PEOM THE KIMMEEIDGE CLAY. 449 



-whicli meets the chevron-facet at an acute angle. (I have obtained 

 similar caudal vertebrse in the Isle of Wight.) The sides of the 

 centrum are vertically slightly convex. The narrow under surface is 

 more encroached on by the anterior than by the posterior chevron- 

 articulation. The transverse process arises rather lower than in 

 JSTo. 28. The spinous process has a strong backward slant. 



Through the next four vertebrae, ISTos. 30-33, the transverse 

 process sinks on the arch towards the neuro-central suture, and it 

 also becomes smaller. In No. 30 its size is greatly reduced, and 

 it arises in the plane of the neuro-central suture, which is very 

 indistinct. In No. 35 a slight swelling is the only vestige of the 

 transverse process; and even this is absent from the succeeding 

 vertebrae. 



The suppression of the transverse process is soon followed by the 

 disappearance of the neuro-central suture, which ceases to be 

 recognizable in No. 37. Of the succeeding centra many have been, 

 much squeezed in by laterally applied pressure. To this they have 

 yielded in such a manner as to suggest that the middle of the 

 -centrum was very imperfectly ossified, and perhaps permanently 

 cartilaginous, as was thought characteristic of the caudal vertebrae 

 referred to PoiJcilopleuron, but is now known to obtain in Mega- 

 losaurus. In No. 37, the 10th in the caudal series, the posterior 

 chevron-facet is notched in front ; and from this notch a slight groove 

 passes forwards along the under surface of the centrum. 



In No. 41 (PI. XIX. fig. 11) the anterior chevron-facet has almost 

 disappeared. On the side of the centrum is an angular longitudinal 

 ridge ; between this and the situation of the neuro-central suture 

 is a shallow depression ; and below the ridge a small better-marked 

 hollow. The ridge is gradually lost in the succeeding centra. The re- 

 duction in bulk is attended with diminution of length of the centrum. 



In the smallest vertebrae, towards the end of the tail, the centrum 

 has a simple cylindroid form (PI. XIX. figs. 12, 13). In these the 

 arch is reduced to extreme simplicity, a mere hoop bearing an 

 anterior and posterior pair of articular processes. The spinous 

 process ceases in No. 51. A slight rugosity marks the situation of 

 the chevron-joint, so conspicuous in the first half of the tail. The 

 anterior chevron-facet first disappears. Both articular surfaces in all 

 the caudal vertebrae are concave. 



The change in form of the articular surfaces of the vertebral centra, 

 traceable through the column, is highly instructive. In the neck 

 these surfaces are convexo-concave, opisthocoelous ; at the root of the 

 neck the anterior ball is less convex, the posterior cup less deep ; in 

 the fore-trunk the anterior surface is plane, the posterior slightly 

 concave ; in the loins the anterior surface is very slightly concave, 

 the posterior surface more so; and in the tail both surfaces are 

 concave. Variation in length of centrum, shown by the annexed 

 measurements, is not less worthy of notice ; for it had been asserted 

 that the length of centrum was constant for the same vertebral 

 column, although this is not borne out by the skeletons of extant 

 reptiles. 



Q, J.G. S. No. 143. 2ii 



