Seabirds in the bird order Procellariiformes. They occur in four families within that group, which also includes the Albatross family, Diomedeidae. The family Procellariidae is the main radiation of medium-sized 'true petrels', characterised by united nostrils with medium septum, and a long outer functional primary. It is dominant in the Southern Oceans, but not so in the Northern Hemisphere.