The description given by Mr. Audubon of the egg of this species, and also that in my North American Oölogy, of the drawing of an egg said to be of this bird, taken in Louisiana by Dr. Trudeau, do not correspond with an egg in the cabinet of the Boston Society of Natural History, formerly in that of the late Dr. Henry Bryant. This egg measures 1.50 inches in length by 1.32 in breadth, is very nearly globular, but is also much more rounded at one end, and tapering at the other. It is entirely unspotted and of a uniform chalky whiteness, with an underlying tinge of a bluish green. It was found by Mr. C. S. McCarthy in the Indian Territory, on the north fork of the Canadian River, June 25, 1861. The nest was made of a few sticks, and was in the fork of a horizontal branch, fifteen feet from the ground. There were two eggs in the nest. It was also found breeding by Mr. J. H. Clark at Trout Creek, Indian Territory, June 21, and by Dr. E. Palmer at the Kiowa Agency (S. I. 13,534). GENUS ROSTRHAMUS, LESSON. _Rostrhamus_, LESS. 1831. (Type, _Falco hamatus_, ILLIG.) GEN. CHAR. Wings and tail large, the latter emarginated. Bill very narrow, the upper mandible much elongated and bent, the tip forming a strong pendent hook; lower mandible drooping terminally, the gonys straight; the upper edge arched, to correspond with the concavity of the regular commissure. Nostril elongate-oval, horizontal. Tarsus short, about equal to middle toe, with a continuous frontal series of transverse scutellæ; claws extremely long and sharp, but weakly curved; inner edge of the middle claw slightly pectinated. Third to fourth quills longest; outer five with inner webs sinuated. [Illustration: =53081=, ♀. ¼ =53081=, ♀. ½ =53081=, ♀. ½ _Rostrhamus sociabilis._] The species of this genus are two in number, and are peculiar to the tropical portions of America, one of them being confined to the Amazon region, the other extending to Florida in one direction and Buenos Ayres on the other. Their nearest allies are the species _Circus_ and _Elanus_, like them inhabiting marshy localities, where their food is found, which consists, in large part, of small mollusca. Species and Races. COMMON CHARACTERS. _Adult._ Prevailing color plumbeous-black, or bluish-plumbeous; the tail and primaries black. Entirely concolored, or with white tail-coverts. Cere and feet orange-red. _Young._ Spotted with blackish-brown and ochraceous, the former prevailing above, the latter beneath. 1. =R. sociabilis.= Tail-coverts, with terminal and basal zones of the tail, white; that of the tail more or less shaded with grayish-brown. _Adult._ Uniform blackish-plumbeous, darker on the head, quills, and tail. _Hab._ South America, West Indies, and Florida.