Aves. Skin covered wholly or in part with feathers. Anterior pair of limbs, converted into wings, generally used in flight; sometimes rudimentary. Occiput with a single condyle. Jaws encased in horny sheaths, which form a bill; lower jaw of several elements and articulated behind with a distinct quadrate bone attached to the skull. Heart with double auricle and double ventricle. Air-spaces connected to a greater or less extent with the lungs; the skeleton more or less pneumatic. Diaphragm incomplete. Pelvis generally open. Reproduction by eggs, fertilized within the body, and hatched externally, either by incubation or by solar heat; the shells calcareous and hard.
[4] Methodi naturalis avium disponendarum tentamen. Stockholm, 1872-73.
[5] This group is insusceptible of definition. The wading birds, as usually allocated, do not possess in common one single character not also to be found in other groups, nor is the collocation of their characters peculiar.
[6] Corresponding closely with the Linnæan and earlier Sundevallian acceptation of the term. Equivalent to the later Oscines of Sundevall.
[7] As remarked by Sundevall, exceptions to the diagnostic pertinence of these two characters of hind claw and wing-coverts taken together are scarcely found. For, in those non-passerine birds, as Raptores and some Herodiones, in which the claw is enlarged, the wing-coverts are otherwise disposed; and similarly when, as in many Pici and elsewhere, the coverts are of a passerine character, the feet are highly diverse.
[8] Laminiplantares of Sundevall plus Alaudidæ.
[9] Scutelliplantares of Sundevall minus Alaudidæ.
[10] Nearly equivalent to the Linnæan Picæ. Equal to the late (1873) Volucres of Sundevall.
[11] A polymorphic group, perfectly distinguished from Passeres by the above characters in which, for the most part, it approximates to one or another of the following lower groups, from which, severally, it is distinguished by the inapplicability of the characters noted beyond. My divisions of Picariæ correspond respectively to the Cypselomorphæ, Coccygomorphæ, and Celeomorphæ of Huxley, from whom many of the characters are borrowed.
[12] Groups G., H., and I. are respectively equal to the Charadriomorphæ, Pelargomorphæ, and Geranomorphæ of Huxley.