 2, P l, and PL XXVIII, fig. 1) ; 

 beyond this they shorten, but continue as free elements, though short and straight 

 (PL 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 — 11, dp) 

 on each side : with the disappearance of the di-parapophysis the ribs cease. All the ribs 



1 ' Quarterly Journal of the Geological Society,' vol. xxxii, p. 43 (1875). 



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