Mr. Gould, at the time of describing this species, entertained some doubts whether it might not eventually prove to be the _Phalcobænus montanus_ of D’Orbigny, in a state of change. I have carefully compared it with the description of the _P. montanus_, and certainly, with the exception of the one great difference of _M. albogularis_ having a white breast, whilst that part in the _P. montanus_ is black, the points of resemblance are numerous and exceedingly close. The _M. albogularis_, appears to be rather larger, and the proportional length of the wing feathers are slightly different; the cere and tarsi are not of so bright a colour; the middle toe has fifteen scales on it instead of having sixteen or seventeen. The black shades of the upper surface are pitchy, instead of having an obscure metallic gloss, and the feathers of the shoulders are terminated with brown, so as to form a collar, which is not represented in the figure of _P. montanus_, given by M. D’Orbigny. Although the main difference between the two birds, is the colour of their breasts, yet it must be observed, that in the _M. albogularis_ there is some indication of an incipient change from white to brown in the plumage of that part. But as M. D’Orbigny, who was acquainted with the young birds of the _P. montanus_, (of which he has given a figure), does not mention so remarkable a modification in its plumage, as must take place on the supposition of _M. albogularis_ being an immature bird of that species; and as the geographical range of the two is so very different, I am induced to consider them distinct. Moreover, on the plains of Santa Cruz, I saw several birds, and they appeared to me similar in their colouring. The _M. albogularis_ is remarkable from the confined locality which it appears to frequent. A few pair were seen during the ascent of the river Santa Cruz, (Lat. 50° S.) to the Cordillera; but not one individual was observed in any other part of Patagonia. They appeared to me to resemble, in their gait and manner of flight, the _P. Brasiliensis_; but they were rather wilder. They lived in pairs, and generally were near the river. One day I observed a couple standing with the Carranchas and _M. pezoporus_, at a short distance from the carcass of a guanaco, on which the condors had commenced an attack. These peculiarities of habit are described by M. D’Orbigny in almost the same words, as occurring with the _P. montanus_; both birds frequent desert countries; the _P. montanus_, however, haunts the great mountains of Bolivia, and this species, the open plains of Patagonia.