race would degenerate, and would only be saved from filial extinction by some individuals occasionally dropping their eggs in the nests of other species, perhaps of a _Molothrus_, as _M. rufoaxillaris_ still does, rather than of birds of other genera. Certainly in this way the parasitic instinct may have originated in _M. bonariensis_ without that species ever having acquired the habit of breeding in the covered dark nests of other birds. I have supposed that they once possessed it only to account for the strange attraction such nests have for them, which seems like a recurrence to an ancestral habit. 95. MOLOTHRUS RUFOAXILLARIS, Cassin. (SCREAMING COW-BIRD.) [Plate VI. Fig. 2.] [Illustration: Fig. 1. MOLOTHRUS BADIUS. ad. } " 2. " RUFO-AXILLARIS. pull. }] +Molothrus rufoaxillaris+, _Scl. et Salv. Nomencl._ p. 37; _Hudson, P. Z. S._ 1874, p. 161 (Buenos Ayres); _Durnford, Ibis_, 1877, p. 174 (Buenos Ayres); _White, P. Z. S._ 1882, p. 601 (Catamarca); _Barrows, Bull. Nutt. Orn. Cl._ viii. p. 134 (Entrerios); _Scl. Cat. B._ xi. p. 338. _Description._--Silky black, washed with purple; wings and tail with a slight greenish gloss; a chestnut spot on the axillaries; bill and feet black: whole length 8·0 inches, wing 4·5, tail 3·3. _Female_ similar, but somewhat smaller. _Hab._ Argentina and Uruguay. This bird has no vulgar name, not being distinguished from the Common Cow-bird by the country people. The English name of Screaming Cow-bird, which I have bestowed on it, will, I think, commend itself as appropriate to those who observe this bird, for they will always and at any distance be able to distinguish it from the species it resembles so nearly by listening to its impetuous screaming notes, so unlike anything in the language of the Common Cow-bird. The Screaming Cow-bird is larger than the allied species. The female is less than the male in size, but in colour they are alike, the entire plumage being deep blue-black, glossy, and with purple reflections; and under the wing at the joint there is a small rufous spot. The beak is very stout, the plumage loose, and with a strong, musky smell; the œsophagus remarkably wide.