224- PALEONTOLOGY 



of different species, Lave since been discovered, fully confirm- 

 ing the sagacious restorations by the original discoverers of 

 the Plesiosaurus. 



Vertebral Column. — The vertebral bodies have their ter- 

 minal articular surfaces either flat or slightly concave, or with 

 the middle of such cavity a little convex. In general the 

 bodies present two pits and holes at their mider part. The 

 cervical vertebra? consist of centrum, neural arch, and pleur- 

 apophyses. The latter are wanting in the first vertebra ; but 

 both this and the second have the hypapophyses. The cervical 

 ribs are short, and expand at their free end, so as to have 

 suggested the term " hatchet-bones " to their first discoverers. 

 They articulate by a simple head to a shallow pit, which is 

 rarely supported on a process, from the side of the centrum ; 

 but is commonly bisected by a longitudinal groove, a rudi- 

 mental indication of the upper and lower processes which 

 sustain the cervical ribs in Crocodilia. 



The body of the atlas articulates with a large hypapophysis 

 below, with the neurapophysis above, with the body of the 

 axis behind, and with part of the occipital condyle in front ; 

 all the articulations save the last become, in Plcsiosaurus 

 'pacliyomus, and probably with age in other species, obliterated 

 by anchylosis. The hypapophysis forms the lower two-thirds, 

 the neurapophysis contributes the upper and lateral parts, and 

 the centrum forms the middle or bottom of the cup for the 

 occipital condyle. The second hypapophysis is lodged in the 

 inferior interspace between the bodies of the atlas and axis ; 

 it becomes anchylosed to these and to the first hypapophysis. 

 The first pleurapophysis, or rudimental rib, is developed from 

 the centrum of the axis. 



As the cervical vertebrae approach the dorsal, the lower 

 part of the costal pit becomes smaller, the upper part larger, 

 until it forms the whole surface, gradually rising from the 

 centrum to the neurapophysis (fig. 71). 



