The birds of this family are readily distinguished from the _Paridæ_ by the slender bill, notched and decurved at tip; much bristled gape, sharp-ridged culmen, exposed oval nostrils, less adherent toes, etc. They are much smaller than the _Turdidæ_ and _Saxicolidæ_, with much more slender, depressed bill, longer rictal bristles, etc. The short outer primary, with the primaries ten in number, distinguish them from the _Sylvicolidæ_. The following synopsis will serve to characterize the American forms of their respective subfamilies. The species are all among the most diminutive in size with the exception of the Humming-Birds:— A. Wings longer than the nearly even and emarginate tail. Scutellæ of tarsus scarcely or not at all appreciable. General color olivaceous above. No white on tail. Nostrils naked. Scutellæ distinct on inner face of tarsus only. Head plain. … _Sylviinæ._ Nostrils overhung by bristly feathers. Scutellæ of tarsus not appreciable. Head with a colored central crest … _Regulinæ._ B. Wings about equal to the graduated tail. Tarsal scutellæ distinct. Above bluish; tail with white spots or patches. Nostrils uncovered. Head plain; either bluish or black above.… _Polioptilinæ._ SUBFAMILY SYLVIINÆ. CHAR. Size and form of _Sylvicolinæ_, but with a spurious first primary about one third the second quill. Wings considerably longer than the nearly even or emarginate tail. Feathers of frontal region with bristly points; but not covering the nostrils. Tarsi scutellate anteriorly, but indistinct externally. (Characters drawn with reference to the American form.) The introduction of this subfamily into the present work is required to accommodate a species of _Phyllopneuste_ collected on the Yukon by the Russian Telegraph Expedition, the first known instance of the existence in North America of a group of birds characteristic of the northern parts of the Old World. Among the smallest of the class, they are eminently sociable, and feed entirely on insects, which they capture mostly on the wing, like flycatchers. The nest is placed on the ground, and is of an oval or spherical form with a round opening on one side. The sexes are similar, and the young differ very little from the parents. GENUS PHYLLOPNEUSTE, MEYER & WOLF. _Phyllopneuste_, MEYER & WOLF, Taschenbuch, 1822.—DEGLAND et GERBE, Ornith. Europ. I, 1867, 543. [Line drawing: _Phyllopneuste borealis._ 45909] GEN. CHAR. Bill shorter than the head; straight, slender, and depressed, notched at tip. Nostrils open. Tarsi lengthened; exceeding the middle toe; scutellate anteriorly, but with the plates indistinct, claws short, much curved. Wings pointed, longer than tail, and reaching at least to its middle; spurious quill extending farther than the upper covert. Tail emarginate. Olivaceous above; yellowish or whitish beneath.