462 H. G. SEELEr ON THE AXIS OF A WBALDEN DINOSAUR. 



middle, but becomes narrower at its two extremities, which are 

 lj inch apart. 



The dentata (figs. 1 and 2, d) projects anteriorly beyond the vertical 

 articular surface for If inch, and apparently for ■£■ inch in front of 



Fig. 2. — Axis of a Dinosaur from the Wealden of Brook. 

 (Front view. Half natural size.) 



a. Wedge-bone articulation. Jc. Neural canal. 

 The other letters as in fig. 1. 



the articular surface for the atlas. The end of the dentata is an arti- 

 cular surface, semicircular, being convex below and flattened above; 

 but it is scarcely rounded enough to be a quarter of a sphere, and 

 has a slight depression in the middle. It is 1^ inch broad, and 

 appears to have joined the basioccipital bone as in birds ; it is mar- 

 gined below by a prominent border, between which and the articu- 

 lation for the atlas is the non-articular surface (fig. 1, c), concave from 

 front to back, |- inch wide on the middle of the underside, and 

 widening laterally to f- inch at the sides, which converge anteriorly 

 towards the median line of the neural canal. The upper surface of 

 the dentata is slightly concave. 



The neural canal (fig. 2, Jc) is large, concave from side to side, and 

 behind the dentata forms a shallow cup-like depression, at the bottom 

 of which are two nutritive foramina : a few other small foramina 

 occur on the neural surface near the anterior termination of the 

 dentata. 



The posterior articular surface of the vertebra (fig. l,e) is trans- 

 versely elliptical ; it is concave in the middle and slightly flattened 

 above. The concave depression is slightly roughened, and resembles 

 that seen in a Plesiosaurian vertebra ; only the deepest part, instead 

 of being a point, is a short transverse line. The surface is 2| inches 

 broad, and 1^ inch deep. Its outer and inferior margin is slightly 

 convex; and below and just behind this convexity runs a sharp 

 slightly elevated border. 



The lengths of the centrum between the anterior and posterior 

 articular surfaces are 2^ inches at the sides, and 2| inches in the 



