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PROCEEDINGS OF THE NATIONAL MUSEUM 



come readily, at least in Gallus domesticus, and the theoretical objec- 

 tions to retraction of the upper jaw below the resting position thus 

 are eliminated. While watching a night heron (Calherodias leueonotus) 

 at close range in the New York Zoological Park, I observed retrac- 

 tion of the upper jaw when the bird was yawning. Just before 



a 



b 



Figure 12. — Diagrams showing positions of tomial edges and of particular points (dots) 

 on the tomia during motions of the two jaws: a, when centers of jaw rotation lie close to 

 tomial planes; b, when centers of jaw rotation lie far from tomial planes. 



Figure 13. — Diagram of the avian skull showing retraction of the upper jaw beyond the 

 normal closed position. Because the lower jaw articulates well behind the cranial bending 

 axis of the upper jaw, the manipulative possibilities of the jaw are increased. Biting with 

 the bill tip as shown would not be possible if the jaws were of equal length. 



raising the depressed mandible to its resting position, the bird re- 

 tracted its upper jaw below the resting position with the result that 

 only the tips of the jaws were touching and the gap between the 

 bases of the jaws was clearly visible. Herons have the postorbital 

 ligament well developed and are also "coupled" through the jaw- 



