II. Division Odontolcae.—Marine, flightless, without sternal keel. Upper and lower jaws with teeth in furrows. Cretaceous epoch. Enaliornis, England, vertebrae chiefly biconcave; Hesperornis, North America, vertebrae heterocoelous.
III. Division Carinatae.—With keeled sternum.
1. Order Ichthyornithes.—Power of flight well developed. Vertebrae still amphicoelous. With small pygostyle. Incisura ischiadica. With alveolar teeth. Cretaceous of Kansas. Ichthyornis, Apatornis.
2. Order Colymbiformes.—Plantigrade, nidifugous, aquatic. All toes webbed, fourth largest, hallux short; metatarsus laterally compressed; tibia with high, pyramidal crest. Bill straight, pointed, with simple sheath.
Sub-order 1. Colymbi, Divers. Front toes completely webbed. Holarctic. Colymbus.
Sub-order 2. Podicipedes, Grebes. Toes lobated. Cosmopolitan.
3. Order Sphenisciformes.—Nidicolous, marine. Flightless, wings transformed into rowing paddles. Sphenisci, penguins. Antarctic and southern temperate coasts. Since the Eocene.
4. Order Procellariiformes.—Well flying, pelagic, nidicolous. Hallux absent or vestigial. Rhamphotheca compound. Cosmopolitan. Tubinares, petrels and albatrosses.
5. Order Ciconiiformes.—Swimmers or waders. Desmognathous, without basipterygoid processes; with one pair of sterno-tracheal muscles.
Sub-order 1. Steganopodes.—Well flying, aquatic, nidicolous; with all the four toes webbed together. Rhamphotheca compound; cosmopolitan. Phaëthon, tropic-bird; Sula, gannet; Phalacrocorax, cormorant and Plotus, snake-bird; Fregata, frigate-bird; Pelecanus. Here also Pelagornis, Miocene of France; Argillornis and probably Odontopteryx from the London Clay.