_R. pallide fusca, plumâ singulâ distinctâ semilunari notâ candidâ terminatâ; capite collo, femoribusque pallidioribus: rostri culmine augusti, ad apicem latiore, frontes plumis parvis setosis anticè directis et supra nares extensis; tarsi lateribus in dimidiam partem plumis parvis mollibus tectis; tarso ⅔ anticè posticeque toto, squamis reticulatis tecto._ Long. tot. 52 unc.; _alæ_, 30; _tarsi_, 11; _rostri_, 2. The whole of the plumage light brown, each feather with a decided crescent-shaped mark of pure white at the extremity; head, neck, and thighs lighter; base of the neck blackish; culmen of the bill narrow, becoming a little broader towards apex; front with small bristly feathers, pointing forwards and reaching over the nostrils. Tarsus with small downy feathers on sides, extending half way downwards; upper two-thirds of front of tarsus, and whole hinder side, with reticulated scales. Habitat, Eastern Patagonia (Lat. 40° S. to 54° S.) This species, which Mr. Gould, in briefly characterizing it at a meeting of the Zoological Society, has done me the honour of calling after my name, differs in many respects from the _Rhea Americana_. It is smaller, and the general tinge of the plumage is a light brown in place of grey; each feather being conspicuously tipped with white. The bill is considerably smaller, and especially less broad at its base; the culmen is less than half as wide, and becomes slightly broader towards the apex, whereas in the _R. Americana_ it becomes slightly narrower; the extremity, however, of both the upper and the lower mandible, is more tumid in the latter, than in the _R. Darwinii_. R. Darwinii. R. Americana, inches inches Length of beak, from edge of membrane at base to the apex 2 2⁶⁄₈ Length, from anterior margin of eye to apex 3⁴⁄₁₂ 5⁶⁄₁₂ Width of upper mandible, measured across middle of nostrils 1¹⁄₂₀ 1⁶⁄₂₀