AVES. 9) 
neludes all living birds in which ihe vertebra supporting the 
rectrices have become so abbreviated that the tail-feathers have 
to be arranged fan-wise on either side of a fused mass of bones 
known as the ‘“ pygostyle.” 
As regards the Neornithes, the palate affords a much more satis- 
factory basis of division than the sternum. According to this, 
living birds are divisible into two further groups, the Paleognathe 
and Neoynathe, the former in substitution for the Ratite and the 
latter for the Carinate. 
In the Paleognuthe the vomer is large, and articulates by 
squamous suture with the pterygoid, while the palatine is applied 
to the outer margin of the vomero-pterygoid articulation, 
In the Ne ognathee the palatines have shifted inwards, under 
the vomero-pterygoid articulation, to meet one another in the 
median line. The pterygoids, in early post-embryonic life, undergo 
a striking process of segmentation, inasmuch as that portion of 
their shafts which rests upon the proximal end of the palatine snaps 
off, as it were from the main shaft, and fuses with the palatine. 
Later, at the point of fracture a cup-and-ball joint is formed, 
affording the strongest possible contrast with the squamous 
suture found in the Palwognathe. 
Where the vomer still retains some semblance of its former 
size, its proximal bifurcated end may just reach the extreme tip 
of the anterior end of the pterygoid, but it now depends for its 
support not upon the pterygoid, but upon the palatine, as, for 
example, in the Penguins. But among the Neognathe the vomer 
displays a striking series of stages in degeneration, becoming more 
and more divorced from the pterygoid, until it finally assumes 
the form of a minute nodule of bone, andat last, in the Galline, 
it becomes a mere spicule of bone held by a few tendinous fibres 
to the anterior border of the expanded ends of the palatines, and 
in some, as in the Falconide for example, vanishes altogether. If 
nothing were known of the early post-embryonic developmental 
stages of the Neognathine vomer, it would have been impossible 
to divine that the Neognathine was a direct derivative from the 
Palwoguathine palate. 
These two orders, the Palwognathe and Neognathe, must be 
divided further, for the Class Aves, in the course of its evolution, 
has split up into a vast number of different forms. The genetic 
relation of these forms or types to one another, and the precise 
affinities of the individual members of the various groups, should 
as far as possible find expression in any system of classification. 
These divisions may be known as Orders, which are again divided 
into Sub-Orders, Families, Genera and Species. 
