all living birds in which the vertebræ 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 satisfactory basis of division than the sternum. According to this, living birds are divisible into two further groups, the Palæognathæ and Neognathæ, the former in substitution for the Ratitæ and the latter for the Carinatæ.
In the Palæognathæ 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 Neognathæ 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 Palæognathæ.
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 Neognathæ 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, and at last, in the Gallinæ, 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 Falconidæ 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 Palæognathine palate.
These two orders, the Palæognathæ and Neognathæ, 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.