LIASSIC ICHTHYOSAURS. 49 



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 (Tab. XVIII) 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, Tab. XVII, fig. 3, i^l) are developed, as free movable 

 elements, over a larger proportion of the vertebral column than in most other four-limbed 

 Meptilia, extinct or existing (Tab. XXIV, fig. 1). They commence at the foremost seg- 

 ment as shown in the description of the ' atlas ' (Tab. XIX, 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 (Tab. XVII, fig. 2, vh and Tab. XXIV, fig. 1) ; 

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

 XVII, fig. 4), along a great part of the caudal region ; their existence, as such, being 

 attested in detached centrums by the single sessile process (Tab. XVIII, figs. 9 — 11, dp) 

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

 are comparatively slender, commonly subcylindrical ; the longer ones, or those of the trunk, 

 are longitudinally grooved on their outer surface (Tab. XVII, fig. 7), as if each rib con- 

 sisted of two confluent more slender ones.^ This structure is not common to all the 

 species. A transverse section which I made exposed a small central cavity. 



The haemal or costal arch is complete along the major part of the trunk. Here the 

 hsemapophyses are each in two partially overlapping pieces (ib., fig. 2, h, K) ; they are more 

 slender than the pleurapophyses.* The median piece (haemal spine, u) is transversely 

 extended, symmetrical, slightly produced at its thickest midpart, forward and backward, 

 but more extended laterally, there becoming slender and diminishing to a point. A 

 similar slender piece, pointed at both ends (A'), is spliced as it were, to the fore part of each 



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



2 Clift, 'Phil. Trans.,' mdcccxiv, pi. xix (1814, Ichthyosaurus platyodon). 

 8 lb., 'Phil. Trans.,' mdcccxix, pi. xiv (1819, Ichthyosaurus communis). 



7 



