Birds form a large and extremely homogeneous class of the vertebrata, and are readily distinguished from all other animals by the possession of an epidermal exoskeleton having the form of feathers. Feathers differ from hairs in the fact that they grow from papillae formed of both the horny and the Malpighian layer of the epidermis, which papillae at first project from the surface, and only subsequently become imbedded in pits of the dermis. A dermal exoskeleton does not occur in birds.

The endoskeleton is characterised by its lightness, the large bones being generally hollow; but the pneumaticity does not vary in proportion to the power of flight. The cervical part of the vertebral column is very long and flexible, while the post-cervical portion is generally very rigid, owing to the fusion of many of the vertebrae, especially in the lumbar and sacral regions. The vertebrae are generally without epiphyses to their centra. The cervical vertebrae in living forms have saddle-shaped articulating surfaces, and many of them bear ribs. The thoracic ribs in almost all birds have large uncinate processes. The sternum is very large, and the ribs are always attached to its sides, not as in many reptiles to any backwardly-projecting process or processes. The sternum ossifies from two or more centres.

The skull is extremely light, and its component bones show a great tendency to fuse together completely. The facial part of the skull is prolonged into a beak, chiefly formed of the premaxillae; this beak is in all modern birds devoid of teeth, and is coated externally with a horny epidermal sheath. The quadrate is large and freely movable. The supratemporal arcade[1] is imperfect, while the infratemporal arcade[95] is complete. There are no postorbital or postfrontal bones. Neither parotic processes nor an interparietal foramen occur. There are commonly large pre-orbital vacuities. The palatines and pterygoids never form a secondary bony palate as in Crocodiles. Part of the floor of the skull is formed by a wide basitemporal (paired in the embryo) which is continued in front as a long slender rostrum; these structures have replaced the parasphenoid of Ichthyopsids. Cartilage or bone is always developed in the sclerotic. The first branchial arch is well developed, the hyoid arch but slightly. The coracoids are large, and the clavicles are nearly always united forming the furcula. There is no separate interclavicle and hardly any trace of a precoracoid.

The anterior limbs form wings, and the manus is in the adult always much modified, never having more than three digits. The three bones of the pelvis are, except in Archaeornithes, always ankylosed together in the adult, and the ilium is greatly prolonged in front of the acetabulum, which is perforated. The ilia are not connected with the sacrum by ossified sacral ribs. The pubes and ischia are directed backwards parallel to one another, and except in a very few forms never meet their fellows in ventral symphyses. The fibula is generally much reduced. The proximal tarsal bones are always ankylosed to the tibia, and the distal tarsals to the metatarsals, so that the ankle joint is intertarsal. The first metatarsal is nearly always free. The pes never has more than four digits in the adult.

The class Aves is most conveniently divided into two subclasses: 1. Archaeornithes. 2. Neornithes.

Subclass I. Archaeornithes.

The only form referred to this subclass of extinct birds is Archaeopteryx[96], the earliest known bird. In this animal the skeleton does not seem to be pneumatic. The cervical and trunk vertebrae are generally thought to be flat, certainly their articulating surfaces are not saddle-shaped. There is no long compound sacrum as in modern birds. The tail is longer than the whole body, the caudal vertebrae are twenty in number, they gradually taper as traced away from the trunk, and each bears a pair of feathers. The posterior caudal vertebrae are not united together to form a pygostyle. The upper jaw bears thirteen pairs of conical teeth, planted in distinct sockets in the maxillae and premaxillae, but the mandible has only three pairs. The presence of these teeth forms the most essential difference between the skull of Archaeopteryx and that of modern birds, and the fact that they occur on the premaxillae renders it improbable that a horny beak was present. There is a ring of ossifications in the sclerotic. The ribs do not show uncinate processes, and articulate with the vertebrae by single heads not divided into capitula and tubercula. Abdominal ribs appear to have been present. The furcula is large, and the scapula has a well developed acromion. The sternum is unknown. The radius and ulna are approximately equal in size. In the manus the first, second and third digits[97] are present, each terminated by a claw. The second digit is considerably the longest, while the third includes four phalanges. The three bones of the pelvis probably remained distinct throughout life. The tarsals are ankylosed respectively to the tibia and metatarsals as in other birds. The metatarsals are ankylosed together, and the pes has four digits.

Subclass II. Neornithes.

To this subclass may be referred all known birds except Archaeopteryx. They all agree in having a short tail whose component vertebrae are commonly ankylosed together forming a pygostyle. The three metacarpals do not all remain distinct. The bones of the pelvis are ankylosed together, and to a large though variable number of vertebrae. There are three orders, the Ratitae, Odontolcae, and Carinatae.

Order 1. Ratitae.

The Ratitae differ from Archaeopteryx and the great majority of Carinatae in being flightless. The bones are generally not pneumatic, containing marrow instead of air, in the Ostrich however they are very pneumatic. The tail is short and the posterior caudal vertebrae are generally ankylosed together forming a pygostyle. The pectoral girdle has comparatively a much smaller size than in Carinatae, clavicles are small or absent, and the scapula and coracoid lie nearly in the same straight line. The ilium and ischium do not as in Carinatae unite posteriorly, and enclose a foramen except in very old Rheas and Emeus. The quadrate articulates with the cranium by a single head. The vomers unite and form a broad plate, separating the palatines, pterygoids and basisphenoidal rostrum.

The anterior limbs are greatly reduced in size or even absent, while the posterior limbs are greatly developed and adapted for running. The tibia and fibula are quite distinct.

Many ornithologists agree that the various forms grouped together as Ratitae are not all very closely allied to one another, that they resemble one another mainly in having lost the power of flight, and do not form a natural group.

The Ratitae include the following groups:—

Æpyornithes[98], huge extinct birds from Madagascar.

Apteryges, including the Apteryx of New Zealand.

Dinornithes[99], the Moas, huge extinct birds from New Zealand, and some of the neighbouring islands.

Megistanes, including the Cassowaries (Casuarius) of Australia, New Guinea, and some of the neighbouring islands; and the Emeus (Dromaeus) of Australia.

Rheornithes, including the Rheas of S. America.

Struthiornithes, including the Ostriches (Struthio) now living in Africa, and found fossil in N. India and Samos.

Order 2. Odontolcae.

This order includes only an extinct N. American bird Hesperornis[100]. The jaws are provided with a series of sharp teeth placed in continuous grooves, but the premaxillae are toothless, and were probably sheathed in a horny beak. The rami of the mandible are not ankylosed together in front. The skeleton is not pneumatic. The cervical vertebrae have saddle-shaped articulating surfaces as in ordinary birds, and the thoracic vertebrae are not ankylosed together. The tail is comparatively long, and formed of twelve vertebrae with only slight indications of a pygostyle. The ribs have uncinate processes. The anterior limb is quite vestigial, being reduced to a slender humerus. The posterior limb is very powerful and adapted for swimming.

Order 3. Carinatae.

This order includes the vast majority of living birds. The cervical vertebrae have saddle-shaped articulating surfaces (except in the Ichthyornithiformes). The posterior caudal vertebrae are ankylosed forming a pygostyle. The quadrate articulates with the cranium by a double head. In all except the Tinamidae the vomers are narrow behind and not interposed between the palatines, pterygoids and basisphenoidal rostrum. The sternum has a median keel, and the anterior limbs are in the great majority of cases adapted for flight. Clavicles are well developed, and the scapula and coracoid are nearly at right angles to one another. The various groups into which the Carinatae are divisible are shown in the table on pp. 40-42. Their special characters will not be dealt with.