22 FOSSIL REPTILIA OF THE 



the length of the protruding summits of the new teeth before they displace the 

 old, when they are pushed, causing absorption of the intervening osseous bar, 

 into the large sockets of the teeth they replace. 



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

 the teeth of Polyptychodon (fig. 3) resembles in the ridged enamelled sur- 

 face and sub-circular transverse section ; but the teeth of true Plesiosauri are pro- 

 portionally longer and more slender, whilst those of Polyptychodon in the 

 proportions of the crown more resemble the teeth of the crocodilian genera 

 Goniopholis and Madrimosaurus. 



The microscopic structure agrees equally with the plesiosauroid and cro- 

 codilian modifications of the dental tissues. In Tab. I, fig. 3, b shows the shape 

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

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

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



Cervical Vertebra (Tab. V, 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 vertebras I believe to belong to that genus. 



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

 sand (figured in Tab. V, 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, 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, pi). 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-vertebras of Nothosaurus and Plesiosaurus, and must 

 be referred to the order Sauropterygia.* The specimen is preserved in the 

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



* See the "Classification of Reptilia," 'Reports of the British Association,' 1859, p. 159, and ' Pake- 

 ontology,' 8vo, 1S60, p. 209. 



