PELONEUSTES. 



51 



widens out and projects forwards over the body of the atlas, bearing on its upper face 

 the zygapophysial facets, with which the hinder ends of the two halves of the arch of 

 the atlas articulate. From these surfaces a ridge runs upwards and backwards, and 

 forms the outer edge of the posterior zygapophyses which project considerably behind 

 the posterior surface of the centrum. The neural spine (n.s.) is low and stout, it slopes 

 backwards, its anterior edge being convex, the posterior slightly concave ; the upper 

 end is considerably thickened. In the succeeding cervicals (text-fig. 17) the neural 

 arch is similar, uniting with the whole length of the centrum and, in some cases, even 

 forming a small portion of the upper edge of the articular surface. Both anterior and 



Middle cervical vertebra of Peloneustes pMlarchus, from left side. (R. 3313, § nat. size.) 

 a.z., anterior zygapophysis ; n.s., neural spine ; p.z., posterior zygapophysis ; )•./., facets for ribs. 



posterior zygapophyses are large, and their articular surfaces are flat and oval in 

 outline ; the anterior pair are directed somewhat upwards, the posterior downwards, 

 and in each case they project considerably beyond the centrum. The base of the neural 

 spine (n.s.) extends the whole length of the arch. The lower part of the anterior edge 

 is sharp, but towards the top it thickens and becomes roughened ; at the same time 

 the width of the spine decreases. In the third vertebra, the spine (n.s.) is short and 



it 2 



