Sterjium, and Shoulder -Girdle o/^pyornis. 383 



closer examination shows that this is almost certainly not so. 

 In the first place, the two halves of the posterior horder are 

 symmetrical, which would hardly be the case if the ontliue 

 was the result of accidental fractures. Moreover, for about 

 25 mm. in the middle line the hinder border forms a gently 

 concave curve, with a sharp clean edge, clearly the natural 

 margin of the bone ; on either side of this the edge is trun- 

 cate, and in life was evidently bordered with cartilage, the 

 extent and form of which it is, of course, impossible to 

 determine. 



The anterior edge of the sternum is thin and sharp. Its 

 middle portion, between the coracoid grooves, is deeply 

 concave; on either side, where it forms the upper lip of these 

 grooves, it is convex, and externally it passes into the base 

 of the very prominent and stout antero-lateral processes 

 [ant dat .pr .) . The coracoid grooves are about 60 mm. long, 

 and are separated by an interval of 66 mm. The middle 

 portion of their lower lip forms a very prominent plate of 

 thin bone, about 14 mm. high and 25 wide. 



The lateral border is formed in front by the antero-lateral 

 processes, in the middle by a thickened, area, bearing four 

 facets for the articulation of the sternal ribs, while pos- 

 teriorly it becomes a comparatively thin edge, which passes 

 into the hinder border in the rounded postero-lateral angle. 

 The first of the costal processes is the most prominent; 

 it rises at the base of the antero-lateral process, above 

 the level of which it lies. On its anterior surface there 

 is a large pneumatic foramen. The articular surface is 

 obliquely elongated, and slopes downward and backward. 

 The next two processes are similar in form, and are separated 

 from the first and from each other by deep pits. The 

 fourth stands some distance behind the others, and its 

 articular surface is nearly circular. 



The form of the hinder border is shown in fig. 1, and the 

 structure of its edges has already been referred to. 



The body of the bone is for the greater part very thin, but 

 in the middle line it is somewhat thickened, the visceral 

 surface being raised in a broad transverse ridge, which dies 

 away as it is traced towards the costal borders. 



