Bill slender, acute, slightly curved, of variable length, base thick, broad, trigonal, the sides compressed; upper mandible near the tip slightly notched; nostrils basal, oval, short, covered by a naked membrane, in the middle of which is an oval aperture; tongue long, bifid, not extensible, the tip fibrous; the three first quills of nearly equal length, and longer than the rest; tail moderate, emarginate, of 12 nearly equal feathers. Generic Types, Div. 1. _Certhia cyanea, cayana._ Div. 2. _Certhia spiza, &c._ Linn. * * * * * SPECIFIC CHARACTER. _N. (mas.) cyanea; jugulo, dorso, cauda alisque nigris, remigibus margine cyaneo ornatis. (Fem.) Viridis; capite, genis scapulisque caerulescentibus; jugulo cano._ (Male.) Changeable blue; throat, back, tail, and wings black; the quills edged with blue. Female green; head, cheeks, and scapulars bluish; throat grey. _Male._ Motacilla cayana. _Linn. Gmelin_, 1. 990. Sylvia cayana. _Lath. In. Orn._ 2. 545. _Gen. Zool._ 10. 655. Pepit bleu de Cayenne? _Brisson, Ois._ _vol._ 3. _pl._ 28. 1. Cayenne Warbler. _Lath. Syn._ 4. 502. _Gen. Zool._ 10. 655. Sylvia Cayenensis caerulea. _Brisson, Orn._ 1. _p._ 455. _Female._ Motacilla cyanocephala. _Gmelin_, 1. 990. Sylvia cyanocephala. _Lath. In. Orn._ 2. 546. _Gen. Zool._ 10. 684. Sylvia viridis. _Brisson, Orn._ 1. _p._ 455. Le Pepit verd. _Brisson, Ois._ 3. _pl._ 28. _f._ 4. Blue-headed Warbler. _Lath. Syn._ 4. _p._ 503. Blue-headed Creeper? _Lath. Syn._ 2. _p._ 727. * * * * * Few birds require more illustration than this very beautiful though common species; described by most ornithological writers, but hitherto so little understood, that the two sexes stand as distinct species in a family of birds to which they have, in reality, no natural affinity. According to the Linnaean system it should have been rather placed with the Creepers than the Warblers; an error which has been continued by every subsequent writer, even by Professor Temminck, whose skilful and accurate perception of natural affinities is, in general, so remarkably correct. That these two birds, however dissimilar in colour, are the sexes only of one species, repeated dissections in their native country have put beyond all doubt; and that it is a genuine _Nectarinia_ (or _Caereba_ of Temminck) will appear from submitting it to a rigorous comparison with the characters the Professor himself has laid down for that genus.