CRETACEOUS FORMATIONS. 9 



Plesiosaurus Bernardi. 

 In. lines. 

 Antero-posterior diameter or length ....... I 3 



Transverse diameter or breadth ....... 1 7 



Vertical diameter or height ........ 1 4 



In a more posterior cervical vertebra, from the same Russian locality, the 

 terminal articular surfaces are deeper towards the centre, with the out-turned or 

 " evase " borders very thick. The base of the neurapophysis was here also par- 

 tially anchylosed, and the rib more completely so ; it presented a rhomboid form, 

 being inclined backward as well as outward, with the anterior angle rounded, and 

 the posterior one produced. The inferior medial ridge was well marked. The 

 breadth of the centrum was relatively greater than in the preceding vertebra. 



In the vertebrae from the Cambridge Green-sand (Tab. IV, figs. 9 and 10), which 

 have succeeded one another from about the same part of the neck, anchylosis of 

 the pleurapophysis has not been completed ; but that of the neurapophysis (np) has 

 been so to a degree sufficient for preserving their base in connection with the 

 centrum, although the summit has undergone fracture. The line of suture is, 

 however, very distinct. 



The terminal surface of the centrum presents the same degree of concavity, with 

 a slight central horizontal linear depression, Tab. V, fig. 1 , as shown in Tab. IV, fig. 8. 

 The base of the neurapophysis (np) extends to the anterior margin of the centrum, but 

 not quite to the posterior one. The outer surface of the neurapophysis presents a 

 low obtuse ridge or rising, extending from near the infero-posterior angle to the 

 outer side of the prezygapophysis (Tab. IV, figs. 9, 10, 11, z) ; the aspect of the arti- 

 cular surface of this process is obliquely upward and inward. The posterior border 

 of the neurapophysis is thicker, or more obtuse, than the anterior one ; the 

 internal surface is smooth and even. Rather less than the vertical diameter of 

 the pleurapophysial pit (figs. 10 and 11, pi) intervenes between it and the base of 

 the neurapophysis {np)- The inferior surface of the centrum presents the ridge 

 between the two depressions into which the venous vertical canals open. 



In the vertebra (Tab. IV, fig. 11), from a more posterior part of the neck, or 

 from a larger Plesiomurus, a greater proportion of the neural arch {np) is preserved, 

 partially anchylosed to the centrum ; the sides are strengthened by the same 

 oblique thickening, extending to the prezygapophysis ( z ) ; this is larger than the 

 postzygapophysis (2'), and the breadth of the arch across the prezygapophyses is 

 nearly twice that across the posterior pair (Tab. V, fig. 6). The neural spine 

 appears to have been a thin plate ; its base (Tab. V, fig. 6) extends from the notch 

 between the postzygapophyses («') to within 3 lines of that between the prezyga- 

 pophyses ( z ). This vertebra has been compressed laterally, and rather obliquely, 

 by posthumous pressure; yet under such general support that the neural arch, 



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