Head, exposed ends of secondary quills, and entire under-parts, light-cinereous, palest and nearly white on the tips of the secondaries. Back, wing-coverts, and rump dark lead-color; primaries and tail brownish-black, the latter with a tinge of bluish. Bill dark; tarsi and feet lighter. Dimensions. Total length, _female_, about 15 inches; wing, 11 to 11½; tail, 6½ inches. _Male_—smaller. Hab. Southern States; Texas (Mr. Audubon); South Carolina (Prof. Gibbes). Spec. in Mus. Acad., Philada. Obs. This bird is quite different from the South American _I. plumbea_, for which it has been mistaken by American naturalists; in fact, it is so little like it, that a comparison of specimens of the two species would render a suspicion of their identity quite impossible. IV. GENUS ROSTRHAMUS. Lesson, Traité d’Orn., I. p. 55. (1831.) Bill long, very slender, hooked, and sharp at the tip; wings long, pointed; tail rather long, emarginate; tarsi and toes rather long; claws very long, slender, acute. The present is the only species. 1. Rostrhamus sociabilis. (Vieill.) The Hooked-billed Hawk. Herpetotheres sociabilis. Vieill., Nouv. Dict., XVIII. p. 318. (1817.) Cymindis leucopygius. Spix, Av. Bras., I. p. 7. (1824.) Rostrhamus niger. Less., Traité, I. p. 56. (1831.) “Falco hamatus. Illiger.” Lesson, as above. Temm., Pl. col. I. 61, 231; Spix, Av. Bras., I. pl. 2; Guerin, Mag. de Zool., 1834, pl. 20. Adult. Tail at base, and under tail-coverts, white; all other parts black. Naked space before the eye yellow, which is also the color of the feet; bill and claws black. Tail usually tipped with pale-cinereous. Younger. Throat and line over and behind the eye yellowish-white; general plumage brownish-black, mixed with yellowish-white on the under-parts of the body. Young. Forehead—stripe behind the eye and throat reddish or ferruginous-white; upper-parts brown, many feathers edged with pale-ferruginous; under-parts yellowish, with longitudinal stripes of black; tail at tip and base, and under tail-coverts, yellowish-white. Legs yellowish-green. Dimensions. _Female_—total length (of skin), 16 inches; wing, 14; tail, 7½ inches. _Male_—smaller. Hab. Florida (Mr. Harris, Dr. Heermann). Spec. in Mus. Acad., Philada. Obs. This bird is remarkable for its slender and hooked bill, unlike that of any other Falcon. It is well known as a South American bird, and was first noticed in Florida by Mr. Edward Harris; subsequently by Dr. Heermann. Both these gentlemen having obtained specimens of young birds, it is probably a constant resident of that State. V. GENUS CIRCUS. Lacepede, Mem. d’Inst., III. p. 506. (1803.) STRIGICEPS. Bonap., Comp. List., p. 5. (1838.)