FIVE members of the subgenus Vireosylva Bonaparte that have at one time or another been considered full species (namely gilvus of Vieillot, Baird's swainsonii, Lafresnaye's leucophrys, Sclater's josephae, and amauronotus of Salvin and Godman) are now thought to be conspecific (see in particular, Ridgway, Amer. Journ. Sci., (3) 4: 456, 1872, and Hellmayr, 'Cat. Birds of the Americas,' Publ. Field Mus. Nat. Hist., zool. ser., 13: no. 347, 152, footnote, 1935). They are alike in being dull in general appearance and in having no striking markings such as wing bars, sharply defined superciliary line, eye ring, or mystacial streaking. Their habits apparently are similar. Their breeding ranges do not, so far as we know, overlap. Since Vieillot's name gilvus antedates the four others, the group may properly be called the Warbling Vireos. The several races of the Warbling Vireo fall into two general groups, however-a gilvus group, with gray or grayish-olive upper parts and white under parts; and a leucophrys group, with brown or brownish-olive upper parts and yellow under parts. In color, as well as on geographical grounds, amauronotus of Mexico (with the white under parts of gilvus and the brown upper parts of leucophrys) might be thought to represent a third, and intermediate group. At any rate, when placed side by side, the races gilvus, swainsonii, brewsteri, connectens, amauronotus, strenuus, chiriquensis (including Ridgway's costaricensis), mirandae, leucophrys, josephae, and laetissimus lead from one more or less directly to another without abrupt break. However, the extreme in white-breastedness is not found in the most northward-ranging races, gilvus and swainsonii of the United States and southern Canada, any more than the extreme in yellow-breastedness is to be found in the most southward-ranging race, laetissimus of Bolivia. We have not examined V. g. leucopolia Oberholser, named from the Warner Mountains of Oregon, but it is said to be close to swainsonii. In the mountains of Hidalgo lives an undescribed form that, like connectens of Guerrero, falls between or connects amauronotus and the 'gilvus group'. It is grayish brown on the aown, grayish brown (of a lighter shade than the crown) on the back, and has little more than a trace of yellow on the under parts. This bird we propose to call