Note 3 — INTRODUCTION.

Carus and Gerstaecker (Handbuch der Zoologie, 1868, 191) present the following definition of birds as a class:—

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.