Bill wide at base, curved; nostrils basal, with large membrane; wings very long; pointed shafts of the primaries strong and rigid; first and second primaries longest; secondaries short; tail moderate, with the shafts of its feathers strong and rigid; legs short, strong; tarsus usually feathered; toes short, strong; claws strong, curved, and sharp; hind toe very short, reversible; general form robust; head broad. This genus includes about twenty species, which are inhabitants of the countries of the old world, except six American and one Australian species. They are remarkable for great swiftness of flight, and live habitually in the crevices of rocks, in caverns, or in hollow trees. The bird now about to be described is the only species of this genus yet observed within the limits of the United States, the others of this continent being confined to South America. 1. Cypselus melanoleucus. Baird, Proc. Acad., Phila., VII. p. 118. (1854.) Not figured. Wings very long, exceeding the tail; second primary longest; first primary tapering towards the end; secondaries short, obliquely incised at their ends; tail moderate, emarginate; shafts of the tail-feathers very stiff at their bases; tarsi and toes feathered, short, and very strong; entire plumage above dark fuliginous, paler on the head, nearly black on the back and rump; throat, breast, wide longitudinal stripe down the middle of the abdomen, and large spot on the flank, pure white; sides and under-coverts of the tail dark fuliginous, which is also the color of the plumage of the tibiæ, tarsi, and toes; wings and tail dark fuliginous; secondaries tipped with white; wings at the shoulders edged with white; under-wing-coverts ashy fuliginous; claws light-colored. Dimensions. Total length of skin, from tip of bill to end of tail, about 5½ inches; wing, 5½; tail, 2½ inches. Female very nearly the same. Hab. New Mexico (Dr. Kennerly, Dr. Heermann). Only noticed, as yet, by the two naturalists just mentioned. This is a handsome bird, and an interesting addition to the ornithology of the United States, being the first species of the genus _Cypselus_ discovered within our limits. According to the gentlemen above mentioned, this bird habitually frequents the crevices in rocks, in which it builds its nest and rears its young. Its flight is represented to be exceedingly rapid and long-continued. This bird is evidently of the same group of species as _Cypselus montivagus_, D’Orbigny, Voyage dans L’Amerique Meridionale, Ois., p. 357, pl. 42, fig. 1 (Paris, 1844), which inhabits the mountains of Bolivia. The white stripe down the middle of the abdomen distinguishes the present bird. II. GENUS ACANTHYLIS. Boie, Isis, 1826, p. 971. CHÆTURA. Stephens. HIRUNDAPUS. Hodgson.