OSTEOLOGY OF BIRDS 73 



raised above the surrounding parts, which articulates with a broad 

 surface of simiHar form on the outer side of the expanded head of 

 the clavicle ; this latter surface looks directly backward, a special 

 recess being made for it. The meeting of the two bones is ex- 

 tensive and very intimate, as I have elsewhere pointed out. 



The rounded tuberous head of the coracoid rises but little above 

 the broad surface of the anterior end of the clavicle, and this pro- 

 jection arches over a recess at its inner aspect in which is hidden 

 large pneumatic foramina that communicate with the hollow shaft 

 and other parts of the bone. 



The furcida or the united clavicles are likewise highly pneumatic 

 bones ; the foramina that enter them being found upon the nonartic- 

 ulating surface, opposite the foramina just described as perforating 

 the inner side of the head of the coracoid. . When the two bones 

 are /;/ situ, these two surfaces form the anterior walls of a fossa 

 that lies immediately beyond the " tendinal canal " and really a part 

 of the same inclosure. 



Above, the clavicles are broad and articulate with the sides of the 

 heads of the coracoids, and the clavicular process of either scapula 

 in a manner already described. Viewed from in front they present 

 the extreme type of the U-shaped style of the bone, the internal 

 periphery of the arch being nearly a semicircle. The bones are 

 compressed from side to side, and diminish in bi^eadth as they ap- 

 proach the point of union below. 



Here the clavicles support a small tul:)erous hypoclcidium, which, 

 owing to the backward curvature of the fourchette, is about opposite 

 the coracoidal beds on the sternum. A well developed os huinero 

 scapulare is supported in the usual manner at the back part of the 

 shoulder joint. It is quite a characteristic of the raptorial as well 

 as other groups of birds, and is of great service in increasing the 

 osseous articular surface for the humerus. 



Pectoral limb. Circus in common with many other Raptores has 

 but one pneumatic bone in the skeleton of its wing, and this is the 

 humerus [fig. 29]. This bone is thoroughly permeated with air, and 

 although of good size, is very light indeed. The pneumatic fossa 

 is of an elliptical outline, occupying its usual site, and at its base 

 numerous pneumatic perforations occur. Over it curls the ulnar 

 tuberosity, forming for it contracted margins on three sides, mak- 

 ing the entrance smaller than the fossa inside. 



The articular tuberosity for the glenoid cavity is spindle-shaped 

 and not very extensive. A decided valley divides it from the ulnar 



