CRETACEOUS FORMATIONS. 5 



probable one, although in some species of Plesiosaurus, as e.g. the present, the 

 " hatchet bones " or cervical ribs might only commence on the third or fourth 

 vertebra, beyond the coalesced atlas and axis. As a general rule, they begin on 

 the second cervical. 



Thus, the characters of the Plesiosaurus planus are exemplified, so far as they 

 are shown by the vertebral centres, from all the chief regions or parts of that 

 column. The majority of the vertebrae which have served for the comparisons 

 and illustrations leading to the above-given information as to the species, have 

 been kindly confided to ine for that purpose by the Rev. Adam Sedgwick, F.R.S.j 

 Woodwardian Professor in the University of Cambridge. 



These vertebrae differ from those of all the previously described species of 

 Cretaceous Plesiosaurs in the proportions of breadth to length, especially in the 

 cervical region, and in the flatness of the terminal articular and of some other 

 surfaces of the centrum. 



Several larger vertebrae have reached me singly, as though from a more 

 scattered disposition of parts of the dislocated skeleton in the phosphatic Green- 

 sand bed of Cambridgeshire, which agree in character with the Plesiosaurus planus. 



In the Plesiosaurus pachyomus the centrum increases in breadth as it approaches 

 the back, whilst some of the dorsal vertebrae offer almost the same proportions as 

 those in the above-described series. 



But the difference in the corresponding cervical vertebrae is very striking, as is 

 exemplified in the following comparative admeasurements. 



Admeasurements of vertebral centrum : — 



Anterior cervical. Middle cervical. 



PI. pachyomus. PL planus. PI. pachyomus. PI. planus. 

 In. Hues. In. lines. In. lines. In. lines. 



Antero-posterior diameter or length . 19 . Oil . 2 . 13 



Transverse diameter or breadth . 23. 110 . 23 . 15 



Vertical diameter or height . . 19 . 11 . 23 . 13 



The centrum (Tab. II, figs. 1 and 2) is of a vertebra from the posterior part of 

 the neck. The anterior articular surface presents a transversely elongate elliptical 

 form (fig. 1), contrasting with the almost circular contour of the same part in 

 Plesiosaurus pachyomus (Monogr. 1851, Tab. XX, fig. 2). It is very slightly, but 

 uniformly, concave. The neurapophysial pits (fig. 2, np), of a triangular form, and 

 coextensive with the fore-and-aft extent of the centrum, are divided by a neural 

 canal (fig. 2, n), of about 4 lines in breadth, and their lower angle, which is rounded 

 off, projects from the side of the centrum, which is not the case in Plesiosaurus 

 pachyomus. The costal pit (Tab. I, fig. I, pi) is much smaller than in Plesiosaurus 



