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.
In cormorants (Phalacrocoracidae), and especially in Sula, where the nasal slits become completely closed up, and the greater portion of the nasal cavity is also abolished, being restricted to the olfactory region with its unusually wide choanae.
The existing genera include Anas, Aquila, Bubo, Columba, Cypselus, Lanius, Picus, Phalacrocorax, Sula, &c. Very interesting is the fact that Serpentarius, Psittacus and Trogon are amongst this list of birds, which are now restricted to the tropics.
In Colorado and New Mexico Marsh has detected bones of Meleagris, Puffinus, Sula and Uria, all existing genera; but the first is especially suggestive, since it is one of the most characteristic forms of the New World.
Dvina, and the Pripet, both very important for navigation - as well as several smaller tributaries on which rafts are floated; on the left the Sozh, the Desna, one of the most important rivers of Russia, navigated by steamers as far as Bryansk, the Sula, the Psiol and the Vorskla.
The more common sea-birds are the Sula variegate or guano-bird, a large gull called the Larus modestus, the Pelecanus thayus, and the Sterna Ynca, a beautiful tern with curved white feathers on each side of the head.
SULA ISLANDS (Sulla, Xulla; Dutch Soela), a chain of islands forming a prolongation of the eastern peninsula of Celebes and the Banggai Islands, Dutch East Indies.
Of the numerous considerable islands which lie north-east, east and south of Celebes (those off the west coast are few and small), the chief are prolongations of the four great peninsulas - the Sangir and Talaut islands off the north-east, the Banggai and Sula off the east, Wuna and Buton off the south-east, and Saleyer off the south.
The residency consists of the following groups of islands: the Halmahera group, the Bachian and the Obi group, the Sula Islands, the islands near the western half of New Guinea (Gebeh, Vaigeu, Salawati, Misol, collectively called the Papuan Islands), the western half of New Guinea as far as 141° E., with the islands in Geelvink Gulf on the north coast of New Guinea (Schouten Islands, Yapen, &c.), along with others on the south coast.