Warm-blooded Sauropsida in which the epidermis developes feathers clothing the head, neck, body, fore-limbs and the upper part of the hind-limbs (cf. pp. 51-2). The fore-limbs are modified into wings. The sacral series of vertebrae is long and includes 1-3 dorsal, all the lumbar, the two true sacral and a variable number of caudal ( = urosacral) vertebrae, all anchylosed together. The bones of the pelvis are anchylosed: the ilium of great anteroposterior extent: and the pubes (postpubes) and ischia do not meet in a ventral symphysis. There is a carpo-metacarpus; a tibio-tarsus and tarso-meta-tarsus formed by the union of both proximal and distal tarsalia to the tibia and metatarsals respectively. The jaws are covered by a horny epidermic sheath, and there is a more or less muscular gizzard in the digestive tract. The heart is quadrilocular and the right auriculo-ventricular valve muscular, consisting of two flaps. The aorta is single and crosses the right bronchus. The lungs are firmly fixed to the back of the thorax, and there are air-sacs developed from the ends of certain of the bronchia.

The right ovary is atrophied.

Epidermic scales are found covering the tarso-metatarsus and the toes, but these parts may in exceptional cases be feathered. There are claws on the last phalanges of the toes, frequently on the thumb and first finger. A thickened epidermis covers the upper and lower jaw. It is subject to a partial moult and renewal every year in the Puffin; see Zoologist, 1878, p. 233. There is no dermal exo-skeleton unless the bony spur on the metacarpus in a few birds, e. g. Megapodius, the Mound-bird, and on the posterior aspect of the tibio-tarsus in many Gallinae ( = Alectoromorphae) is to be considered a dermal structure. The corium or derm is remarkably thin, very vascular, rich in Pacinian bodies, and contains numerous bundles of non-striated muscle fibres connected to the follicles of the feathers. There is but one skin-gland, the uropygial or oil gland, situated on the dorsal aspect of the tail.

The leading features of the skeleton are as follows1. The surface of the skull is polished, and the sutures between the bones obliterated at an early period; the interorbital septum well ossified. The praemaxillae are very large and the maxillae small, their palatal plates variably developed. The parasphenoid is very large in the embryo and ossifies as an anterior azygos bone, the rostrum, and posterior paired bones, the basi-temporals. The quadrate is free, and usually articulates by two heads, both with the cranium and the lower jaw: it is connected to the maxillae by jugo-quadrato-jugal bones, the rami of the lower jaw are anchylosed at the symphysis at an early period, and the constituent bones are also anchylosed. The hyoid arch is rudimentary, but the first branchial well-developed. The neck is long and mobile, and contains a variable number of vertebrae. The centra of the praesacral vertebrae are procoelous, cylindroidal and saddle-shaped posteriorly. They articulate by synovial joints with a cartilaginous meniscus interposed between successive centra.

Certain of the dorsal vertebrae are opisthocoelous in the Penguins and Auks, and all the vertebrae are amphicoelous in the extinct Archaeopteryx and Ichthyornis. Some of the dorsal vertebrae are occasionally anchylosed and they always possess strong spines and ligaments. The terminal 4-6 caudals fuse as a rule and form a ploughshare bone or pygostyle. The cervical ribs are small, provided with double articulations, and anchylosed to the vertebrae with the exception of the 2-3 last which are large and free. The dorsal ribs are divided into a vertebral and sternal section, both well ossified. The posterior ribs fail to reach the sternum. Ossified plates or 'processus uncinati' are attached to the posterior edges of certain of the vertebral sections of the dorsal ribs. The sternum is very large, convex ventrally, and the original cartilage replaced by membrane bone. It has in the majority of living birds, in Archaeopteryx and Ichthyornis, a prominent ventral keel, probably derived from the posterior part of the interclavicle: hence Carinatae. In a few living birds and the extinct Hesperornis this keel is absent and the sternum smooth and raft-shaped: hence Ratitae. The scapula is thin, narrow, sabre-shaped, either anchylosed to the coracoid (Ratitae) or united to it by ligament (Carinatae). The ventral ends of the coracoids are received into grooves of the ventral surface of the sternum.

The clavicles are rarely absent (most Ratitae) and are generally fused at their lower extremities, forming the furcula. The interclavicle is present apparently in development: its anterior end either becoming entirely fibrous, or else partially ossified as hypocleidium and fused to the furcula, and its posterior either remaining fibrous (Ratitae) or ossified as the keel of the sternum (Carinatae). The pubes form a ventral symphysis in the Ostrich (Struthio), and the ischia a dorsal symphysis in Rhea. The conformation of the limb-bones is characteristic. The fibula has a pointed lower extremity and is shorter in the adult than the tibia. The scaphoid is present in the carpus; the lunar and cuneiform fused. The hand has but three digits, first, second, and third, with fused metacarpals (except in Archaeopteryx): the foot, four digits, the fifth toe being always absent. In Struthio the third and fourth are alone present, but the tarso-metatarsus retains a trace of the articulation for the second toe.

In Aves as in Lacer-tilia the phalanges of the toes increase typically in number from the first to the fourth toe in the series 2, 3, 4, 5.

1 For detailed description see Preparation ii. pp. 58-67.

The cerebral hemispheres are large and made up chiefly of corpus striatum. They touch the cerebellum posteriorly, and the two solid optic lobes are thus thrust aside laterally. The olfactory lobes are small. The cerebellum has a median foliate lobe, showing in longitudinal section an arbor vitae as in Mammalia, and two small floccular lobes. The extinct Cretaceous birds had large olfactory lobes, and small hemispheres, their brain resembling to a certain extent that of the Alligator. The spinal cord has a lumbar swelling with the posterior fissure widely open and filled by gelatinous tissue. The cervical sympathetic is double, one part accompanying the common carotids, the other running in the vertebrarterial canal. The fore-part of the sclerotic is obtusely conical, and contains a ring of bones; the hind-part spheroidal, and there is in some birds a bony plate in it near the entrance of the optic nerve. A vascular pigmented process, the pecten or marsupium, projects into the vitreous humour in the line of the choroidal fissure and is absent only in Apteryx. The muscular fibres of the iris are striated, and the radial portion of the striated ciliary muscle, known as Crampton's muscle, large, a bird having apparently exceptional powers of accommodation.