As originally described, this species differs from _swainsoni_ in larger size, longer bill, feet, and wings especially, straighter and narrower bill. The back is of a greener olive. The breast and sides of the head are entirely destitute of the buff tinge, or at best this is very faintly indicated on the upper part of the breast. The most characteristic features are seen on the side of the head. Here there is no indication whatever of the light line from nostril to eye, and scarcely any of a light ring round the eye,—the whole region being grayish-olive, relieved slightly by whitish shaft-streaks on the ear-coverts. The sides of body, axillars, and tibiæ are olivaceous-gray, without any of the fulvous tinge seen in _swainsoni_. The bill measures .40 from tip to nostril, sometimes more; tarsi, 1.21; wing, 4.20; tail, 3.10,—total, about 7.50. Some specimens slightly exceed these dimensions; few, if any, fall short of them. In autumn the upper surface is somewhat different from that in spring, being less grayish, and with a tinge of rich sepia or snuff-brown, this becoming gradually more appreciable on the tail. A specimen from Costa Rica is undistinguishable from typical examples from the Eastern United States. HABITS. This species, first described in the ninth volume of the Pacific Railroad Surveys, bears so strong a resemblance to the Olive-backed Thrush (_T. swainsoni_), that its value as a species has often been disputed. It was first met with in Illinois. Since then numerous specimens have been obtained from the District of Columbia, from Labrador, and the lower Mackenzie River. In the latter regions it was found breeding abundantly. It was also found in large numbers on the Anderson River, but was rare on the Yukon, as well as at Great Slave Lake, occurring there only as a bird of passage to or from more northern breeding-grounds. In regard to its general habits but little is known. Dr. Coues, who found it in Labrador, breeding abundantly, speaks of meeting with a family of these birds in a deep and thickly wooded ravine. The young were just about to fly. The parents evinced the greatest anxiety for the safety of their brood, endeavoring to lead him from their vicinity by fluttering from bush to bush, constantly uttering a melancholy _pheugh_, in low whistling tone. He mentions that all he saw uttered precisely the same note, and were very timid, darting into the most impenetrable thickets.