CRETACEOUS ENALIOSAURS. 457 



The crown of the teeth of Plesiosaurus is, moreover, one wliich that of 

 the teeth of Polyptychodon (fig. 3) resembles in the ridged enamelled surface and 

 subcircular transverse section ; but the teeth of true Plcsiosauri are proportionally 

 longer and more slender, whilst those oi Polyptychodon in the proportions of the crown 

 more resemble the teeth of the crocodilian genera Go/iiopho/is and Madrimosaurus. 



The microscopic structure agrees equally with the plesiosauroid and cro- 

 codilian modifications of the dental tissues. In PI. 14, fig. 3, j shows the shape of 

 the base of the deeply implanted tooth, at the part where it had been broken in 

 one of the specimens (a), accompanying the portion of cranium from the Lower 

 Chalk at Frome. Fig. 3 is a more entire tooth of the same individual. 



Cervical Vertebra {Enallosauria, PI. 31, figs. 1 and 2). 



I next proceed to offer other evidences tending to show the affinity of Poly- 

 ptychodon to Plesiosaurus. In the Upper Green-sand deposits near Cambridge and 

 in the Neocomian formations of similar age at Kursk, south of Moscow, large 

 vertebrae of the Plesiosauroid type have been discovered, together with teeth of 

 Polyptychodon, which vertebrae I believe to belong to that genus. 



The centrum of a cervical vertebra, from the Cambridgeshire Upper Green-sand 

 (figured in Enaliosauria, PI. 31, figs. 1 and 2), measures 4 inches 3 lines in length, 5 

 inches 3 lines across the terminal articular surface, and 7 inches in total breadth, 

 including the transverse processes (;)/, pi). Each of these projects about an inch 

 from the side, rather nearer the fore than the back part, of the vertebra, and 

 terminates in a flattened surface for the ligamentous articulation of the cervical rib, 

 which surface measures 2 inches 3 lines by 2 inches in its diameters (fig. 1, jil)' 

 The articular surfaces of the centrum are nearly flat. 



This vertebra, with which no other teeth save those of Polyptychodon, 

 from the same formation and locality, agree in size, thus presents the essential 

 characters of the neck-vertebrte o{ Nothosaurus and Plesiosaurus, and must be referred 

 to the order Sauropteryyia.* The specimen is preserved in the Woodwardian 

 Museum at Cambridge. It was obtained from the Green-sand phosphatic-nodule 

 works at Haslingfield, about four miles from the town of Cambridge. 



In a collection of Upper Greed-sand fossils from the vicinity of that town, 

 lately purchased by the British Museum, there is the centrum of a dorsal vertebra 

 of corresponding dimensions. It presents the usual characters of the Plesiosauroids ; 

 the articular ends are very slightly concave, with a moderate prominence in the 

 middle, of a subcircular form, about the size of a crown-piece. The sides are gently 

 concave lengthwise; the under surface is so in a less degree; this non-articular 



* See the " Classification of Eeptilia," ' Reports of the British Association,' 1859, p. 159, and Owen's 

 ' Palaeontology,' 8vo, 1860, p. 209. 



