_Description._--Above clear greyish blue; wing-coverts, bastard-wing, and primary-coverts dusky brown, with greyish-blue edges; quills dusky; upper tail-coverts and tail-feathers black, the third outer tail-feather white at the tip, the outer tail-feather nearly entirely white; from the base of the forehead a black line extends backwards over the eye; under surface delicate ashy grey, white on the abdomen and under tail-coverts; bill and feet bluish-black: total length 4·5 inches, wing 2·1, tail 2·0. _Female_ similar, but without the black eye-streak. _Hab._ Paraguay and Northern Argentina. This little bird strongly resembles some species of that division of the Tyrannidæ which includes the genera _Stigmatura_, _Serpophaga_, and _Anæretes_; but the likeness, strange to say, is even more marked in habits and voice than in coloration and general appearance. It is found in open thorny woods and thickets; and in Buenos Ayres seems to have a partial migration, as it is much more common in summer than in winter. At all times male and female are found together, and probably pair for life, like several of the species in the groups just mentioned. They are seen continually hopping about among the twigs in a leisurely deliberate manner, all the time emitting a variety of low short notes, as if conversing together; and at intervals they unite their voices in a burst of congratulatory notes, like those uttered by the small Tyrant-birds they resemble. They have no song. I have not found the nest, but Dr. Burmeister says that it is made in bushes, and that the eggs are white. Fam. IV. TROGLODYTIDÆ, or WRENS. The Troglodytidæ, or Wren family, are of wide distribution, and are found alike under the tropics and in temperate latitudes. In South America nearly 100 species altogether are known to occur. Of these two are familiar inhabitants of the whole Argentine Republic, and a third, belonging to the water-loving genus _Donacobius_, is met with in the eastern provinces on the Paraná. A fourth species has been lately described from Tucuman. 11. DONACOBIUS ATRICAPILLUS (Linn.). (BLACK-HEADED REED-WREN.) +Donacobius atricapillus+, _Scl. Cat. A. B._ p. 16; _Scl. et Salv. Nomencl._ p. 5; _Sharpe, Cat. B._ vi. p. 364; _Burm. La-Plata Reise_, ii. p. 475. +Donacobius brasiliensis+, _d'Orb. Voy., Ois._ p. 213 (Corrientes). _Description._--Above dark chocolate-brown; cap black; wings black, with a large white patch on the inner primaries; tail black; lateral rectrices broadly ended with white; beneath ochreous buff; sides of breast and flanks with cross lines of dusky brown: total length 7·5 inches, wing 2·9, tail 3·7. _Female_ similar. _Hab._ Guiana, Amazonia, Brazil, Paraguay, and Northern Argentina.