CRETACEOUS FORMATIONS. 



Vertebrae of Ichthyosaurus Campylodon. T. XXII. 



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 which he might have 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 Ichthyosaurian vertebrce are manifested by the specimen 

 figured in T. XXII. 



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

 Cliff as the jaws and teeth figured in T. XXIII, 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, wh'ch 

 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, (T. XXII, 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 campyhdon 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., MiR.CS. 



