A B Y, A Y, A B, X Y and B do not occur at all, some of them for obvious reasons, Occasionally there is an instructive progressive evolution expressed in these formula; for instance Phaethon, in various other respects the lowest of the Steganopodes, has A X Y, Sula and Phalacrocorax have A X, Fregata, the most specialized of these birds, has arrived at the reduced formula A.
Phaethon, the tropic-bird, he would place with the " Larides " and not with the " Pelecanides," which it only resembles in its feet having all the toes connected by a web.
Genera: (a) Anas, Mergus, Phaethon, Plotus; (b) Rhyncops, Diomedea, Alca, Procellaria, Pelecanus, Larus, Sterna, Colymbus.
This arch appears on Roman coins from Augustus to Commodus; according to Pausanias it bore two four-horse chariots, one driven by Helios and the other by Phaethon, his son, all in gilded bronze.
The first literary evidence is a fragment of Euripides (Phaethon), in which it is especially characterized as an innovation.