The Petrels were formerly classed with the Gulls, to which they have a considerable external resemblance, although they differ in many important characters, and they appear, on the whole, to be as nearly allied to the Steganopodes as to any other order. They may be at once distinguished from all other birds by the nostrils terminating externally in tubes, separate or united. The rhamphotheca or horny covering of the bill is divided into several pieces by deep grooves, as in some Steganopodes, and the upper mandible is generally much hooked at the end. The anterior toes are webbed throughout, the hallux is small, rudimentary or absent, being frequently represented by the claw-phalanx alone. The wings are long in the typical forms, the primaries 11, the tilth secondary wanting. Oil-gland tufted. Spinal feather-tract well-defined ou the neck by lateral bare tracts; forked on the upper back.
Petrels are schizognathous and holorhinal. The vomer is large, broad, depressed, and pointed. Nostrils impervious. Cervical vertebras 15. Large supraorbital glands ; two carotids ; caeca rudimentary or wanting. Femoro-caudal and semitendinosus muscles always present, ambiens and accessory femoro-caudal generally, but wanting in a few genera.
The majority of the species lay a single egg in a burrow or under stones, without any nest. Some, as the Albatrosses, make a nest in the open. The egg is either entirely white or has a faint zone of reddish spots near the larger end. The young are helpless, and clad with down till fully grown. Sexes alike in Coloration.
The Petrels are birds of the ocean, passing the greater part of their life far from land, resting on the water at times, and only visiting the shore, as a rule, for breeding purposes. They feed on floating crustacea, mollusca, small fish, alive or dead, and similar aliment. Some of them, as the Fulmars and Daption, follow ships and feed on any refuse, especially fat, that may be thrown overboard. Most of the Petrels are swift and powerful flyers, and may be seen skimming over the waves, almost without moving their wings, whilst some of them, and especially the small Stormy Petrels, appear to aid their flight by striking the water with their feet. Hence, as Newton points out in his ' Dictionary of Birds,' their name of Petrel was derived, for they were supposed to be walking on the sea as St. Peter is recorded to have done.
Many Petrels are crepuscular or nocturnal, especially during the breeding-season. The majority of them, on being captured, vomit a small quantity of clear oil with a disagreeable smell.
The classification of the Petrels, like their systematic position, is still unsettled. By many the Albatrosses are placed in a separate family, whilst Forbes*, to whom we owe by far the best account hitherto published of their anatomy, only assigned distinct family rank to Oceanites and its allies. Seebohm and Salving do not recognize this distinction, but attach weight to the presence or absence of basipterygoid processes, and the last-named divides the order into four families, of which Procellariidae and Diomedeidae (Stormy Petrels and Albatrosses) want the processes, whilst Puffinidae and Pelecanoideae possess them, the other distinctions being founded on characters of the nostrils, sternum, furcula, coracoids, and primaries. These families may be worthy of distinction, but there is so much doubt that, in arranging the few species of which, in each case, from one to three specimens have been obtained in the seas around India, the simple plan of leaving all the Indian genera in one family is most convenient. '