CRETACEOUS ICHTHYOSAURS. 233 



Vertebra of Ichthyosaurus Campylodon. 'Enaliosauria,' Plate 7. 



Had no other part of the Ichthyosaurus been discovered in the Chalk Formations 

 than the centrum of a vertebra, that alone would have sufficed to convince the 

 investigator, who had commenced his researches by descending from the more recent 

 to the older Formations, that some marine Saurian had existed totally distinct from any 

 other Reptile the remains of which he might have previously met with in the chalk ; 

 if, indeed, a vertebra so far departing from those of the Reptilia in general had not 

 been mistaken for the vertebra of a Fish. The most fish-like character of the 

 Ichthyosaurus is the deep concavity of both articular extremities of the centrum, 

 fig. 3, and the shortness of the vertebra, fig. 1, as compared with its breadth and 

 height, fig. 2, in which proportion it resembles the vertebrae of the shark tribe. But 

 the peripheral non-articular or free surface of the vertebra is smooth and entire : 

 the articular depressions for the neurapophyses are shallow, and those for the ribs 

 are situated on either one or two tubercles on each side of the centrum. Such pair 

 of costal tubercles would alone suffice to distinguish the vertebra of the Ichthyosaurus 

 from the biconcave vertebra of any fish. All the general characters of the Ichthyo- 

 saurian vertebrae are manifested by the specimen figured in PL 7. 



It was discovered in the same mass of grey chalk at the base of Shakspeare's 

 Cliff as the jaws and teeth figured in PI. 4, and forms with these part of the 

 collection of W. H. Taylor, Esq. It corresponds in its dimensions with those fine 

 fragments of jaw, and might well have formed part of the vertebral column, which 

 supported a head four feet in length. 



The substance of the bone is decomposed, and the surface studded with firmly 

 adherent pyritic matter. It appears to have come from the base of the tail, where the 

 costal tubercles become single. The surface of the articular concavity has the gentle 

 undulating disposition, convex at the periphery, before the deeper central concavity is 

 scooped out, as shown in the section, (PI. 7, fig. 3,) which is common to some other 

 species of Ichthyosaurus ; but no specific character could have been deduced from this 

 fragment of the skeleton. 



The vertebra figured measures 4 inches vertically across the articular concavity; 

 and 1 inch 10 lines longitudinally across the side. A smaller vertebra from the middle 

 of the tail measures 3^ inches transversely, and 1^ inch in antero-posterior extent. 

 The concavity deepens rather suddenly towards the centre. 



Three more or less mutilated bodies of vertebrae, having similar proportions to 

 those of the Ichthyosaurus campylodon from the Dover Chalk, have been obtained from 

 the Upper Green-sand near Cambridge, where they are also associated with teeth of 

 the same species. They are preserved in the cabinet of James Carter, Esq., M.R.CS. 



