8 AVES. 
presence or absence of a median keel to the sternum. But these 
groups, though accepted by Blanford and Oates, are very un- 
satisfactory, since in some flightless Cwrinate the sternum has 
become reduced to the Ratite condition. 
Taking into consideration birds both living and extinct, we have 
two well-defined sub-classes, the Archwornithes and Neornithes. 
NXP 
sph PLA 
Under view of the skull of a Raven. vo, vomer; sxp, maxillo-palatine 
process ; pa, palatine ; ptg, pterygoid; g, quadrate; b.sph, basi-sphenoid ; 
sph.r, sphenoidal rostrum. 
The first is reserved for the Archwopteryx with an elongated 
reptilian chain of caudal vertebre, each bearing a pair of rectrices 
and having the jaws armed with teeth. The sub-class Neornithes 
