AND PELVIC BONES OF PLTOSAURUS EVANSI. 



717 



bone is deeply concave from side to side, nearly flat from within 

 outward, with the articular surface directed outward and downward 

 so as to meet the inferior external surface in a rounded border. Its 

 antero-posterior extent in the middle is 1^ inch, but becomes less 

 laterally. The line in which it joins the atlas is concave, but is in 

 a vertical plane. 



On each side the wedge bone terminates superiorly in a flattened 

 triangular surface, to which were attached the bases of the neural- 

 arch elements. The neural-arch bones are not preserved. Each 

 had a broad union with the atlas, which widens laterally (b, fig. 2) 

 from | inch below to the entire width of the centrum superiorly. 

 These surfaces look obliquely outward, forward, and upward, and 

 therefore have a tendency to converge upward and forward, where 

 they are -Z- inch apart at the anterior margin of the neural canal (c?, 

 fig. 1). 



Fig. 1. — Anterior aspect Fig. 2. — Left side view of Atlas 



of Atlas, | nat. size. 



and A.vis, ^ nat. size. 



L e 



Articular face of centrum. c. 



Surface from which the neural d. 



arch has come away. e. 



Subvertebral wedge-bone. 

 Base of neural canal. 

 Articulation for rib. 



The neural surface is 1^ inch wide posteriorly, slightly concave 

 from side to side, and has vascular foramina on its lateral margins — 

 three conspicuous foramina on the left side, and smaller foramina 

 on the right side. 



The anterior face of the centrum (a) is necessarily small and four- 

 sided ; but it has a subtriangular aspect, owing to the shortness of the 

 superior margin of the neural canal and the width of the wedge bone 

 forming its curved base. It is concave and deepest in the centre, 

 though not much impressed. 



The extreme width of the centrum, just behind the neurapophysial 

 facets, is rather less than 3| inches. The lateral non-articular 

 surfaces of the centrum (fig. 2) are below theso points ; they are 

 small and subtriangular, about -J inch from side to side, and 1| 

 inch from above downward. 



