BIRDS agree in having their atlantal extremities (with very few exceptions) formed for flying; in having a sin gle condyle to the occipital bone ; a very large sternum; a brain that completely fills the cavity of the skull, but is not very complicated ; three eyelids; no external ear, and the internal ear with only one loose bone, and a co nical, but not spiral cochlea. Their jaws or mandibles arc covered by a horny substance ; their chyle is trans parent; they have no mesenteric glands, and no omen turn ; their lungs are attached to the cavity of the chest; there is no diaphragm; their trachea is furnished with two larynxes ; they have no epiglottis, no urinary blad der, and a single ovary and oviduct.
The principal differences according to which birds are arranged, respect the position and connection of the toes; the form and consistence of the bill; the struc ture of the head and neck ; or that of the wings. From the first of these circumstances, Cuvier divides them into six orders; viz. ACCIPITRES, having short feet, the toes furnished with strong claws, and a hooked bill ; PASSERES, having three toes before, and one behind, with the fibular toe of the former either wholly or par tially united to the next ; SCANSORES, having two toes before and two behind ; GALLINE, having the rotular or fore toes united at their base by a short membrane ; GRALLE, having elevated and naked tarsi, and the two fibular toes united; and ANSERES, having the toes uni ted by broad membranes. The first four of these orders
are land-birds, the two last, water-birds. The following table skews the orders of Blumenbach and Linne that correspond to the above divisions of Cuvier.
The subordinate divisions of Cuvier's orders are very numerous, and cannot, with propriety, be given here. They will be noticed under ORNITHOLOGY.
In the general table of classification, we have arran ged reptiles and serpents under distinct classes, be cause we think, that the general differences in their anatomical structure warrant such a separation. Most naturalists, however, consider them only as orders of the same class, and this class is by Linnxus and Blumen bach, denominated AMPHIBIA ; by CUVIER, REPTILIA.