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PROCEEDINGS OF THE NATIONAL MUSEUM 



jaw of a completely cleaned skull in place, as described by Bohm 

 (1930) and others. Partly because of the gripping flanges but pri- 

 marily because of the structure of the crests, trough, and lateral 

 articular surfaces, the lower jaw can be depressed or raised only if 

 the quadrate and articular surfaces slide along each other in the 

 direction dictated by the crests and troughs. During depression of 

 the jaw, this could be accomplished either by a passive spreading of 

 the rami of the jaw as they slide backward and upward along the 

 condyles of fixed quadrates; or the quadrates could move forward 

 and inward while the jaw was depressed without producing a lateral 

 spreading of the rami (fig. 10). Whether or not there is a flange grip 

 and a deep and well-defined trough of the articular as in Balaeniceps, 

 the condyles of many diverse orders of birds are arranged in such a 



a 



b 



Figure 10. — Diagrams of relative positions from dorsal view of ramus of lower jaw and 

 quadrate (black) when the jaw is closed (left ramus), and when depressed (right ramus): 

 a, quadrate moves anteromedially; b, ramus moves posterolateral!/. 



way that the planes of motion within the two jaw-quadrate articula- 

 tions converge anteriorly like those of Balaeniceps and downward 

 rotation of the mandible therefore requires either posterolateral 

 spreading of the rami or anteromedial motion of the quadrates. 

 Protraction of the upper jaw thus will accompany depression of the 

 lower jaw if resistance to spreading of the rami is greater than resist- 

 ance to forward and inward rotation of the quadrates (which of course 

 depends upon the combined resistance of all flexible portions of the 

 upper jaw and palate). Protraction would be prevented if the quad- 

 rates were held in place, but depression of the lower jaw could still 

 occur through spreading of the rami. Birds vary greatly in the ease 

 of motion of the palate and upper jaw and in the lateral flexibility of 

 the rami; the extent to which the rami may contribute to protraction 



