The home of the bird now before us, appears to be the mountainous regions of Oregon and California, from which it descends in the winter season to the lower districts, in the valleys of the rivers, and on the shores of the Pacific. In the former country, near the sources of the Platte river, the first specimen that we ever saw, was obtained by the late Dr. John K. Townsend during his trip across the continent in the year 1834, and in whose collection, now belonging to the Philadelphia Academy, it yet remains. Since that period it has been observed in the Rocky Mountains and on the Columbia river, by the naturalists attached to the United States Exploring Expedition, in the Vincennes and Peacock. In the fine zoological collection made by this expedition, is preserved the only female specimen, in the dark plumage, described below, yet known to have been brought to the attention of naturalists. In California, Dr. Heermann particularly noticed this bird, and his collection contains several specimens which he obtained in the plains near Sacramento city. DESCRIPTION AND TECHNICAL OBSERVATIONS. Genus Falco. Linnæus, Syst. Nat., I. p. 124. (1766.) General form compact and strong; bill strong, short, with a distinct and sharp tooth in the upper mandible; wings long, pointed; tail moderate, or rather long; tarsi short, robust; toes long, claws large, curved, sharp; tarsi covered with hexagonal or circular scales. A genus of birds remarkable for their courage and very rapid flight, species of which are found in all countries. Falco polyagrus. Cassin, Birds of California and Texas, I. p. 88. (1853.) Form robust; wings rather long, second and third quills longest, and nearly equal; tail rather long; bill short, rather wide at base; tooth in the upper mandible prominent. Dimensions. Female. Total length of skin, about 20 inches; wing, 14; tail, 8 inches. Colors. Female nearly adult. Narrow frontal band, line over the eye, and entire under parts white; narrow stripe from the corner of the mouth running downwards, dark brown; some feathers on the breast, and abdomen with longitudinal stripes and spots of brown, which color forms a large and conspicuous spot on the flank. Entire upper parts brown, paler on the rump, many feathers with rufous edgings; tail above pale grayish-brown, with transverse bars of white, and narrowly tipped with white; quills dark grayish-brown, with numerous bars of white On their inner-webs; under wing-coverts dark brown; edge of the wing at the shoulder and below, white, spotted with brown. The brown of the back extending somewhat on to the breast at the wing. Bill, bluish horn color, under mandible yellow at its base. Large space around the eye, bare, with a narrow edging of brown on the first plumage by which it is encircled.