This species is found in the western provinces of the Argentine Republic, and, like _C. anthracinus_, which it closely resembles, is a summer visitor in Patagonia, where it makes its appearance in October. The plumage is intense black, with the inner webs of the remiges at their base white, but the wing-band, which is over an inch in breadth, shows only when the bird flies. There is also a small white spot on the flank, scarcely visible, and excepting for this speck the bird at rest appears entirely black. When it flies the white band appears suddenly, producing a curious effect, for the wings are opened and shut successively and with great rapidity, making the white band appear like a succession of flashes. All the movements of the bird are eccentric to a degree. It selects a dead twig on the summit of a bush, and this perch it occupies during many hours every day. Occasionally it darts after a passing insect, but I believe it feeds principally on the ground, like _Lichenops perspicillatus_. At intervals it quits its perch very suddenly and revolves round it with the rapidity of a moth whirling round the flame of a candle, the wings producing a loud humming sound, and the bird uttering a series of sharp clicking notes. During this performance the white wing-band appears like a pale mist surrounding the bird. This fantastical dance over, it resumes its perch, and, until moved to a second display, sits as motionless as a bird carved out of jet. 128. CNIPOLEGUS CYANIROSTRIS (Vieill.). (BLUE-BILLED TYRANT.) +Cnipolegus cyanirostris+, _Burm. La-Plata Reise_, ii. p. 457 (Mendoza): _Scl. et Salv. Nomencl._ p. 43; _Barrows, Bull. Nutt. Orn. Cl._ vol. viii. p. 141 (Entrerios). _Description._--Uniform dull black; inner webs of the wing-feathers margined with dull white; bill plumbeous, feet dark brown: whole length 5·7 inches, wing 3·0, tail 5·6. _Female_ dark cinnamomeous brown, brighter on the head and rump; wings blackish, wing-coverts and secondaries edged with pale rufous; tail blackish brown, outer margins of outer rectrices and inner webs of all the rectrices, except the two middle, bright rufous; below pale fulvous, densely striated with blackish; bill and feet brown. _Hab._ S. Brazil, Paraguay, Uruguay, and La Plata. In size this species of Black Tyrant is but slightly smaller than the one described, and the colour is also of an intense black, but the white wing-band is less conspicuous, the borders only of the inner webs of the quills being white. It also differs in having a bright blue beak and red eyes. Azara discovered it in Paraguay, and described the brown-plumaged female as a distinct species. He merely says that it sits on the summit of a bush and darts after passing insects, but it will, no doubt, be found to possess some curious performing habits like the other species of the genus.