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SWAN  . M . the shining steel-blue upper plumage, and the dusky white —in some cases reddening so as almost to vie with the frontal and gular patches—of the lower parts are well known to every person of observation, as is the markedly forked tail, which is become proverbial of this bird . Taking the word swallow in a more extended sense, it is used for all the members of the family Hirundinidae,' excepting a few to which the name martin (q.v.) has been applied, and this family includes from 8o to 10o species, which have been placed in many different genera . The true swallow has very many affines, some of which range- almost as widely as itself does, while others seem to have curiously restricted limits, and much the same may be said of several of its more distant relatives . But altogether the family forms one of. the most circumscribed and therefore one of the most natural groups of Oscines, having no near allies; for, though in outward appearance and in some habits the swallows bear a considerable resemblance to swifts (q.v.), the latter belong to a different order, and are not Passerine birds at all, as their structure, both internal and external, proves . It has been sometimes stated that the Hirundinidae have their nearest relations in the flycatchers (q.v.) ; but the assertion is very questionable, and the supposition that they are allied to the Ampelidae (cf . Waxw1NG), though possibly better founded, has not been confirmed . An affinity to the Indian and Australian Artamus (the species of which genus are often known as wood-swallows or swallow-shrikes) has also been suggested but has not been accepted . (A . N.) SWALLOW-HOLE, in physical geography the name applied to a cavity resulting from the solution of rock under the action of water, and forming, or having at some period formed, the entrance to a subterranean stream-channel . Such holes are common in calcareous (limestone or chalky) districts, or along the line of outcrop of a limestone belt among non-calcareous strata .

These cavities are also known as sinks, dolinas or

butter-tubs, and by other local names, and sometimes as pot-holes; the last term, however, is also synonymous with Giant's Kettle (q.v.) . See CAVE .