184 ]\Ir. IT, G. Seeley on a new Species o/'Plesiosaurus. 



the centrum and the neural arch the characters are Plesio- 

 saurian. 



The height of the neural arch from the suture to the summit 

 of the spine, measured at the side, is 4f inches. The height 

 from the base of the centrum to the posterior zygapophysis is 

 3 inches. These articular facets are flat, and look downward 

 and outward. A moderately elevated transverse ridge on each 

 side connects them with the anterior zygapophyses. The 

 neural spine may be f of an inch thick where thickest, but it 

 is compressed to a sharp edge at both the anterior and poste- 

 rior margins ; towards its base it measures If inch from front 

 to back; but it naiTOws to about 1^ inch at the free end, 

 which is truncated so as to be convex from front to back, and 

 parallel to the base of the centnim. 



The neural canal is small, ovate, broader than high, being 

 about ^ an inch high and | of an inch broad. The greatest 

 width of the neural arch, from side to side in front across the 

 neural canal, is If inch. 



The distance, at the side of the vertebra, from the base of the 

 neural arch to the facet for the rib is | of an inch. It is diffi- 

 cult to refer the specimen to its coiTcct position in the neck ; 

 but I regard it as a late cervical, probably about the 7th from 

 the end of the neck. 



The species appears to be distinct from any yet described — 

 though, from tlie uncertainty of its exact position in the neck, 

 the specific value of its characters cannot be accm-ately esti- 

 mated. The species to which it approaches most closely is 

 PL megadeiriis, from which it appears to differ in the centrum 

 being longer and flatter on the articular surface, with a larger 

 lateral nuirgin to the articulation and a relatively shorter arti- 

 culation for the cervical rib, which is placed further from the 

 anterior margin of the centrum. 



In the 31st cervical vertebra of PL megadeirus the measure- 

 ments of the centrum are : — 



Length of centnim at its base 2 inches. 



Length of centrum through the centre. ... 1| inch. 

 Width of centrum over posterior articular 



surface 3i inches. 



Width of centrum in front 3 „ 



Depth of centrum 2h „ 



The corresponding measurements of tliis vertebra are : — 



Length of centrum 2 inches. 



Width of centrum 2| „ 



Depth of centnim 2 ., 



