THE STERNUM IN BIRDS. 



241 



The extent to which the keel of the lophosteon is devel- 

 oped in the carinate birds varies very much. In Strigo2ys it 

 is rudimentary ; in birds of powerful flight, as well as in those 

 which use their wings for swimming, it is exceedingly large. 



F'iG. 81. — Front and side views of the sternum of a Fowl, r., rostrum, or manubrium : c. p^ 

 costal process; pi. o., pleurosteon (the line from the letter goes to the point of juno 

 tion between the pleurosteon and the metosteon) ; m. a?., the middle xiphoid process ; 

 ca.., the carina or keel. 



In the bird's skull (Fig. 82), the brain-case is more arched 

 and spacious, and is larger, in proportion to the face, than in 

 any RejJtilia^ with the exception of the Pterosauria. There 

 is a well-marked interorbital septum, but the extent to which 

 it is ossified varies greatly. As a general rule, the superior 

 temporal bar is incomplete, and there is no distinct post-frontal 

 bone. The inferior temporal bar, formed by the jugal and 

 quadrato-jugal, on the other hand, is always complete. There 

 ttre no long parotic processes, nor any post-temporal fossas, 

 the whole of each parietal bone being, as it were, absorbed in 

 the roof of the skull. 



The nasal apertures are almost always situated far back 

 near the base of the beak. In the dry skull (above Mx. in 

 11 



