The fourteen orders of the Carinatae are further congregated into four " Legions ".
The Ratitae branched off, probably during the Eocene period, from that still indifferent stock which gave rise to the Tinami+Galli+Gruiformes, when the members of this stock were still in possession of those archaic characters which distinguish Ratitae from Carinatae.
It follows that new groups of Ratitae can no longer be developed since there are no Carinatae living which still retain so many low characters, e.g.
The most novel feature, and one the importance of which most ornithologists of the present day are fully prepared to admit, is the separation of the class A y es into two great divisions, which from one of the most obvious distinctions they present were called by its author Carinatae' and Ratitae, 2 according as the sternum possesses a keel (crista in the phraseology of many anatomists) or not.
The separation of the Ratitae from the other birds, and their seemingly fundamental differences, notably the absence of the keel and of the power of flight, induced certain authors to go so far as to derive the Ratitae from the Dinosaurian reptiles, whilst Archaeopteryx (q.v.) and the Carinatae were supposed to have sprung from some Pterosaurian or similar reptilian stock.