AND AVES OF NORTH AMERICA. 



51 



the others more oval, the more anterior are sub-triangular, and the rib pits on the first dorsals are sub-round or 

 vertically oval. Thus the heads of the ribs also vary. The shafts are all flat, probably more so from pressure. They 

 are frequently curved in the direction of the compression, which suggests a vertical head. They however are probably 

 more or less distorted, and the plane of compression changed. No well defined distal extremity of a rib can be made 

 out, nor have anything like abdominal ribs been preserved. 



The scapular arch is remarkable for its large clavicles (or procoracoids). As preserved, the latter are quite con- 

 vex downwards both antero-posteriorly and transversely, while the coracoids are equally concave in both directions. 



Fig. 7. 



Scapular Arch. 



The clavicles have a remarkable external flat projection, which is separated from the glenoid cavity by a deep sinus. 

 The glenoid cavity is bounded by an elevated ridge, which sends a branch along the claviculo-coracoid suture to the 

 precoracoid foramen. This foramen is relatively of small size, and is a longitudinal oval; the two are separated by 

 an isthmus composed equally of processes of clavicle and coracoid. The coracoids are very thin except in a transverse 



