CRETACEOUS FORMATIONS. 7 



depression, of the same relative extent as in fig. 6, Tab. II. The under surface 

 (Tab. Ill, fig. 4) is almost flat, both lengthwise and transversely; the venous 

 outlets present the same relative position, and the non-articular surface of the 

 centrum shows the same degree of smoothness and flatness as in the smaller ver- 

 tebra? (Tab. I, figs. 2, 6). The present centrum belongs to the same species o£ Ple- 

 siosaurus as those of the more regular elliptical form, and is merely indicative of a 

 diffei'ent position in the region of the neck. 



A centrum with the surface much abraded (Tab. Ill, fig. 8) appears to have 

 presented the same inferior expansion, and consequent triangular form, as fig. ^; 

 but in the under surface (fig. 9) the venous canals have opened into well-marked 

 depressions. Other differences, as in the character of the neurapophysial surfaces 

 (fig. 7, np), may be due to the degree of abrasion to which the present fossil has 

 been subject. 



Plesiosaurus Bernardi, Owen. Cervical Vertebrae. Tab. IV. 



In my ' Monograph of the Fossil Reptilia of the Cretaceous Formations,' 

 Volume of the Palseontographical Society for 1851, p. 60, I characterised aspecies 

 of Plesiosaurus from a cervical vertebra then in the museum of my esteemed 

 friend, Frederic Dixon, Esq., of Worthing, under the name of Plesiosaurus 

 Bernardi, which vertebra v, as figured in Plate XVIII of the above-cited Mono- 

 graph. I have subsequently had the opportunity of examining several other 

 vertebra? of a Plesiosaurus from the Green-sand of Reach, near Cambridge, which 

 are referable to the same species, but most of them to an individual of smaller 

 size, and probably of immature age. 



The specimen (Tab. IV, figs. 1, 2, 3, 4) is an anterior cervical vertebra, which 

 agrees with the more posterior one above figured in the degree of concavity of 

 the articular surfaces of the centrum, in the extent of the peripheral border of 

 that cavity, which is convexly bevelled off ("evase"), and in the relative position 

 of the neur- and pleur-apophyses ; the breadth of the centrum is not. so much 

 greater proportionally to the length ; but this difference I believe to be due to the 

 more anterior position in the vertebral series from which the present specimen 

 has been derived. 



The neurapophysial depression (np) is deep and smooth, encroaching further on 

 the convex border of the centrum at its back than at its fore part ; they are 

 divided at the upper surface of the centrum by a neural tract (fig. 3, n ), about 2 

 lines broad at its narrowest part. The non-articular surface of the centrum is 

 moderately smooth, especially at the sides between the neur- (np) and pleur- (pi) 



