XVI. 21 



JAWS 



465 



Fig. 278. Skull of young gosling (Anser). 

 A. angular; Ar. articular; As. alisphenoid; Bo. basioccipital; Bsh. basisphenoid ; D. dentary; 

 E. ethmoid; Eo. exoccipital; F. frontal; Ip. interparietal; J. jugal; L. lachrymal; Mx. 

 maxillary; A r . nasal; O. supra-occipital; Op. opisthotic; P. parietal; Pa. palatine; Pm. pre- 

 maxillary; Po. postorbital; Pt. pterygoid; Q. quadrate; Qj. quadrato-jugal; R. rostrum of 

 basisphenoid; S. squamosal; Sa. sur-angular; V. vomer. (From Heilmann, The Origin of 

 Birds, H. F. & C. Witherby, Ltd.) 



has presumably been derived from the diapsid archosaurian condition. 

 Typically, there is a single large fossa, communicating with the orbit, 

 but this is often partly subdivided by bony processes; occasionally, 

 there is a complete post-orbital bar (parrots). There are moderately 

 large temporal and pterygoid muscles, but the jaws are not usually 



