Several new genera have lately been established to receive certain species of the sub-family of _Polyborinæ_, and consequently great confusion exists in their arrangement. Mr. George R. Gray has been kind enough to give me the following observations, by which it appears he has clearly made out, that Spix’s genus _Milvago_, is that which ought to be retained. M. D’Orbigny has made two sections in the genus _Polyborus_, according as the craw is covered with feathers, or is naked, and he states that the _P. Brasiliensis_ is the only species which comes within the latter division; but we shall afterwards see that the _Falco Novæ Zelandiæ_, Auct. (the _Milvago leucurus_ of this work) has a naked craw, which is largely protruded after the bird has eaten. M. D’Orbigny has also instituted the genus _Phalcobænus_, to receive a bird of this sub-family, with the following characters: “Bec fortement comprimé, sans aucune dent ni sinus, à commissure trèsarquée à son extrémité; cire alongée et droite; un large espace nu entourant la partie antérieure et inférieure de l’œil, et s’étendant sur toute la mandibule inférieure; tarses emplumés sur un tiers de leur longueur, le reste réticulé; doigts longs, semblables à ceux des gallinacés, terminés par les ongles longs, deprimés et élargis, très-peu arqués, toujours à extrémité obtuse ou fortement usée; ailes de la famille, la troisième penne plus longue que les autres.” Mr. George R. Gray, however, has pointed out to me that Spix, (in his Avium Species Novæ) ten years since, made a division in this sub-family, from the rounded form of the nostril of one of the species, namely, the _M. ochrocephalus_ of his work, or the _Chimachima_ of Azara. And Mr. Gray thinks, that all the species may be grouped much more nearly in relation to their affinities by this character, than by any other: he further adds;—“The only difference which I can discover between this latter genus (_Milvago_), and D’Orbigny’s (_Phalcobænus_), is, that in the latter the bill is rather longer, and not quite so elevated in the culmen as in the former; and these characters must be considered too trivial for the foundation of a generic division. I, therefore, propose to retain Spix’s genus, _Milvago_, for all those _Polyborinæ_ which possess _rounded nostrils with an elevated bony tubercle in the centre_. They were once considered to form three distinct genera, viz.—Milvago, _Spix._ (Polyborus, _Vieill._ Haliaëtus, _Cuv._ Aquila, _Meyen._)—Senex, _Gray_. (Circaëtus, _Less._)—Phalcobænus, _D’Orb._ but a careful comparison of the several species, shows a regular gradation in structure from one to the other, which induces me to consider them as only forming two sections of one genus. Those which have the bill short, with the culmen arched, and are of small size, slender form, and with the tarsi rather long and slender, are— 1. Milvago ochrocephalus, _Spix._ Polyborus chimachima; _Vieill._ (young). Falco degener, _Licht._