OPHTHALMOSAUEUS. 



43 



vertebrae were perhaps more mobile and more directly affected by definite strains 

 and stresses from the outside. In them the centra are deeper than wide, evenly 

 concave, without rounded borders, and have a neural surface wider than it is in some 

 of the larger vertebrae in front ; at the same time the neurapophysial ridges are less 

 prominent. No pleurapophyses are present in this region. 



In all cases the neural arch is found separated from the centrum clearly in conse- 

 quence of the persistence of cartilage at the junction between the two ; in a few 

 instances it has been possible to refer the arches to the centra to which they actually 

 belonged, but in most cases this is not possible. 



In the first few cervical vertebr;x3 the arch consists of two distinct halves, which 



Left half of the neural arch of an anterior cervical vertebra of Ophihalmosaurus : A, inner side ; 

 B, from front ; C, outer side. (E. 2180, | nat. size.) 



a.s., anterior surface on inner face of neural spine for union with opposite half ; a.z., anterior zygapophysis ; 

 n.f., facet for articulation with centrum ; n.sp., neural spine ; p.s., posterior surface of inner face of 

 spine ; p.z., posterior zygapophysis. 



unite above to form the neural spine, and it seems possible that there may have been 

 a median element in this region, though this is not certain. Each of the two halves 

 (text-fig. 29) consists of the pedicle terminating below in a rounded roughened facet 

 for union with the centrum. This widens out above and bears the anterior and 

 posterior zygapophyses, the former forming a strong prominence, while the latter 

 forms an oblique surface on the postero-ventral border of the spine. This latter is a 

 high compressed plate of bone, the outer face of which is smooth, while the inner bears 

 two surfaces separated by a slight ridge : the anterior suiface is roughened, apparently 

 for union with the other half of the arch ; the other surface indicates that this posterior 

 portion overlaps on to the outer surface of the anterior half of the arch behind. The 



